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                                                                     Exhibit 4.7

                          FIRST SUPPLEMENTAL INDENTURE

This First Supplemental Indenture (this "SUPPLEMENTAL INDENTURE"), dated as of
February 7, 2006, is entered into among Altra Industrial Motion, Inc., a
Delaware corporation (the "COMPANY"), the Guarantors signatory hereto (the
"GUARANTORS") and The Bank of New York Trust Company, N.A., as trustee under the
indenture referred to below (the "TRUSTEE").

                                   WITNESSETH

WHEREAS, the Company has heretofore executed and delivered to the Trustee an
indenture (the "INDENTURE"), dated as of November 30, 2004, providing for the
issuance by the Company of up to an aggregate principal amount of $165,000,000
of 9% Senior Secured Notes due 2011 (the "NOTES");

WHEREAS, capitalized terms used herein but not defined shall have the meanings
ascribed to them in the Indenture;

WHEREAS, Section 9.01 of the Indenture provides, among other things, that the
Company, the Guarantors and the Trustee may amend or supplement the Indenture
and the Notes without the consent of the Holders to cure any ambiguity, defect
or inconsistency contained therein;

WHEREAS, the Company desires to execute and deliver an amendment to the
Indenture for the purpose of clarifying an ambiguity in Section 4.14
(Limitations on Transactions with Affiliates);

WHEREAS, the execution and delivery of this Supplemental Indenture has been duly
authorized and all conditions and requirements necessary to make this
Supplemental Indenture a valid and binding agreement of the Company have been
duly performed and complied with; and

WHEREAS, pursuant to Sections 9.01 and 9.06 of the Indenture, the Trustee is
authorized to execute this Supplemental Indenture.

NOW, THEREFORE, in consideration of the foregoing and for other good and
valuable consideration, the receipt of which is hereby acknowledged, the Company
and the Trustee mutually covenant and agree for the equal and ratable benefit of
the holders of the Notes as follows:

                                   ARTICLE I
                             AMENDMENTS TO INDENTURE

      Section 1.01. AMENDMENT OF ARTICLE FOUR. Section 4.14(b)(2) (Limitations
on Transactions with Affiliates) is hereby amended by deleting the reference to
"Wholly Owned Restricted Subsidiaries" and inserting in lieu thereof "Wholly
Owned Subsidiaries".

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                                   ARTICLE II
                                 EFFECTIVENESS

      Section 2.01. EFFECTIVENESS. This Supplemental Indenture shall become
effective and binding on the Company, the Trustee and the Holders upon execution
and delivery of this Supplemental Indenture by the parties hereto.

                                  ARTICLE III
                                 MISCELLANEOUS

      Section 3.01. INDENTURE RATIFIED. Except as otherwise provided herein, the
Indenture is in all respects ratified and confirmed, and all of the terms,
provisions and conditions thereof shall be and remain in full force and effect.

      Section 3.02. CONSTRUCTION OF SUPPLEMENTAL INDENTURE. This Supplemental
Indenture is executed as and shall constitute an indenture supplemental to the
Indenture and shall be construed in connection with and as part of the
Indenture.

      Section 3.03. TRUST INDENTURE ACT CONTROLS. If any provision of this
Supplemental Indenture limits, qualifies or conflicts with any other provision
of this Supplemental Indenture or the Indenture that is required to be included
by the Trust Indenture Act of 1939, as amended, as in force at the date this
Supplemental Indenture is executed, the provision required by said Act shall
control.

      Section 3.04. COUNTERPARTS. This Supplemental Indenture may be executed in
any number of counterparts, each of which shall be an original, but such
counterparts shall together constitute but one and the same instrument.

      Section 3.05. TRUSTEE NOT RESPONSIBLE. The Trustee shall not be
responsible in any manner whatsoever for or in respect of the validity or
sufficiency of this Supplemental Indenture or for or in respect of the recitals
contained herein, all of which are made solely by the Company.

      Section 3.06. GOVERNING LAW. This Supplemental Indenture shall be governed
by, and construed in accordance with, the laws of the State of New York but
without giving effect to applicable principles of conflicts of law to the extent
that the application of the laws of another jurisdiction would be required
thereby.

      Section 3.07. SUCCESSORS. All covenants and agreements in this
Supplemental Indenture by the Company or the Trustee shall bind their respective
successors and assigns, whether so expressed or not.

      Section 3.08. SEVERABILITY. In case any provisions in this Supplemental
Indenture shall be invalid, illegal or unenforceable, the validity, legality and
enforceability of the remaining provisions shall not in any way be affected or
impaired thereby.

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IN WITNESS WHEREOF, the parties hereto have caused this Supplemental Indenture
to be duly executed and attested, all as of the date first above written.

                          ALTRA INDUSTRIAL MOTION, INC.

                          By: /s/ MICHAEL L. HURT
                              ------------------------------------
                          Name: Michael L. Hurt
                          Title: Chief Executive Officer

                          THE BANK OF NEW YORK TRUST COMPANY, N.A., as Trustee

                          By: /s/ SANDEE PARKS
                              ------------------------------------
                     Name: Sandee Parks
                     Title: Vice President

                          AMERICAN ENTERPRISE MPT CORP.
                          AMERICAN ENTERPRISES MPT HOLDINGS, LLC
                          AMERIDRIVES INTERNATIONAL, LLC
                          BOSTON GEAR LLC
                          FORMSPRAG LLC
                          THE KILIAN COMPANY
                          KILIAN MANUFACTURING CORPORATION
                          NUTTALL GEAR L L C
                          WARNER ELECTRIC INTERNATIONAL HOLDING, INC.
                          WARNER ELECTRIC LLC
                          WARNER ELECTRIC TECHNOLOGY LLC

                          By: /s/ MICHAEL L. HURT
                              ------------------------------------
                          Name: Michael L. Hurt
                          Title: Chief Executive Officer