QUESTIONS AND ANSWERS ABOUT THE MERGER AND THE SPECIAL MEETINGS

        The following questions and answers are intended to address briefly some commonly asked questions regarding the merger, the merger agreement and the special meetings. These questions and answers highlight only some of the information contained in this joint proxy statement/prospectus and may not contain all the information that is important to you. You should read carefully this entire joint proxy statement/prospectus, including the Annexes and the documents incorporated by reference into this joint proxy statement/prospectus, to understand fully the merger and the voting procedures for the special meetings. See "Where You Can Find More Information" beginning on page 196.

Q1:

What is the merger?

A:

American Axle & Manufacturing Holdings, Inc. (which we refer to in this joint proxy statement/prospectus as AAM), Alpha SPV I, Inc., a wholly-owned subsidiary of AAM (which we refer to in this joint proxy statement/prospectus as Merger Sub), and Metaldyne Performance Group Inc. (which we refer to in this joint proxy statement/prospectus as MPG) have entered into an Agreement and Plan of Merger, dated as of November 3, 2016, as it may be amended from time to time (which we refer to in this joint proxy statement/prospectus as the merger agreement). A copy of the merger agreement is attached as Annex A to this joint proxy statement/prospectus. The merger agreement contains the terms and conditions of the proposed acquisition of MPG by AAM. Under the merger agreement, subject to satisfaction or, where permissible under applicable law, waiver of the conditions to the merger set forth in the merger agreement and described hereinafter, Merger Sub will merge with and into MPG, with MPG continuing as the surviving corporation and as a wholly-owned subsidiary of AAM (which we refer to in this joint proxy statement/prospectus as the merger). The merger will be effective at the time (which we refer to in this joint proxy statement/prospectus as the effective time of the merger) the certificate of merger is filed with the Secretary of State of the State of Delaware (or at a later time, if agreed upon by the parties and specified in the certificate of merger filed with the Secretary of State of the State of Delaware).

Q2:

Why am I receiving this joint proxy statement/prospectus?

A:

You are receiving this joint proxy statement/prospectus in connection with the proposed acquisition of MPG by AAM because you were a stockholder of record as of the record date for the AAM special meeting and/or MPG special meeting, as applicable. The proposed acquisition is contemplated by the merger agreement.

In order to complete the transactions contemplated by the merger agreement, the stockholders of AAM must approve the issuance of shares of common stock, par value $0.01 per share, of AAM (which we refer to in this joint proxy statement/prospectus as AAM common stock) as part of the merger consideration to be paid to stockholders of MPG pursuant to the merger agreement (which we refer to in this joint proxy statement/prospectus as the AAM share issuance), and the stockholders of MPG must adopt the merger agreement and approve the transactions contemplated by the merger agreement. AAM and MPG will hold separate special meetings of their stockholders to obtain such required approvals, and you are receiving this joint proxy statement/prospectus in connection with those special meetings. For a summary of certain provisions of the merger agreement, see the section entitled "The Merger Agreement" beginning on page 123. In addition, a copy of the merger agreement is attached to this joint proxy statement/prospectus as Annex A. We urge you to read carefully this joint proxy statement/prospectus and the merger agreement in their entirety.

Q3:

What are AAM stockholders being asked to vote on and how many votes are required?

A:

AAM stockholders are being asked to vote to:

•

approve the AAM share issuance (which we refer to in this joint proxy statement/prospectus as the AAM share issuance proposal), which requires approval by the affirmative vote of holders of a majority in voting power of the shares of AAM common stock present in person or represented by proxy at the AAM special meeting and entitled to vote on the AAM share issuance; and

•

approve the adjournment of the AAM special meeting, if necessary or appropriate, to permit further solicitation of proxies if there are not sufficient votes at the time of the AAM special meeting to approve the AAM share issuance (which we refer to in this joint proxy statement/prospectus as the AAM adjournment proposal), which requires the affirmative vote of holders of a majority in voting power of the AAM common stock present in person or represented by proxy, and entitled to vote on the proposal at the AAM special meeting.

Q4:

What are MPG stockholders being asked to vote on and how many votes are required?

A:

MPG stockholders are being asked to vote to:

•

adopt the merger agreement and approve the transactions contemplated by the merger agreement, which requires the affirmative vote of the majority of the shares of common stock, par value $0.001 per share, of MPG (which we refer to in this joint proxy statement/prospectus as MPG common stock) that are outstanding as of the record date for the MPG special meeting and entitled to vote on the adoption of the merger agreement and the approval of the transactions contemplated by the merger agreement at the MPG special meeting;

•

approve, on an advisory non-binding basis, the compensation that may be paid or become payable to MPG's named executive officers in connection with the merger (which we refer to in this joint proxy statement/prospectus as the MPG merger-related compensation proposal), which requires the affirmative vote of the majority of the shares of MPG common stock present in person or represented by proxy and voting on the proposal at the MPG special meeting; and

•

approve the adjournment of the MPG special meeting, if necessary or appropriate, to permit further solicitation of proxies if there are not sufficient votes at the time of the MPG special meeting to adopt the merger agreement and approve the transactions contemplated by the merger agreement (which we refer to in this joint proxy statement/prospectus as the MPG adjournment proposal), which requires the affirmative vote of the majority of the shares of MPG common stock present in person or represented by proxy and voting on the proposal at the MPG special meeting.

Contemporaneously with the execution of the merger agreement, ASP MD Investco L.P. (which we refer to in this joint proxy statement/prospectus as the AS stockholder), a stockholder of MPG and an affiliate of American Securities LLC (which we refer to in this joint proxy statement/prospectus as American Securities), entered into a voting agreement with AAM (which we refer to in this joint proxy statement/prospectus as the voting agreement) with respect to all shares of MPG common stock owned by the AS stockholder, which constituted approximately 77% of the issued and outstanding shares of MPG common stock as of the date of the voting agreement. Under the voting agreement, the AS stockholder has agreed to, among other things and subject to the terms and conditions of the voting agreement, vote shares of MPG common stock owned by the AS stockholder constituting approximately 38% of the issued and outstanding shares of MPG common stock as of the date of the voting agreement in favor

of the adoption of the merger agreement and the approval of the transactions contemplated by the merger agreement, and vote all other shares of MPG common stock owned by the AS stockholder in the same proportion as the manner in which the shares of MPG common stock not owned by the AS stockholder are voted. As a result, assuming approximately 20% of the outstanding shares of MPG common stock not owned by the AS stockholder vote in favor of the adoption of the merger agreement and approval of the transactions contemplated thereby, that proposal will be adopted by the requisite vote. You should read "The Voting Agreement" beginning on page 155 for a more complete discussion of the terms and conditions contained in the voting agreement. A copy of the voting agreement is attached to this joint proxy statement/prospectus as Annex B.

Q5:

What will MPG stockholders receive in the merger?

A:

At the effective time of the merger, each share of MPG common stock (other than MPG excluded shares, as described below) will be converted into the right to receive the following (which we collectively refer to in this joint proxy statement/prospectus as the merger consideration):

•

$13.50 in cash, without interest (which we refer to in this joint proxy statement/prospectus as the cash consideration); and

•

0.5 (which we refer to in this joint proxy statement/prospectus as the exchange ratio) of a share of AAM common stock (which we refer to in this joint proxy statement/prospectus as the stock consideration).

In lieu of the issuance of any fractional share of AAM common stock to which an MPG stockholder would otherwise be entitled, an MPG stockholder will be entitled to receive an amount in cash, without interest and rounded down to the nearest whole cent (and subject to applicable withholding of taxes), equal to the product obtained by multiplying (a) the fractional share of AAM common stock to which the stockholder would otherwise be entitled (after taking into account all fractional share interests then held by the stockholder) by (b) the average of the volume weighted averages of the trading prices of shares of AAM common stock on the NYSE on each of the 5 consecutive trading days ending on the trading day that is 2 trading days prior to the closing date of the merger (which we refer to in this joint proxy statement/prospectus as the closing date).

Shares of MPG common stock held in the treasury of MPG or that are owned by AAM or any direct or indirect subsidiary of AAM (including Merger Sub), which will automatically be canceled in the merger, and shares of MPG common stock with respect to which appraisal rights are properly exercised and not withdrawn (which we refer to collectively in this joint proxy statement/prospectus as the MPG excluded shares) will not be converted into the right to receive the merger consideration.

Q6:

How will AAM pay the cash consideration component of the merger consideration?

A:

AAM's obligation to complete the merger is not conditioned upon its obtaining financing. AAM anticipates that approximately $990 million will be required to pay the aggregate cash consideration to the MPG stockholders. AAM intends to fund the cash consideration of the merger through sources of debt financing.

For a more complete description of sources of funding for the merger and related costs, see "Financing Relating to the Merger" beginning on page 121.

Q7:

Does the AAM board of directors support the merger and the AAM share issuance?

A:

Yes. The AAM board of directors unanimously (i) determined that the merger is fair to, and in the best interests of, AAM and its stockholders, (ii) approved the merger agreement and the transactions contemplated thereby, including the merger and the AAM share issuance, and declared their advisability, and (iii) recommends that AAM stockholders vote "FOR" the AAM share issuance and "FOR" the AAM adjournment proposal.

Q8:

Does the MPG board of directors support the merger and the MPG merger-related compensation proposal?

A:

Yes. The MPG board of directors unanimously (i) determined that entering into the merger agreement and consummating the transactions contemplated thereby (including the merger) is fair to, and in the best interests of, MPG and its stockholders, (ii) approved the merger agreement, the merger and the transactions contemplated by the merger agreement and declared the merger agreement and the merger advisable, and (iii) recommends that MPG stockholders vote "FOR" the adoption of the merger agreement and the approval of the transactions contemplated by the merger agreement, "FOR" the MPG merger-related compensation proposal and "FOR" the MPG adjournment proposal.

Q9:

Are there risks involved in undertaking the merger?

A:

Yes. In evaluating the merger, you should carefully consider the factors discussed in the section of this joint proxy statement/prospectus entitled "Risk Factors" beginning on page 34, as well as the other information about AAM and MPG contained or included in the documents incorporated by reference in this joint proxy statement/prospectus.

Q10:

When and where is the AAM special meeting?

A:

The AAM special meeting will be held on Wednesday, April 5, 2017, at 8:00 a.m., local time, at AAM World Headquarters Auditorium, One Dauch Drive, Detroit, Michigan 48211. AAM stockholders may attend the AAM special meeting and vote their shares in person, or may appoint a proxy by completing, signing, dating and returning the enclosed proxy card for the AAM special meeting. AAM stockholders may also follow the instructions on the proxy card or voting instruction form to appoint a proxy by telephone or via the Internet. If you hold your shares of AAM common stock in "street name" through a bank, broker or other nominee, you should follow the instructions provided to you by that nominee in order to vote your shares.

Q11:

When and where is the MPG special meeting?

A:

The MPG special meeting will be held on Wednesday, April 5, 2017, at 8:30 a.m., local time, at Two Towne Square, Suite 110, Conference Center, Southfield, Michigan 48076. MPG stockholders of record may attend the MPG special meeting and vote their shares in person, or may appoint a proxy by completing, signing, dating and returning the enclosed proxy card for the MPG special meeting. MPG stockholders may also follow the instructions on the proxy card or voting instruction form to appoint a proxy by telephone or via the Internet. If you hold your shares of MPG common stock in "street name" through a bank, broker or other nominee, you should follow the instructions provided to you by that nominee in order to vote your shares.

Q12:

Who can vote at the AAM special meeting?

A:

You can vote at the AAM special meeting if you were the record holder of shares of AAM common stock as of the close of business on Friday, February 24, 2017 (which we refer to in this

joint proxy statement/prospectus as the AAM record date). As of the AAM record date, there were 76,476,788 shares of AAM common stock outstanding.

Q13:

Who can vote at the MPG special meeting?

A:

You can vote at the MPG special meeting if you were the record holder of shares of MPG common stock as of the close of business on Friday, February 24, 2017 (which we refer to in this joint proxy statement/prospectus as the MPG record date). As of the MPG record date, there were 67,917,752 shares of MPG common stock outstanding.

Q14:

What is the quorum requirement for the AAM special meeting?

A:

Stockholders who hold shares representing a majority in voting power of the shares of AAM common stock issued and outstanding and entitled to vote at the AAM special meeting, present in person or represented by proxy, will constitute a quorum for the transaction of business at the AAM special meeting. Abstentions will be treated as present at the AAM special meeting for purposes of determining the presence or absence of a quorum for the transaction of all business. Because all of the matters to be voted on are considered "non-routine" under New York Stock Exchange (which we refer to in this joint proxy statement/prospectus as the NYSE) Rule 452, there can be no broker non-votes at the AAM special meeting. Accordingly, failure to provide instructions to your bank, broker or other nominee on how to vote will result in your shares not being counted as present for establishing a quorum at the AAM special meeting.

Q15:

What is the quorum requirement for the MPG special meeting?

A:

Stockholders of record who hold shares representing a majority in voting power of the shares of MPG common stock entitled to vote at the MPG special meeting, present in person or represented by proxy, will constitute a quorum for the transaction of business at the MPG special meeting. Abstentions will be treated as present at the MPG special meeting for purposes of determining the presence or absence of a quorum for the transaction of all business. Because all of the matters to be voted on are considered to be "non-routine" under NYSE Rule 452, there can be no broker non-votes at the MPG special meeting. Accordingly, failure to provide instructions to your bank, broker or other nominee on how to vote will result in your shares not being counted as present for establishing a quorum at the MPG special meeting.

Q16:

What do AAM stockholders need to do now?

A:

After carefully reading and considering the information contained, or incorporated by reference, in this joint proxy statement/prospectus, please complete, sign and date your proxy card and return it in the enclosed postage-paid return envelope or appoint your proxy by telephone or via the Internet as soon as possible, so that your shares may be represented at the AAM special meeting. If you hold your shares through a bank, broker or other nominee, you should direct the vote of your shares in accordance with the voting instructions received from your bank, broker or other nominee.

If you are an AAM stockholder of record, if you sign and send in your proxy card and do not indicate how you want to vote, the persons named in the proxy card will vote the shares represented by that proxy "FOR" each of the proposals to be voted on at the AAM special meeting described in this joint proxy statement/prospectus, as recommended by the AAM board of directors.

Q17:

What are the consequences of not voting or abstaining from voting at the AAM special meeting?

A:

Approval of the AAM share issuance requires the affirmative vote of holders of a majority in voting power of the shares of AAM common stock present in person or represented by proxy at the AAM special meeting and entitled to vote on the AAM share issuance. Abstentions will have the same effect as a vote "AGAINST" the AAM share issuance, while shares not present at the AAM special meeting (i.e., where there has been a failure to submit a proxy card or to vote in person, or a failure to provide voting instructions to your bank, broker or other nominee) will have no effect on the vote to approve the AAM share issuance.

Approval of the AAM adjournment proposal requires the affirmative vote of holders of a majority in voting power of the shares of AAM common stock present in person or represented by proxy at the AAM special meeting and entitled to vote on such proposal. Abstentions will have the same effect as votes "AGAINST" this proposal, while shares not present at the AAM special meeting will have no effect on the outcome of this proposal.

Q18:

What do MPG stockholders need to do now?

A:

After carefully reading and considering the information contained, or incorporated by reference, in this joint proxy statement/prospectus, please complete, sign and date your proxy card and return it in the enclosed postage-paid return envelope or appoint your proxy by telephone or via the Internet as soon as possible, so that your shares may be represented at the MPG special meeting. If you hold your shares through a bank, broker or other nominee, you should direct the vote of your shares in accordance with the voting instructions received from your bank, broker or other nominee.

If you are an MPG stockholder of record, if you sign and send in your proxy card and do not indicate how you want to vote, the persons named in the proxy card will vote the shares represented by that proxy "FOR" each of the proposals to be voted on at the MPG special meeting described in this joint proxy statement/prospectus, as recommended by the MPG board of directors.

Q19:

What are the consequences of not voting or abstaining from voting at the MPG special meeting?

A:

Approval of the proposal to adopt the merger agreement and approve the transactions contemplated by the merger agreement requires the affirmative vote of the majority of the shares of MPG common stock that are outstanding as of the record date for the MPG special meeting and entitled to vote thereon. Abstentions and shares not present at the MPG special meeting (i.e., where there has been a failure to submit a proxy card or to vote in person, or a failure to provide voting instructions to your bank, broker or other nominee) will have the same effect as a vote "AGAINST" the proposal to adopt the merger agreement and approve the transactions contemplated by the merger agreement.

Approval of each of the MPG merger-related compensation proposal and the MPG adjournment proposal requires the affirmative vote of the majority of the shares of MPG common stock present in person or represented by proxy and voting on such proposals at the MPG special meeting. Abstentions and shares not present at the MPG special meeting will have no effect on the outcome of these proposals.

Q20:

If my shares are held in "street name" by my bank, broker or other nominee, will my bank, broker or other nominee vote my shares for me?

A:

Your bank, broker or other nominee will not vote your shares and your shares will not be present at a meeting unless you provide instructions on how to vote. There will be no broker

non-votes because the only proposals to be voted on at the special meetings are "non-routine" under NYSE Rule 452. Shares for which no instructions have been given will be treated as not present at the respective special meetings. You should follow the directions and instructions provided by your bank, broker or other nominee regarding how to instruct your bank, broker or other nominee to vote your shares.

Q21:

What happens to MPG stock options and other equity-based awards at the effective time of the merger?

A:

Each unvested MPG stock option outstanding immediately prior to the effective time of the merger will be accelerated in full and become fully vested immediately prior to the effective time of the merger and, at the effective time of the merger, all MPG stock options will be cancelled and the holders thereof will receive an amount in cash (without interest and subject to applicable withholding of taxes) equal to the product obtained by multiplying (x) the aggregate number of shares of MPG common stock that were issuable upon exercise of the MPG stock option immediately prior to the effective time of the merger, by (y) the value of the merger consideration (with the AAM common stock issued in the merger valued based on the closing price of a share of AAM common stock as of the trading day immediately preceding the closing date) less the per share exercise price of the MPG stock option. If the exercise price payable upon exercise of an MPG stock option equals or exceeds the value of the merger consideration (with the AAM common stock issued in the merger valued based on the closing price of a share of AAM common stock as of the trading day immediately preceding the closing date), the MPG stock option will be cancelled for no consideration.

Each outstanding share of MPG common stock that is unvested or subject to a repurchase option, risk of forfeiture or other condition under an equity plan of MPG (which we refer to in this joint proxy statement/prospectus as restricted MPG common stock), immediately prior to the effective time of the merger, will be cancelled and terminated at the effective time of the merger and the holder thereof will receive the per share merger consideration for such share of restricted MPG common stock (subject to applicable withholding of taxes).

Each outstanding MPG restricted stock unit award under an MPG equity plan will become fully vested immediately prior to the effective time of the merger and will be cancelled and terminated as of the effective time of the merger, and the holder of the MPG restricted stock unit award will be paid the merger consideration multiplied by the number of shares of MPG common stock subject to the MPG restricted stock unit award (subject to applicable withholding of taxes).

Q22:

What happens if the non-binding advisory proposal to approve compensation that will or may be paid or become payable by MPG to its named executive officers in connection with the merger is not approved?

A:

Approval, on an advisory (non-binding) basis, of compensation that will or may be paid or become payable by MPG to its named executive officers in connection with the merger is not a condition to completion of the merger. The vote is an advisory vote and is not binding. See "Interests of MPG Directors and Executive Officers in the Merger" for a description of the compensation that may be paid or become payable to MPG's named executive officers in connection with the merger.

Q23:

What if I hold shares in both AAM and MPG?

A:

You will receive separate proxy or voting instruction cards for each company and must complete, sign and date each proxy or voting instruction card and return each proxy or voting instruction

card in the appropriate postage-paid envelope or, if available, by submitting a proxy or voting instructions by telephone or via the Internet for each company.

Q24:

What happens if I sell my shares of AAM common stock before the AAM special meeting?

A:

The record date for the AAM special meeting is earlier than the date of the AAM special meeting and the date that the merger is expected to be completed. If you transfer your shares of AAM common stock after the AAM record date, but before the AAM special meeting, then, unless the transferee requests a proxy, you will retain your right to vote at the AAM special meeting.

Q25:

What happens if I sell my shares of MPG common stock before the MPG special meeting?

A:

The record date for the MPG special meeting is earlier than the date of the MPG special meeting and the date that the merger is expected to be completed. If you transfer your shares of MPG common stock after the MPG record date, but before the MPG special meeting, then, unless the transferee requests a proxy, you will retain your right to vote at the MPG special meeting but will have transferred any right to receive the merger consideration applicable to such shares if the merger is completed. In order to receive the merger consideration applicable to your shares of MPG common stock as set out in the merger agreement, you must hold your shares through the completion of the merger.

Q26:

What happens if I sell my shares of MPG common stock after the MPG special meeting, but before the completion of the merger?

A:

If you transfer your shares of MPG common stock after the MPG special meeting, but before the completion of the merger, you will have transferred any right to receive the merger consideration applicable to such shares if the merger is completed. In order to receive the merger consideration for your shares of MPG common stock as set out in the merger agreement, you must hold your shares through the completion of the merger.

Q27:

Do AAM stockholders have to vote on the AAM share issuance at the AAM special meeting if the AAM board of directors has changed its recommendation to vote in favor of such proposal?

A:

Yes. Unless the merger agreement is terminated before the AAM special meeting (which MPG has the right to do in the event the AAM board of directors changes its recommendation with respect to the AAM share issuance), AAM will notify AAM stockholders before the AAM special meeting if the AAM board of directors has changed its recommendation that AAM stockholders vote to approve the AAM share issuance. However, unless the merger agreement is terminated before the AAM special meeting, AAM stockholders will be asked to vote on the AAM share issuance even if the AAM board of directors has so changed its recommendation. Under certain circumstances specified in the merger agreement, the AAM board of directors may terminate the merger agreement in addition to changing its recommendation, in which case the AAM special meeting would not occur and the AAM stockholders would not have to vote on the AAM share issuance. You should read "The Merger Agreement—Termination of the Merger Agreement" beginning on page 134 for a more complete discussion of the rights of MPG and AAM to terminate the merger agreement.

Q28:

Do MPG stockholders have to vote on the adoption of the merger agreement and the approval of the transactions contemplated by the merger agreement at the MPG special meeting if the MPG board of directors has changed its recommendation to vote in favor of such proposal?

A:

Yes. Unless the merger agreement is terminated before the MPG special meeting (which AAM has the right to do in the event that the MPG board of directors changes its recommendation that MPG stockholders vote to adopt the merger agreement and approve the transactions contemplated thereby), MPG will notify MPG stockholders before the MPG special meeting if the MPG board of directors has changed its recommendation that MPG stockholders vote to approve the adoption of the merger agreement and approve the transactions contemplated by the merger agreement. However, unless the merger agreement is terminated before the MPG special meeting, MPG stockholders will be asked to vote on such adoption and approval even if the MPG board of directors has so changed its recommendation. Under certain circumstances specified in the merger agreement, the MPG board of directors may terminate the merger agreement in addition to changing its recommendation, in which case the MPG special meeting would not occur and the MPG stockholders would not have to vote on the adoption of the merger agreement and the approval of the transactions contemplated by the merger agreement. You should read "The Merger Agreement—Termination of the Merger Agreement" beginning on page 134 for a more complete discussion of the rights of MPG and AAM to terminate the merger agreement.

Q29:

If I am an AAM stockholder, can I revoke my proxy after I have mailed my signed proxy?

A:

Yes. You can revoke your proxy before your proxy is voted at the AAM special meeting, including in the event that the AAM board of directors changes its recommendation. You can revoke your proxy in one of three ways:

•

you can send a written notice to AAM's Secretary stating that you would like to revoke your proxy;

•

you can complete and submit a new valid proxy bearing a later date by mail, telephone or via the Internet; or

•

you can attend the AAM special meeting and vote in person.

Attendance at the AAM special meeting will not, in and of itself, constitute revocation of a proxy; you must also vote by ballot at the AAM special meeting to revoke a prior proxy.

If you are an AAM stockholder and you choose to send a written notice of revocation or to mail a new proxy, you must submit your notice of revocation or your new proxy to American Axle & Manufacturing Holdings, Inc., Attention: Secretary, One Dauch Drive, Detroit, Michigan 48211-1198, and it must be received at any time before the start of the AAM special meeting. Any proxy that you submitted via the Internet or by telephone may be revoked by submitting a new proxy via the Internet or by telephone, not later than 11:59 p.m. Eastern Time on Tuesday, April 4, 2017, or by voting in person at the meeting. If your shares are held in the name of a bank, broker or other nominee, you should contact your bank, broker or other nominee to change your vote.

Q30:

If I am an MPG stockholder, can I revoke my proxy after I have mailed my signed proxy?

A:

Yes. You can revoke your proxy at any time before your proxy is voted at the MPG special meeting, including in the event that the MPG board of directors changes its recommendation. You can revoke your proxy in one of three ways:

•

you can send a written notice to MPG's Secretary stating that you would like to revoke your proxy;

•

you can complete and submit a new valid proxy bearing a later date by telephone or via the Internet; or

•

you can attend the MPG special meeting and vote in person.

Attendance at the MPG special meeting will not, in and of itself, constitute revocation of a proxy; you must also vote by ballot at the MPG special meeting to revoke a prior proxy.

If you are an MPG stockholder and you choose to send a written notice or to mail a new proxy, you must submit your notice of revocation or your new proxy to Metaldyne Performance Group Inc., Attention: Secretary, One Towne Square, Suite 550, Southfield, Michigan 48076, and it must be received at any time before the vote is taken at the MPG special meeting. Any proxy that you submitted may also be revoked by submitting a new proxy via the Internet or by telephone, not later than 11:59 p.m. Eastern Time on Tuesday, April 4, 2017, or by voting in person at the meeting. If your shares are held in the name of a bank, broker or other nominee, you should contact them to change your vote.

Q31:

Where can I find the results of the AAM special meeting or the MPG special meeting?

A:

Each of AAM and MPG intends to announce preliminary voting results at the applicable special meeting and publish final results through a Current Report on Form 8-K that each company will file with the U.S. Securities and Exchange Commission (which we refer to in this joint proxy statement/prospectus as the SEC) within four business days of the applicable special meeting.

Q32:

If I am an MPG stockholder and my shares are represented by physical stock certificates, should I send in my stock certificates now?

A:

No. After the merger is completed, you will receive a transmittal form with instructions for the surrender of your MPG common stock certificates. Please do not send in your stock certificates with your proxy card.

Q33:

Is the merger expected to be taxable to MPG stockholders?

A:

The receipt of the merger consideration in exchange for MPG common stock pursuant to the merger will be a taxable transaction for U.S. federal income tax purposes and may also be taxable under state, local and non-U.S. income and other tax laws. For U.S. federal income tax purposes, if you held MPG common stock as a capital asset, you will generally recognize capital gain or loss as a result of the merger measured by the difference, if any, between (1) the sum of (a) the amount of cash you receive in the merger including amounts, if any, withheld from the merger consideration otherwise payable to you and paid to taxing authorities by AAM or other applicable withholding agents and (b) the fair market value, at the effective time of the merger, of the shares of AAM common stock you receive in the merger and (2) the adjusted tax basis in your shares of MPG common stock immediately prior to the effective time of the merger. You should read "The Merger—Material U.S. Federal Income Tax Consequences of the Merger" beginning on page 107 for a more detailed discussion of the material U.S. federal income tax consequences of the merger.

Q34:

When do you expect the merger to be completed?

A:

AAM and MPG are working to complete the merger as quickly as possible. Subject to obtaining the required approvals of our stockholders described in this joint proxy statement/prospectus at our respective special meetings, and satisfaction of the other conditions set out in the merger agreement, AAM and MPG currently expect that the merger will be completed during the first half of 2017. However, it is possible that factors outside of our control could require us to complete the merger at a later time or not complete it at all. Either party may terminate the merger agreement if the merger is not completed by August 3, 2017, subject to an automatic 90 day extension under certain circumstances related to the receipt of certain required regulatory approvals and absence of certain restraints, unless the failure of the party seeking to terminate the merger agreement to have fulfilled any of its obligations under the merger agreement was the principal cause of, or resulted in, the failure of the merger to have occurred by such date. For a description of certain matters that could delay or prevent the completion of the merger, please refer to "Risk Factors" beginning on page 34.

Q35:

What are the conditions to the completion of the merger?

A:

In addition to the approval of the AAM share issuance by AAM stockholders and the adoption of the merger agreement and the approval of the transactions contemplated by the merger agreement by MPG stockholders, the completion of the merger is subject to the satisfaction of a number of other conditions, including receipt of certain regulatory approvals. For additional information on the regulatory approvals required to complete the merger, please see the section titled "The Merger—Regulatory Matters" beginning on page 113. For further information on the conditions to the completion of the merger, please see the section titled "The Merger Agreement—Conditions to the Completion of the Merger" beginning on page 126.

Q36:

Will I be paid dividends prior to the merger?

A:

MPG has historically paid quarterly cash dividends to its stockholders. Under the merger agreement, MPG may continue to make its regular quarterly cash dividends consistent with past practice (including as to the record date, timing of payment and amount thereof) in an aggregate amount per quarter not in excess of $0.0925 per share of MPG common stock. On February 24, 2017, the MPG board of directors declared a dividend of $0.0925 per share of MPG common stock. The dividend is payable on March 24, 2017 to holders of record of MPG common stock as of March 10, 2017. MPG expects to make additional public announcements from time to time prior to the completion of the merger with respect to the declaration and timing of the payment of dividends to its stockholders.

AAM has not paid cash dividends to its stockholders since 2008 and does not intend to pay (and is prohibited under the merger agreement from paying) dividends prior to the merger.

Q37:

Can I seek appraisal of my shares of MPG common stock?

A:

Under Delaware law, holders of shares of MPG common stock who do not vote in favor of adoption of the merger agreement are entitled to demand appraisal of their shares of MPG common stock in accordance with the applicable provisions of Delaware law, and, if such rights are properly demanded, perfected and are not withdrawn or otherwise lost and the merger is completed, such stockholders will receive a payment in cash of the judicially determined fair value of such stockholders' shares of MPG common stock and will not be entitled to receive the merger consideration for their shares. To exercise such rights, MPG stockholders must strictly follow the procedures prescribed by Delaware law. The failure to strictly follow such procedures can result in the loss of such rights. These procedures are summarized under the section entitled

"The Merger—Appraisal Rights" beginning on page 114. In addition, the text of the applicable appraisal rights provisions of Delaware law is included as Annex E to this joint proxy statement/prospectus, and you are encouraged to read those provisions carefully. It is possible that the fair value of shares of MPG common stock as determined by the Delaware Court of Chancery may be more or less than, or the same as, the merger consideration.

Q38:

Where can I find more information about the companies?

A:

You can obtain more information about AAM and MPG from the various sources described under "Where You Can Find More Information" beginning on page 196.

Q39:

Who can help answer my questions?

A:

If you have any questions about the merger or if you need additional copies of this joint proxy statement/prospectus or the relevant proxy card, you should contact:

SUMMARY

        This summary highlights selected information from this joint proxy statement/prospectus and may not contain all of the information that is important to you. For a more detailed description of the merger and the legal terms of the merger agreement, you should carefully read this entire joint proxy statement/prospectus and the other documents to which we refer you, including in particular the copy of the merger agreement that is attached as Annex A to this joint proxy statement/prospectus and as Exhibit 2.1 to the registration statement on Form S-4, of which this joint proxy statement/prospectus forms a part, filed by AAM with the SEC. See also "Where You Can Find More Information" beginning on page 196. We have included page references parenthetically to direct you to a more detailed description of the topics presented in this summary.

General

The Merger (page 57)

        On November 3, 2016, AAM, Merger Sub and MPG entered into the merger agreement, which is the agreement governing the merger. On the terms and subject to the conditions set out in the merger agreement, Merger Sub, a wholly-owned subsidiary of AAM, will merge with and into MPG, with MPG as the surviving corporation in the merger. Following the completion of the merger, MPG will be a wholly-owned subsidiary of AAM and shares of MPG common stock will no longer be publicly traded.

The Companies (page 56)

        AAM is a leader in the manufacturing, engineering, design and validation of driveline and drivetrain systems and related components and modules, chassis systems, electric drive systems and metal-formed products for light trucks, sport utility vehicles, passenger cars, crossover vehicles and commercial vehicles. In addition to locations in the United States (Michigan, Ohio, and Indiana), AAM has offices or facilities in Brazil, China, Germany, India, Japan, Luxembourg, Mexico, Poland, Scotland, South Korea, Sweden and Thailand. AAM has approximately 13,000 employees globally. AAM was incorporated in Delaware in 1998. AAM's principal executive offices are located at One Dauch Drive, Detroit, Michigan 48211-1198, and its telephone number at that address is 313-758-2000. AAM's website is www.aam.com.

        MPG is a leading provider of highly-engineered lightweight components for use in powertrain and suspension applications for light, commercial and industrial vehicles around the world. MPG produces these components and modules using proprietary metal-forming manufacturing technologies and processes for a global customer base of vehicle OEMs and Tier I suppliers. MPG has a global footprint spanning more than 60 locations in 13 countries across North America, South America, Europe and Asia with approximately 12,000 employees. MPG was incorporated in Delaware in June 2014. MPG's principal executive offices are located at One Towne Square, Suite 550, Southfield, Michigan 48076, and its telephone number at that address is 248-727-1800. MPG's website is www.mpgdriven.com.

        Merger Sub, a Delaware corporation and a wholly-owned subsidiary of AAM, was formed on November 2, 2016, solely for the purpose of effecting the merger. To date, Merger Sub has not conducted any activities other than those in connection with its formation and in connection with the transactions contemplated by the merger agreement. Merger Sub's principal executive offices are located at One Dauch Drive, Detroit, Michigan 48211-1198, and its telephone number at that address is 313-758-2000.

Merger Consideration to be Received by MPG Stockholders (page 106)

        At the effective time of the merger, each share of MPG common stock (other than MPG excluded shares) will be converted into the right to receive (a) $13.50 in cash, without interest, and (b) 0.5 of a share of AAM common stock.

Treatment of MPG Stock Options and Other Equity-Based Awards (page 124)

        Each unvested MPG stock option outstanding under an equity plan of MPG will be accelerated in full and become fully vested immediately prior to the effective time of the merger and, at the effective time of the merger, all MPG stock options will be cancelled and the holders thereof will receive an amount in cash (without interest and subject to applicable withholding of taxes) equal to the product of (x) the aggregate number of shares of MPG common stock that were issuable upon exercise of the MPG stock option immediately prior to the effective time of the merger, and (y) the cash value of the merger consideration, less the per share exercise price of each such MPG stock option. For purposes of the cash payment with respect to the MPG stock options, the stock consideration is valued based on the closing price of a share of AAM common stock as of the trading day immediately preceding the closing date of the merger. If the exercise price payable upon exercise of an MPG stock option equals or exceeds the value of the merger consideration, the MPG stock option will be cancelled for no consideration.

        At the effective time of the merger, each outstanding share of restricted MPG common stock outstanding under an equity plan of MPG will be cancelled and terminated, and each holder thereof will receive the merger consideration for each share of restricted MPG common stock (subject to applicable withholding of taxes).

        Immediately prior to the effective time of the merger, each MPG restricted stock unit award outstanding under an equity plan of MPG will become fully vested and will, at the effective time of the merger, be cancelled and terminated in return for an amount equal to the merger consideration multiplied by the number of shares of MPG common stock subject to such cancelled MPG restricted stock unit award (subject to applicable withholding of taxes).

AAM Board of Directors Following Completion of the Merger

        The merger agreement provides that AAM, prior to the effective time of the merger, will increase the number of members of the AAM board of directors from 8 to 11 and appoint 3 individuals designated by American Securities (who we collectively refer to in this joint proxy statement/prospectus as the AS designees) to serve as new directors, each as a member of a different class, on the AAM board of directors (we refer to each AS designee, after being elected to the AAM board of directors, in this joint proxy statement/prospectus as an AS director) as of the effective time of the merger. Other than such additional directors, no changes to the AAM board of directors are expected in connection with the consummation of the merger. One AS director will also be appointed to each of the committees of the AAM board of directors, subject to applicable requirements or qualifications under applicable law or applicable stock exchange rules (including with respect to director independence).

        The AS designees to the AAM board of directors are current MPG directors George Thanopoulos, Kevin S. Penn and Loren S. Easton.

Appraisal Rights (page 114)

        Under Delaware law, holders of shares of MPG common stock who do not vote in favor of adoption of the merger agreement are entitled to demand appraisal of their shares of MPG common stock in accordance with the applicable provisions of Delaware law, and, if such rights are properly demanded, perfected and are not withdrawn or otherwise lost and the merger is completed, those

stockholders will be entitled to obtain payment of the judicially determined fair value of such stockholders' shares of MPG common stock and will not be entitled to receive the merger consideration. To exercise such rights, MPG stockholders must strictly follow the procedures prescribed by Delaware law. The failure to strictly follow such procedures can result in the loss of such rights. It is possible that the fair value of shares of MPG common stock as determined by the Delaware Court of Chancery may be more or less than, or the same as, the merger consideration.

Material U.S. Federal Income Tax Consequences of the Merger (page 107)

        The receipt of the merger consideration in exchange for MPG common stock pursuant to the merger will be a taxable transaction for U.S. federal income tax purposes and may also be taxable under state, local and non-U.S. income and other tax laws. For U.S. federal income tax purposes, if you held MPG common stock as a capital asset, you will generally recognize capital gain or loss as a result of the merger measured by the difference, if any, between (1) the sum of (a) the amount of cash you receive in the merger including amounts, if any, withheld from the merger consideration otherwise payable to you and paid to taxing authorities by AAM or other applicable withholding agents and (b) the fair market value, at the effective time of the merger, of the shares of AAM common stock you receive in the merger and (2) the adjusted tax basis in your shares of MPG common stock immediately prior to the effective time of the merger. You should read "The Merger—Material U.S. Federal Income Tax Consequences of the Merger" beginning on page 107 for a more detailed discussion of the material U.S. federal income tax consequences of the merger. Tax matters can be complicated, and the tax consequences of the merger to you will depend on your particular tax situation. We urge you to consult your tax advisor to determine the tax consequences of the merger to you, including the effect of U.S. federal, state and local and non-U.S. income and other tax laws.

Recommendation of the AAM Board of Directors (page 70)

        The AAM board of directors unanimously (i) determined that the merger is fair to, and in the best interests of, AAM and its stockholders, (ii) approved the merger agreement and the transactions contemplated by the merger agreement, including the merger and the AAM share issuance, and declared their advisability, and (iii) recommends that AAM stockholders vote "FOR" the AAM share issuance and "FOR" the AAM adjournment proposal.

        To review the background of, and AAM's reasons for, the merger, as well as certain risks related to the merger, see "The Merger—Background to the Merger" beginning on page 57, "The Merger—AAM's Reasons for the Merger and Recommendation of the AAM Board of Directors" beginning on page 70 and "Risk Factors" beginning on page 34, respectively.

Recommendation of the MPG Board of Directors (page 74)

        The MPG board of directors unanimously (i) determined that entering into the merger agreement and consummating the transactions contemplated thereby (including the merger) is fair to, and in the best interests of, MPG and its stockholders, (ii) approved the merger agreement, the merger and the other transactions contemplated by the merger agreement and declared the merger agreement and the merger advisable, and (iii) recommends that MPG stockholders vote "FOR" the adoption of the merger agreement and the approval of the transactions contemplated by the merger agreement, "FOR" the MPG merger-related compensation proposal and "FOR" the MPG adjournment proposal.

        To review the background of, and MPG's reasons for, the merger, as well as certain risks related to the merger, see "The Merger—Background to the Merger" beginning on page 57, "The Merger—MPG's Reasons for the Merger and Recommendation of the MPG Board of Directors" beginning on page 74 and "Risk Factors" beginning on page 34, respectively.

Opinion of AAM's Financial Advisor (page 78)

        In connection with the merger, Greenhill & Co., LLC (who we refer to in this joint proxy statement/prospectus as Greenhill), AAM's financial advisor, delivered to the AAM board of directors an oral opinion on November 2, 2016, which was subsequently confirmed in writing, as to the fairness, from a financial point of view and as of the date of such opinion, to AAM of the merger consideration to be paid by AAM to holders of the shares of MPG common stock pursuant to the merger agreement. The full text of Greenhill's written opinion dated November 2, 2016, which describes, among other things, the assumptions made, procedures followed, matters considered and limitations on the review undertaken in connection with the opinion, is attached as Annex C to this joint proxy statement/prospectus and is incorporated by reference herein in its entirety.

        No opinion or view was expressed as to the relative merits of the merger in comparison to any alternative transactions or strategies that might be available to AAM or in which AAM might engage or as to the underlying business decision of AAM to proceed with or effect the merger. Greenhill provided its opinion to the AAM board of directors for the benefit and use of the AAM board of directors in connection with and for purposes of its evaluation of the merger consideration from a financial point of view and Greenhill's opinion does not address any other aspect of the merger. The opinion is addressed to the AAM board of directors only and does not constitute a recommendation to the AAM board of directors as to whether they should approve the merger or the merger agreement and the transactions contemplated thereby, nor does it constitute a recommendation as to whether AAM stockholders or MPG stockholders should approve the merger or the AAM share issuance, as applicable, at any meeting of the stockholders of AAM or MPG, as the case may be, convened in connection with the merger.

        For a more complete description of Greenhill's opinion, please see the section of this joint proxy statement/prospectus entitled "The Merger—Opinion of AAM's Financial Advisor" beginning on page 78. Please also see Annex C to this joint proxy statement/prospectus.

Opinion of MPG's Financial Advisor (page 85)

        In connection with the merger, Merrill Lynch, Pierce, Fenner & Smith Incorporated (which we refer to in this joint proxy statement/prospectus as BofA Merrill Lynch), MPG's financial advisor, delivered to the MPG board of directors on November 2, 2016 an oral opinion, which was subsequently confirmed by delivery of a written opinion, dated November 2, 2016, as to the fairness, from a financial point of view and as of the date of the opinion, of the merger consideration to be received by holders of MPG common stock. The full text of the written opinion, dated November 2, 2016, of BofA Merrill Lynch, which describes, among other things, the assumptions made, procedures followed, matters considered and limitations on the review undertaken, is attached as Annex D to this joint proxy statement/prospectus and is incorporated by reference herein in its entirety. BofA Merrill Lynch provided its opinion to the MPG board of directors (in its capacity as such) for the benefit and use of the MPG board of directors in connection with and for purposes of its evaluation of the merger. BofA Merrill Lynch's opinion does not address any other aspect of the merger and no opinion or view was expressed as to the relative merits of the merger in comparison to other strategies or transactions that might be available to MPG or in which MPG might engage or as to the underlying business decision of MPG to proceed with or effect the merger. BofA Merrill Lynch's opinion does not constitute a recommendation to any stockholder as to how to vote or act in connection with the merger or any other matter.

        For a more complete description of BofA Merrill Lynch's opinion, please see the section of this joint proxy statement/prospectus entitled "The Merger—Opinion of MPG's Financial Advisor" beginning on page 85. Please also see Annex D to this joint proxy statement/prospectus.

Interests of MPG Directors and Executive Officers in the Merger (page 100)

        In considering the recommendation of the MPG board of directors that MPG stockholders vote to adopt the merger agreement and approve the transactions contemplated by the merger agreement, you should be aware that certain of MPG's directors and executive officers have interests in the merger that differ from, or are in addition to, the interests of MPG stockholders generally. The MPG board of directors was aware of, and considered the interests of, MPG's directors and executive officers in approving the merger agreement. These interests are summarized below and in more detail in the section entitled The Merger—Interests of MPG Directors and Executive Officers in the Merger" beginning on page 100.

Indemnification

        The officers and directors of MPG will have the right to indemnification, exculpation from liabilities and advancement of expenses for events occurring at or prior to the effective time of the merger by the surviving corporation following the merger. Those officers and directors will also have coverage equivalent to that provided under MPG's directors' and officers' liability insurance and/or fiduciary liability insurance policy in effect on the date of the merger agreement with respect to claims arising from facts or events that occurred on or prior to the effective time of the merger for a period of 6 years following the effective time of the merger (and until such later date as of which any action commenced during such 6-year period shall have been finally disposed of).

Employee Benefits Matters

Employment Agreements

        MPG maintains employment agreements with certain of its executive officers. If the applicable executive officer's employment with MPG is terminated by MPG without "cause" or by such executive for "good reason," the executive will be entitled to severance benefits as set forth in the executive's employment agreement. The employment agreements provide that these benefits will be available whether or not the termination occurs in connection with a change in control of MPG.

Change in Control Severance Plan

        Certain of MPG's executive officers are eligible to participate in the MPG Change in Control Severance Plan, which was adopted pursuant to the terms of the merger agreement and provides for the payment of severance to such officers in different tiers to the extent such officers are not party to an employment or other agreement that provides for severance benefits. The merger will constitute a "Change in Control" for purposes of eligibility for severance benefits under the MPG Change in Control Severance Plan.

Comparison of Stockholders' Rights (page 183)

        MPG stockholders, whose rights are currently governed by MPG's amended and restated certificate of incorporation and amended and restated bylaws and Delaware law, will, upon the completion of the merger, become stockholders of AAM and their rights will be governed by AAM's certificate of incorporation and second amended and restated by-laws and Delaware law.

        For a more complete description of the comparison of stockholders' rights, please see the section of this joint proxy statement/prospectus entitled "Comparison of Stockholders' Rights" beginning on page 183.

The AAM Special Meeting

        The AAM special meeting will be held on Wednesday, April 5, 2017, at 8:00 a.m., local time, at AAM World Headquarters Auditorium, One Dauch Drive, Detroit, Michigan 48211. At the AAM special meeting, AAM stockholders will be asked to consider and vote upon the following proposals:

•

approval of the AAM share issuance proposal; and



•

approval of the AAM adjournment proposal.

        Only business that is stated in the notice of the AAM special meeting may be conducted at the AAM special meeting. Any action may be taken on the items of business described above at the AAM special meeting on the date specified above, or on any date or dates to which, by original or later adjournment, the AAM special meeting may be adjourned.

Record Date; Shares Entitled to Vote; Quorum (page 46)

        The AAM board of directors has fixed the close of business on Friday, February 24, 2017 as the record date for the AAM special meeting.

        Each share of AAM common stock is entitled to one vote.

        Stockholders who hold shares representing a majority in the voting power of the shares of AAM common stock issued and outstanding and entitled to vote at the AAM special meeting must be present in person or by proxy to constitute a quorum for voting on the AAM share issuance proposal and the AAM adjournment proposal at the AAM special meeting. On the AAM record date, 76,476,788 shares of AAM common stock were outstanding and entitled to vote at the AAM special meeting.

Vote Required (page 47)

        Approval of the AAM share issuance proposal requires the affirmative vote of holders of a majority in voting power of the shares of AAM common stock present in person or represented by proxy at the AAM special meeting and entitled to vote on the AAM share issuance proposal. Under NYSE rules, abstentions will have the same effect as votes "AGAINST" the AAM share issuance. Shares not present at the AAM special meeting will have no effect on the outcome of the vote on the AAM share issuance proposal.

        Approval of the AAM adjournment proposal requires the affirmative vote of holders of a majority in voting power of the shares of AAM common stock present in person or represented by proxy and entitled to vote on such proposal at the AAM special meeting. Abstentions will have the same effect as votes "AGAINST" the AAM adjournment proposal. Shares not present at the AAM special meeting will have no effect on the outcome of the vote on the AAM adjournment proposal.

Shares Owned by AAM Directors and Executive Officers (page 47)

        On the AAM record date, directors and executive officers of AAM beneficially owned and were entitled to vote 332,129 shares of AAM common stock, which represented less than 1% of the outstanding shares of AAM common stock entitled to vote at the AAM special meeting on such date. Each of the directors and executive officers of AAM has advised AAM that, as of the date hereof, he or she currently expects to vote his or her shares, or cause his or her shares to be voted, "FOR" the AAM share issuance proposal and "FOR" the AAM adjournment proposal.

The MPG Special Meeting

        The MPG special meeting will be held on Wednesday, April 5, 2017, at 8:30 a.m., local time, at Two Towne Square, Suite 110, Conference Center, Southfield, Michigan 48076. At the MPG special meeting, MPG stockholders will be asked to consider and vote upon the following proposals:

•

adoption of the merger agreement and approval of the transactions contemplated by the merger agreement;



•

approval, on an advisory, non-binding basis, of the MPG merger-related compensation proposal; and



•

approval of the MPG adjournment proposal.

        Only business that is stated in the notice of the MPG special meeting or otherwise properly brought before the special meeting in accordance with MPG's amended and restated bylaws may be conducted at the MPG special meeting. Any action may be taken on the items of business described above at the MPG special meeting on the date specified above, or on any date or dates to which, by original or later adjournment, the special meeting may be adjourned.

Record Date; Shares Entitled to Vote; Quorum (page 52)

        The MPG board of directors has fixed the close of business on Friday, February 24, 2017 as the record date for the MPG special meeting.

        Each share of MPG common stock is entitled to one vote.

        Stockholders of record who hold shares representing a majority in voting power of the shares of MPG common stock entitled to vote at the MPG special meeting must be present in person or by proxy to constitute a quorum for voting on the proposal to adopt the merger agreement and approve the transactions contemplated by the merger agreement, the MPG merger-related compensation proposal and the MPG adjournment proposal at the MPG special meeting. On the MPG record date, 67,917,752 shares of MPG common stock were outstanding and entitled to vote at the MPG special meeting.

Vote Required (page 53)

        Adoption of the merger agreement and approval of the transactions contemplated thereby requires the affirmative vote of the majority of the outstanding shares of MPG common stock entitled to vote thereon. Abstentions and shares not present at the MPG special meeting will have the same effect as votes "AGAINST" the adoption of the merger agreement and the approval of the transactions contemplated by the merger agreement.

        Approval of the MPG merger-related compensation proposal requires the affirmative vote of the majority of the shares of MPG common stock present in person or represented by proxy at the MPG special meeting and voting on such proposal. Abstentions and shares not present at the MPG special meeting will have no effect on the outcome of the vote on the MPG merger-related compensation proposal.

        Approval of the MPG adjournment proposal requires the affirmative vote of the majority of the shares of MPG common stock present in person or represented by proxy at the MPG special meeting and voting on such proposal. Abstentions and shares not present at the MPG special meeting will have no effect on the outcome of the vote on the MPG adjournment proposal.

Shares Owned by MPG Directors and Executive Officers (page 53)

        On the MPG record date, directors and executive officers of MPG owned and were entitled to vote 2,185,575 shares of MPG common stock, which represented 3.2% of the outstanding shares of MPG common stock entitled to vote at the MPG special meeting on such date. Each of the directors and executive officers of MPG has advised MPG that, as of the date hereof, he or she currently expects to vote his or her shares, or cause his or her shares to be voted, "FOR" the adoption of the merger agreement and the approval of the transactions contemplated by the merger agreement, "FOR" the MPG merger-related compensation proposal and "FOR" the MPG adjournment proposal.

The Merger Agreement

        The following is a summary of the material provisions of the merger agreement. The following summary of the merger agreement does not purport to be complete and may not contain all of the information about the merger agreement that is important to you. The summary of the material terms of the merger agreement below and elsewhere in this joint proxy statement/prospectus is subject to, and qualified in its entirety by, reference to the full text of the merger agreement, which is attached as Annex A to this joint proxy statement/prospectus and is incorporated herein by reference. You should read the entire merger agreement carefully and in its entirety because it, and not the description below or elsewhere in this joint proxy statement/prospectus, is the legal document that governs the merger.

Conditions to the Completion of the Merger (page 126)

        AAM, MPG and Merger Sub are obligated to complete the merger subject to the satisfaction, or, where permissible under applicable law, written waiver, of the following conditions:

•

the effectiveness of the registration statement for the shares of AAM common stock being issued in the merger, the absence of any stop order suspending such effectiveness and no proceeding seeking a stop order being pending before the SEC;



•

the adoption of the merger agreement and the approval of the transactions contemplated by the merger agreement at the MPG special meeting;



•

the approval of the AAM share issuance at the AAM special meeting;



•

no governmental authority of competent jurisdiction having enacted, issued, promulgated, enforced or entered any decision, injunction, decree, ruling, law or order (whether temporary, preliminary or permanent) that is in effect and enjoins or otherwise prohibits or makes illegal the consummation of the merger and the other transactions contemplated by the merger agreement (which we collectively refer to in this joint proxy statement/prospectus as a restraint);



•

the expiration or termination of the waiting period (and any extensions thereof) under the Hart-Scott-Rodino Antitrust Improvements Act of 1976, as amended, and the rules and regulations promulgated thereunder (which we refer to in this joint proxy statement/prospectus as the HSR Act) applicable to the merger in the United States and the receipt of all consents, approvals, non-disapprovals, orders and other authorizations of any governmental authority in certain other jurisdictions agreed by the parties;



•

the shares of AAM common stock to be issued in the merger having been approved for listing on the NYSE, subject to official notice of issuance; and



•

entry into the stockholders' agreement, effective as of the effective time of the merger.

        AAM's and Merger Sub's obligations to consummate the merger are further subject to satisfaction or, where permissible under applicable law, written waiver (by AAM or Merger Sub), of the following additional conditions:

•

certain representations and warranties of MPG set forth in the merger agreement with respect to (i) the capitalization of MPG being true and correct in all respects, except for de minimis inaccuracies and (ii) the organization and qualification of MPG, the capitalization of MPG's subsidiaries and the absence of certain obligations and arrangements with respect to the equity interests in MPG and its subsidiaries, the authority of MPG relative to the merger agreement, the absence of restrictions under certain takeover laws or provisions in MPG's certificate of incorporation or bylaws, and entitlement of brokers to fees, in each case, being true and correct (without giving effect to any materiality or MPG material adverse effect qualifier in such representations and warranties) in all material respects, in each case of clauses (i) and (ii), as of

the date of the merger agreement and as of the effective time of the merger as though made on and as of such date or time (or, in the case of representations and warranties that address matters only as of a particular date, as of such date);

•

all other representations and warranties of MPG set forth in the merger agreement being true and correct (without giving effect to any MPG material adverse effect, materiality or other similar qualifiers in such representations and warranties) as of the date of the merger agreement and as of the effective time of the merger as though made on and as of such date or time (or, in the case of representations and warranties that address matters only as of a particular date, as of such date) except where the failure of such representations and warranties to be so true and correct does not have, and would not reasonably be expected to have, an MPG material adverse effect;



•

MPG having performed or complied, in all material respects, with all obligations required to be performed or complied with by it under the merger agreement at or prior to the effective time of the merger;



•

the receipt of a certificate from an executive officer of MPG certifying that the above conditions have been satisfied; and



•

the absence of an MPG material adverse effect since the date of the merger agreement.

        MPG's obligation to consummate the merger is further subject to satisfaction or, where permissible under applicable law, written waiver (by MPG), of the following additional conditions:

•

certain representations and warranties of AAM set forth in the merger agreement with respect to (i) the capitalization of AAM being true and correct in all respects, except for de minimis inaccuracies and (ii) the organization and qualification of AAM, the capitalization of AAM's subsidiaries and the absence of certain obligations and arrangements with respect to the equity interests in AAM and its subsidiaries, the authority of AAM relative to the merger agreement, brokers and proposals relating to an alternative transaction with AAM, in each case, being true and correct (without giving effect to any materiality or AAM material adverse effect qualifier in such representations and warranties) in all material respects, in each case of clauses (i) and (ii), as of the date of the merger agreement and as of the effective time of the merger as though made on and as of such date or time (or, in the case of representations and warranties that address matters only as of a particular date, as of such date);



•

all other representations and warranties of AAM set forth in the merger agreement being true and correct (without giving effect to any AAM material adverse effect, materiality or other similar qualifiers in such representations and warranties) as of the date of the merger agreement and as of the effective time of the merger as though made on and as of such date or time (or, in the case of representations and warranties that address matters only as of a particular date, as of such date) except where the failure of such representations and warranties to be so true and correct does not have, and would not reasonably be expected to have, an AAM material adverse effect;



•

AAM having performed or complied, in all material respects, with all obligations required to be performed or complied with by it under the merger agreement at or prior to the effective time of the merger;



•

the receipt of a certificate from an executive officer of AAM certifying that the above conditions have been satisfied;



•

the absence of an AAM material adverse effect since the date of the merger agreement; and

•

the increase of the size of the AAM board of directors from 8 to 11, the appointment of each of the AS designees to a different class on the AAM board of directors and the appointment of one of the AS designees to each of AAM's Executive Committee, Audit Committee, Compensation Committee, and Nominating/Corporate Governance Committee, in each case, effective as of the effective time of the merger.

Termination of the Merger Agreement; Payment of Termination Fees (pages 134 and 138)

        The merger agreement contains provisions addressing the circumstances under which AAM and/or MPG may terminate the merger agreement. In certain circumstances, upon termination of the merger agreement, AAM will be required to pay a termination fee to MPG ranging from $50,897,000 to $101,794,000, depending on the circumstances giving rise to the termination of the merger agreement, and MPG will be required to pay to AAM a termination fee of $50,897,000. In addition, in the event the merger agreement is terminated, in certain circumstances, either party may be required to reimburse the other party for its transaction-related expenses, subject to a $15 million limit. The amount of any expenses paid by either AAM or MPG to the other party will be credited against any termination fee to be paid by such party if the termination fee subsequently becomes payable.

Regulatory Matters (page 113)

        AAM, Merger Sub and MPG have agreed to use their respective reasonable best efforts to consummate and make effective the merger and the other transactions contemplated by the merger agreement, including, among other things, using reasonable best efforts to obtain (and cooperating with the other party to obtain) all regulatory approvals that may be or become necessary for the consummation of the transactions.

        However, in no event will AAM or Merger Sub or their subsidiaries be obligated to take any action in connection with obtaining any required regulatory approval that would require the divestiture of any assets of AAM or MPG or any of their subsidiaries, would limit AAM's freedom of action with respect to, or its ability to retain, MPG and its subsidiaries or any portion thereof or any of AAM's or its affiliates' other assets or businesses, or would, in AAM's reasonable judgment, be expected to have, either individually or in the aggregate, a material adverse impact on any of AAM's businesses or the MPG businesses to be acquired in the merger, except that, if necessary to obtain the regulatory consents or approvals discussed below, AAM will agree to the divestiture of the assets or businesses or products or product lines of MPG and its subsidiaries that, individually or in the aggregate, generated total worldwide revenues of up to $150,000,000 in the twelve month period ended September 30, 2016.

        Under the HSR Act and the rules promulgated thereunder by the Federal Trade Commission (which we refer to in this joint proxy statement/prospectus as the FTC), the merger may not be completed until notification and report forms have been filed by AAM and MPG with the FTC and the Antitrust Division of the Department of Justice (which we refer to in this joint proxy statement/prospectus as the DOJ) and the applicable waiting period has expired or been terminated. AAM and MPG filed their respective notification and report forms under the HSR Act with the FTC and the DOJ on November 18, 2016. To provide the FTC additional time to review the proposed transaction, AAM withdrew its HSR filing effective December 19, 2016 and refiled it on December 21, 2016. On January 20, 2017, at 11:59 p.m. Eastern Standard Time, the HSR Act waiting period expired.

        The merger is also subject to antitrust review by governmental authorities in several foreign jurisdictions in which the companies have a sufficient market presence to require filings. As of the date of this joint proxy statement/prospectus, the parties have made filings in Austria, Germany, Mexico and South Korea. On December 22, 2016, the merger was granted early clearance in Germany. On January 10, 2017, the merger was cleared in Austria. On January 26, 2017, the merger was granted approval in South Korea. On February 16, 2017, the parties refiled their application with respect to the merger with the Federal Economic Competition Commission of Mexico (which we refer to in this joint

proxy statement/prospectus as COFECE) to include additional information requested by COFECE. Approval by COFECE is the only remaining required governmental approval.

No Solicitation (page 129)

        The merger agreement contains restrictions on each of MPG's and AAM's ability to solicit or engage in discussions or negotiations with a third party regarding certain alternative transactions involving each company and/or their respective subsidiaries. Notwithstanding these restrictions, under certain circumstances and subject to certain conditions, prior to the receipt of their respective stockholder approvals, each of MPG and AAM may respond to, and engage in discussions and negotiations with respect to, an unsolicited acquisition proposal. In addition, at any time prior to the receipt of their respective stockholder approvals and subject to certain conditions, in response to an unsolicited written acquisition proposal that the party's board of directors determines provides for a superior proposal, the board of directors of such party may change its recommendation that its stockholders vote to adopt the merger agreement and approve the transactions contemplated thereby, in the case of MPG, or approve the AAM share issuance, in the case of AAM, approve an alternative transaction and/or terminate the merger agreement if such party's board of directors determines that the failure to do so would be inconsistent with its fiduciary duties under applicable law.

Accounting Treatment (page 179)

        The merger will be accounted for using the acquisition method of accounting with AAM as the acquiror.

Timing of the Transaction

        The merger is currently expected to be completed during the first half of 2017. However, it is possible that factors outside of each company's control could require them to complete the merger at a later time or not to complete the merger at all.

The Voting Agreement

        Contemporaneously with entering into the merger agreement, AAM entered into the voting agreement with the AS stockholder with respect to all shares of MPG common stock owned by the AS stockholder, which constituted approximately 77% of the issued and outstanding shares of MPG common stock as of the date of the voting agreement. Under the voting agreement, the AS stockholder has agreed to, among other things and subject to the terms and conditions of the voting agreement, vote shares of MPG common stock owned by the AS stockholder constituting approximately 38% of the issued and outstanding shares of MPG common stock as of the date of the voting agreement in favor of the adoption of the merger agreement and the approval of the transactions contemplated by the merger agreement, and vote all other shares of MPG common stock owned by the AS stockholder in the same proportion as the manner in which the shares of MPG common stock not owned by the AS stockholder are voted. For a more complete description of the voting agreement, see "The Voting Agreement" beginning on page 155.

The Stockholders' Agreement

        AAM, the AS stockholder and, for certain limited purposes, American Securities will enter into a stockholders' agreement (which we refer to in this joint proxy statement/prospectus as the stockholders' agreement) in connection with, and as a condition to, the consummation of the merger, effective as of the effective time of the merger. The stockholders' agreement will contain, among other things, certain voting obligations and transfer and standstill restrictions with respect to the shares of AAM common stock owned following the effective time of the merger by the AS stockholder and any other controlled affiliate of American Securities to whom shares of AAM common stock are transferred. The stockholders' agreement will also grant certain director nomination rights, registration rights and preemptive rights to the AS stockholder. For a more complete description of the stockholders' agreement, see "Stockholders' Agreement" beginning on page 158.

Selected Historical Consolidated Financial Data of AAM

        The selected historical consolidated financial data of AAM for each of the years ended December 31, 2016, 2015 and 2014 and as of December 31, 2016 and 2015 has been derived from AAM's audited consolidated financial statements included in AAM's Annual Report on Form 10-K for the year ended December 31, 2016, filed with the SEC on February 10, 2017, which is incorporated by reference herein. The selected historical consolidated financial data of AAM presented for each of the years ended December 31, 2013 and 2012 and as of December 31, 2014, 2013 and 2012 has been derived from AAM's audited consolidated financial statements not incorporated by reference herein. The results for any historical period are not necessarily indicative of future results of AAM or the combined company following completion of the merger.

        This financial data should be read in conjunction with the consolidated financial statements and related notes incorporated by reference in this joint proxy statement/prospectus, as well as other information that has been filed by AAM with the SEC. See "Where You Can Find More Information" beginning on page 196 for more information on where you can obtain copies of this information.

Selected Historical Consolidated Financial Data of MPG

        The selected historical consolidated financial data of MPG for each of the years ended December 31, 2016, 2015 and 2014 and as of December 31, 2016 and 2015 has been derived from MPG's audited consolidated financial statements included in MPG's Current Report on Form 8-K, filed with the SEC on March 2, 2017, which is incorporated by reference herein. The selected historical consolidated financial data of MPG presented for each of the years ended December 31, 2013 and 2012 and as of December 31, 2014, 2013 and 2012 has been derived from MPG's consolidated financial statements not incorporated by reference herein. The results for any historical period are not necessarily indicative of future results of MPG or the combined company following completion of the merger.

        MPG was formed through the combination of ASP HHI Holdings, Inc. (which, together with its subsidiaries, we refer to in this joint proxy statement/prospectus as HHI), ASP MD Holdings, Inc. (which, together with its subsidiaries, we refer to in this joint proxy statement/prospectus as Metaldyne), and ASP Grede Intermediate Holdings LLC (which, together with its subsidiaries, we refer to in this joint proxy statement/prospectus as Grede) on August 4, 2014 (which we refer to in this joint proxy statement/prospectus as the combination). Each of the three operating groups was owned primarily by certain private equity funds affiliated with American Securities. American Securities acquired its interest in HHI in October 2012, Metaldyne in December 2012, and Grede in June 2014. Each of these acquisitions was accounted for using the acquisition method of accounting, with the respective purchase price allocated to the identifiable assets and liabilities of the acquired entity, with any excess purchase price recorded as goodwill. Effective December 12, 2014, MPG completed an initial public offering of its common stock.

        HHI is presented as the predecessor to MPG for financial reporting purposes. We refer to the period prior to October 6, 2012 as the predecessor period in this joint proxy statement/prospectus and we refer to the period from October 6, 2012 to December 31, 2016 as the successor period in this joint proxy statement/prospectus. The combination was accounted for as a reorganization of entities under common control in a manner similar to a pooling of interests, and, as such, the bases of accounting of HHI, Metaldyne and Grede were carried over to MPG. The selected historical consolidated financial statement data reflects the retrospective application of MPG's capital structure and consolidated presentation of the combination for the successor period.

        As a result of the above transactions, the selected historical consolidated financial data includes:

•

the results of HHI for the five years ended December 31, 2016;



•

the results of Metaldyne from December 18, 2012 through December 31, 2012 and for the years ended December 31, 2013, 2014, 2015 and 2016; and



•

the results of Grede from June 2, 2014 through December 31, 2014 and for the years ended December 31, 2015 and 2016.

        This financial data should be read in conjunction with the consolidated financial statements and related notes incorporated by reference in this joint proxy statement/prospectus, as well as other information that has been filed by MPG with the SEC. See "Where You Can Find More Information" beginning on page 196 for more information on where you can obtain copies of this information.