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                                                                     Exhibit 4.1


                          FIFTH SUPPLEMENTAL INDENTURE

            This Fifth Supplemental Indenture (this "Supplemental Indenture"),
dated as of April 5, 2007, is entered into among Altra Industrial Motion, Inc.,
(the "Company"), the Guarantors signatory hereto (the "Guarantors"), TB Wood's
Incorporated ("TB Wood's"), Plant Engineering Consultants, LLC ("PEC"), TB
Wood's Corporation ("TB Wood's Corp.") and TB Wood's Enterprises, Inc. ("TB
Wood's Enterprises" and together with TB Wood's, PEC and TB Wood's Corp., the
"New Guarantors") and The Bank of New York Trust Company, N.A., as trustee under
the Indenture referred to below (the "Trustee"). Capitalized terms used herein
but not defined shall have the meanings ascribed to them in the Indenture (as
defined below).

                             W I T N E S S E T H

            WHEREAS, the Company has heretofore executed and delivered to the
Trustee an Indenture, dated as of November 30, 2004 (as amended by the First
Supplemental Indenture, dated as of February 8, 2006, the Second Supplemental
Indenture, dated as of February 8, 2006 and the Third Supplemental Indenture,
dated as of April 24, 2006, the "Indenture"), among the Company, the Guarantors
and the Trustee, that provides 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, 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 allow any
Subsidiary or any other Person to guarantee the Notes;

            WHEREAS, the Company, the Guarantors and the New Guarantors desire
to execute and deliver this Supplemental Indenture to the Indenture for the
purpose of allowing the New Guarantors to guarantee the Notes;

            WHEREAS, the execution and delivery of this Supplemental Indenture
has been duly authorized by the Company, the Guarantors and the New Guarantors
and all conditions and requirements necessary to make this Supplemental
Indenture a valid and binding agreement of the Company, the Guarantors and the
New Guarantors 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, the Guarantors, the New Guarantors 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. AGREEMENT TO GUARANTEE. Each of the New Guarantors
hereby fully, jointly and severally, unconditionally and irrevocably guarantees
with each of the Guarantors, to each of the Holders, the Trustee and the
Collateral Agent and their respective

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successors and assigns that (i) the principal of, premium, if any, and interest,
and Additional Interest, if any, on the Notes shall be promptly paid in full
when due, subject to any applicable grace period, whether upon redemption
pursuant to the terms of the Notes, by acceleration or otherwise, and interest
on the overdue principal, if any, and interest on any interest, if any, to the
extent lawful, of the Notes and all other Obligations of the Company to the
Holders, the Trustee and the Collateral Agent hereunder, thereunder or under any
Collateral Agreement shall be promptly paid in full or performed, all in
accordance with the terms of the Indenture or thereof; and (ii) in case of any
extension of time of payment or renewal of any of the Notes or of any such other
obligations, the same shall be promptly paid in full when due or performed in
accordance with the terms of the extension or renewal, subject to any applicable
grace period, whether at stated maturity, by acceleration or otherwise, subject,
however, in the case of clauses (i) and (ii) above, to the limitations set forth
in Section 10.03 (Limitation of Guarantor's Liability) of the Indenture.

            Section 1.02. GUARANTOR BOUND BY TERMS OF INDENTURE. In accordance
with Section 4.15 (Additional Subsidiary Guarantees) of the Indenture, each of
the New Guarantors hereby acknowledges and agrees that it is subject to the
provisions of the Indenture as a Guarantor.

                                   ARTICLE II
                                  EFFECTIVENESS

            Section 2.01. EFFECTIVENESS. This Supplemental Indenture shall
become effective and binding on the Company, the Guarantors, the New Guarantors,
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


                                       2
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Indenture or for or in respect of the recitals contained herein, all of which
are made solely by the Company, the Guarantors and the New Guarantors.

            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, the Guarantors, the New Guarantors 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.

                                    TB WOOD'S INCORPORATED

                                    By: /s/ William T. Fejes, Jr.
                                        --------------------------------
                                    Name: William T. Fejes, Jr.
                                    Title: President and Chief Executive Officer

                                    PLANT ENGINEERING CONSULTANTS, LLC

                                    By: /s/ William T. Fejes, Jr.
                                        --------------------------------
                                    Name: William T. Fejes, Jr.
                                    Title: President

                                    TB WOOD'S CORPORATION

                                    By: /s/ William T. Fejes, Jr.
                                        --------------------------------
                                    Name: William T. Fejes, Jr
                                    Title: President and Chief Executive Officer

                                    TB WOOD'S ENTERPRISES, INC.

                                    By: /s/ Joseph C. Horvath
                                        --------------------------------
                                    Name: Joseph C. Horvath
                                    Title: President and Treasurer


                 SIGNATURE PAGE TO FIFTH SUPPLEMENTAL INDENTURE


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                                    ALTRA INDUSTRIAL MOTION, INC.

                                    By: /s/ David A. Wall
                                        --------------------------------------
                                    Name: David A. Wall
                                    Title: Chief Financial Officer


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

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


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

                                    By: /s/ David A. Wall
                                        --------------------------------
                                    Name: David A. Wall
                                    Title: Chief Financial Officer


                 SIGNATURE PAGE TO FIFTH SUPPLEMENTAL INDENTURE