EX-5.1

Exhibit 5.1

[Letterhead of]

Cravath, Swaine & Moore LLP

[New York Office]

February 8, 2019

Altra Industrial Motion Corp.

Registration Statement on Form S-8

Ladies and Gentlemen:

We have acted as counsel for Altra Industrial Motion Corp., a Delaware corporation (the “Company”), in connection with the registration statement on Form S-8 (the “Registration Statement”), filed by the Company with the Securities and Exchange Commission (the “Commission”) under the Securities Act of 1933, as amended (the “Securities Act”), relating to the Company’s 2,200,000 shares of common stock, par value $0.001 per share (the “Shares”), issuable pursuant to the Altra Industrial Motion Corp. 2014 Omnibus Incentive Plan (as amended, the “2014 Plan”).

In that connection, we have examined originals, or copies certified or otherwise identified to our satisfaction, of such documents, corporate records and other instruments as we have deemed necessary or appropriate for the purposes of this opinion, including: (a) the Second Amended and Restated Certificate of Incorporation of the Company (“Company Articles”); (b) the Certificate of Amendment to the Company Articles; (c) the Second Amended and Restated Bylaws of the Company; (d) certain resolutions adopted by the board of directors of the Company; and (e) the 2014 Plan.

In rendering our opinion, we have assumed the genuineness of all signatures, the legal capacity of all natural persons, the authenticity of all documents submitted to us as originals, the conformity to original documents of all documents submitted to us as certified, conformed or photostatic copies and the authenticity of the originals of such latter documents. As to all questions of fact material to this opinion that have not been independently established, we have relied upon certificates or comparable documents of officers and representatives of the Company.

Based on the foregoing and in reliance thereon, and subject to compliance with applicable state securities laws, we are of opinion that the Shares when, and if, issued pursuant to the terms of the 2014 Plan will be validly issued, fully paid and non-assessable.

We are admitted to practice in the State of New York, and we express no opinion as to matters governed by any laws other than the laws of the State of New York, the General Corporation Law of the State of Delaware and the Federal laws of the United States of America. The reference and limitation to the General Corporation Law of the State of Delaware includes the statutory provisions and all applicable provisions of the Delaware Constitution and reported judicial decisions interpreting these laws.