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                                                                   EXHIBIT 10.31

                                                               EXECUTION VERSION

                               FOURTH AMENDMENT TO
                                CREDIT AGREEMENT

     THIS FOURTH AMENDMENT TO CREDIT AGREEMENT (this "Fourth Amendment") is made
and entered into as of February 16, 2007, by and among Altra Industrial Motion,
Inc., a Delaware corporation, as Administrative Borrower ("Administrative
Borrower") for the Borrowers (as defined below), the lenders listed on the
signatory page hereof (the "Lenders"), and Wells Fargo Foothill, Inc., a
California corporation, in its capacity as the arranger and administrative agent
for the Lenders ("Agent").

                                   WITNESSETH:

     WHEREAS, each of Administrative Borrower, Warner Electric LLC, a Delaware
limited liability company, Kilian Manufacturing Corporation, a Delaware
corporation, Warner Electric Technology LLC, a Delaware limited liability
company, Formsprag LLC, a Delaware limited liability company, Boston Gear LLC, a
Delaware limited liability company, Nuttall Gear L L C, a Delaware limited
liability company, and Ameridrives International L.P., a Delaware limited
partnership (each, a "Borrower" and, collectively, the "Borrowers"), have
entered into a Credit Agreement dated as of November 30, 2004 (as amended as of
December 30, 2004, January 14, 2005, and January 31, 2005, and as may be further
amended, restated, supplemented or otherwise modified from time to time, the
"Credit Agreement"), with the Lenders and Agent; and

     WHEREAS, Borrowers, the Lenders and Agent wish to amend the Credit
Agreement, as provided herein;

     NOW, THEREFORE, in consideration of the agreements and provisions herein
contained, the parties hereto do hereby agree as follows:

SECTION 1. DEFINITIONS. Any capitalized terms used but not otherwise defined
herein shall have the meanings ascribed to such terms in the Credit Agreement.

SECTION 2. AMENDMENT TO CREDIT AGREEMENT. Subject to the satisfaction of the
conditions set forth in Section 4 herein, the Credit Agreement is hereby
amended, effective as of the Effective Date (as defined below), as follows:

     2.01 AMENDMENT TO SCHEDULE 1.1. Schedule 1.1 of the Credit Agreement is
hereby amended by deleting the definition of "Qualified IPO" therein and
inserting the following definition in lieu thereof:

     "Qualified IPO" means an underwritten public offering of common Stock of
     Holdings or Parent pursuant to a registration statement filed with the SEC
     (other than on Form S-8).

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SECTION 3. REPRESENTATIONS AND WARRANTIES. In order to induce Agent and the
Lenders to enter into this Fourth Amendment, Administrative Borrower, for itself
and on behalf of all of the other Borrowers, hereby represents and warrants
that:

     3.01 NO DEFAULT. At and as of the date of this Fourth Amendment, after
giving effect to this Fourth Amendment, no Default or Event of Default has
occurred and is continuing.

     3.02 REPRESENTATIONS AND WARRANTIES TRUE AND CORRECT. At and as of the date
of this Fourth Amendment, each of the representations and warranties contained
in the Credit Agreement and the other Loan Documents is true and correct in all
material respects (except to the extent that such representations and warranties
relate solely to an earlier date).

     3.03 CORPORATE POWER, ETC. Administrative Borrower (a) has all requisite
corporate power and authority to execute and deliver this Fourth Amendment and
to consummate the transactions contemplated hereby for itself and on behalf of
all of the other Borrowers and (b) has taken all action, corporate or otherwise,
necessary to authorize the execution and delivery of this Fourth Amendment and
the consummation of the transactions contemplated hereby for itself and on
behalf of all of the other Borrowers. Administrative Borrower is entering into
this Fourth Amendment on behalf of all of the other Borrowers in accordance with
Sections 14.1 and 16.9 of the Credit Agreement.

     3.04 NO CONFLICT. The execution, delivery and performance by Administrative
Borrower of this Fourth Amendment will not (a) violate any provision of federal,
state, or local law or regulation applicable to any Borrower, the Governing
Documents of any Borrower, or any order, judgment, or decree of any court or
other Governmental Authority binding on any Borrower, (b) conflict with, result
in a breach of, or constitute (with due notice or lapse of time or both) a
default under any material contractual obligation of any Borrower, (c) result in
or require the creation or imposition of any Lien of any nature whatsoever upon
any properties or assets of any Borrower, other than Permitted Liens, or (d)
require any unobtained approval or consent of any Person under any material
contractual obligation of any Borrower.

     3.05 BINDING EFFECT. This Fourth Amendment has been duly executed and
delivered by Administrative Borrower (on behalf of itself and all of the other
Borrowers) and constitutes the legal, valid and binding obligation of
Administrative Borrower (on behalf of itself and all of the other Borrowers),
enforceable against Administrative Borrower (on behalf of itself and all of the
other Borrowers) in accordance with its terms, except as such enforceability may
be limited by (a) applicable bankruptcy, insolvency, reorganization, moratorium
or other similar laws, now or hereafter in effect, relating to or affecting the
enforcement of creditors' rights generally, and (b) the application of general
principles of equity (regardless of whether such enforceability is considered in
a proceeding in equity or at law).


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SECTION 4. CONDITIONS. This Fourth Amendment shall be effective as of December
13, 2006 (the "Effective Date") upon the fulfillment of all of the following
conditions precedent set forth in this Section 4:

     4.01 EXECUTION OF THE FOURTH AMENDMENT. Each of the parties hereto shall
have executed and delivered a counterpart of this Fourth Amendment and shall
have delivered (including by way of telefacsimile or electronic mail) the same
to Agent.

     4.02 REPRESENTATIONS AND WARRANTIES. As of the Effective Date, the
representations and warranties set forth in Section 3 hereof shall be true and
correct.

     4.03 COMPLIANCE WITH TERMS. Borrowers shall have complied in all respects
with the terms hereof and of any other agreement, document, instrument or other
writing to be delivered by Borrowers in connection herewith.

SECTION 5. MISCELLANEOUS.

     5.01 CONTINUING EFFECT. Except as specifically provided herein, the Credit
Agreement and the other Loan Documents shall remain in full force and effect in
accordance with their respective terms and are hereby ratified and confirmed in
all respects.

     5.02 NO WAIVER. This Fourth Amendment is limited as specified and the
execution, delivery and effectiveness of this Fourth Amendment shall not operate
as a modification, acceptance or waiver of any provision of the Credit Agreement
or any other Loan Document, except as specifically set forth herein.

     5.03 REFERENCES.

          (a) From and after the Effective Date, the Credit Agreement, the other
Loan Documents and all agreements, instruments and documents executed and
delivered in connection with any of the foregoing shall each be deemed amended
hereby to the extent necessary, if any, to give effect to the provisions of this
Fourth Amendment.

          (b) From and after the Effective Date, (i) all references in the
Credit Agreement to "this Agreement", "hereto", "hereof", "hereunder" or words
of like import referring to the Credit Agreement shall mean the Credit Agreement
as amended hereby and (ii) all references in the Credit Agreement, the other
Loan Documents or any other agreement, instrument or document executed and
delivered in connection therewith to "Credit Agreement", "thereto", "thereof",
"thereunder" or words of like import referring to the Credit Agreement shall
mean the Credit Agreement as amended hereby.

     5.04 GOVERNING LAW. THIS FOURTH AMENDMENT SHALL BE GOVERNED BY, AND
CONSTRUED IN ACCORDANCE WITH, THE LAWS OF THE STATE OF NEW YORK.

     5.05 SEVERABILITY. The provisions of this Fourth Amendment are severable,
and if any clause or provision shall be held invalid or unenforceable in whole
or in part in any


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jurisdiction, then such invalidity or unenforceability shall affect only such
clause or provision, or part thereof, in such jurisdiction and shall not in any
manner affect such clause or provision in any other jurisdiction, or any other
clause or provision in this Fourth Amendment in any jurisdiction.

     5.06 COUNTERPARTS. This Fourth Amendment may be executed in any number of
counterparts, each of which counterparts when executed and delivered shall be an
original, but all of which shall together constitute one and the same
instrument. Delivery of an executed counterpart of this Fourth Amendment by
telefacsimile or electronic mail shall be equally effective as delivery of a
manually executed counterpart. A complete set of counterparts shall be lodged
with Administrative Borrower and Agent.

     5.07 HEADINGS. Section headings in this Fourth Amendment are included
herein for convenience of reference only and shall not constitute a part of this
Fourth Amendment for any other purpose.

     5.08 BINDING EFFECT; ASSIGNMENT. This Fourth Amendment shall be binding
upon and inure to the benefit of Borrowers, the Lenders and Agent and their
respective successors and assigns; provided, however, that the rights and
obligations of Borrowers under this Fourth Amendment shall not be assigned or
delegated without the prior written consent of Agent.

     5.09 EXPENSES. Borrowers agree to pay Agent for all reasonable expenses,
including reasonable fees of attorneys and paralegals for Agent, incurred by
Agent in connection with the preparation, negotiation and execution of this
Fourth Amendment and any document required to be furnished herewith pursuant to
the terms of the Credit Agreement and the Fee Letter.

                           [Signature page to follow]


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     IN WITNESS WHEREOF, the parties hereto have caused this Fourth Amendment to
be executed by their respective officers thereunto duly authorized, as of the
date first above written.

                                        ALTRA INDUSTRIAL MOTION, INC.,
                                        a Delaware corporation, as
                                        Administrative Borrower on behalf
                                        of itself and all other Borrowers


                                        By: /s/ Michael L. Hurt
                                            ------------------------------------
                                        Name: Michael L. Hurt
                                        Title: Chairman & Chief Executive
                                               Officer


                                        WELLS FARGO FOOTHILL, INC.,
                                        a California corporation, as Agent and
                                        Lender


                                        By: /s/ Vincent J. Egan, Jr.
                                            ------------------------------------
                                        Name: Vincent J. Egan, Jr.
                                        Title: Vice President

                      [SIGNATURE PAGE OF FOURTH AMENDMENT]