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                                                                     Exhibit 4.9

                          THIRD SUPPLEMENTAL INDENTURE

This Third Supplemental Indenture (this "Supplemental Indenture"), dated as of
April 24, 2006, is entered into among Altra Industrial Motion, Inc., (the
"Company"), the Guarantors signatory hereto (the "Guarantors"), Inertia
Dynamics, LLC (the "New Guarantor") 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).

                                   WITNESSETH

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, and the Second Supplemental Indenture,
dated as of February 8, 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 Guarantor desire to execute and
deliver this Supplemental Indenture to the Indenture for the purpose of allowing
the New Guarantor 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 Guarantor and all
conditions and requirements necessary to make this Supplemental Indenture a
valid and binding agreement of the Company, the Guarantors and the New Guarantor
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 Guarantor and the Trustee mutually covenant and
agree for the equal and ratable benefit of the holders of the Notes as follows:

                                   ARTICLE IV
                             AMENDMENTS TO INDENTURE

Section 4.01. AGREEMENT TO GUARANTEE. The New Guarantor 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 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

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

                                   ARTICLE V
                                 EFFECTIVENESS

      Section 5.01. EFFECTIVENESS. This Supplemental Indenture shall become
effective and binding on the Company, the Guarantors, the New Guarantor, the
Trustee and the Holders upon execution and delivery of this Supplemental
Indenture by the parties hereto.

                                   ARTICLE VI
                                 MISCELLANEOUS

      Section 6.01. 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 6.02. 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 6.03. 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 6.04. 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 6.05. 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, the Guarantors
and the New Guarantor.

      Section 6.06. 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.

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      Section 6.07. SUCCESSORS. All covenants and agreements in this
Supplemental Indenture by the Company, the Guarantors, the New Guarantor or the
Trustee shall bind their respective successors and assigns, whether so expressed
or not.

      Section 6.08. 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.

                              INERTIA DYNAMICS, LLC

                              By: /s/ David Wall
                                  --------------------------------------
                              Name: David Wall
                              Title: Manager

                              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 ENTERPRISES 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,
                              as Guarantors

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