EXHIBIT 4.2



                          SECOND SUPPLEMENTAL INDENTURE

            This Second Supplemental Indenture (this "Supplemental Indenture"),
dated as of March 26, 2007, is entered into by and among Altra Industrial
Motion, Inc. (the "Company"), the Guarantors signatory hereto (the
"Guarantors"), and The Bank of New York, 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 February 8, 2006 (the "Indenture"), as
supplemented, among the Company, the Guarantors, the Trustee, The Bank of New
York, as Principal Paying Agent, and The Bank of New York (Luxembourg) S.A., as
Luxembourg Paying Agent, that provides for the issuance by the Company of up to
an aggregate principal amount of (pound)33,000,000 of 11 1/4% Senior Notes due
2013 (the "Notes");

            WHEREAS, Section 9.02 of the Indenture provides, among other things,
that the Company, the Guarantors and the Trustee may amend or supplement the
Indenture and the Notes with the consent of the Holders of at least a majority
in principal amount of the Notes then outstanding (the "REQUISITE HOLDERS");

            WHEREAS, the Company desires to execute and deliver an amendment to
the Indenture for the purposes of permitting the TB Wood's Acquisition (as
defined in Article I hereof) and related transactions;

            WHEREAS, the Company has caused to be delivered to the Holders of
the Notes a Consent Solicitation Statement, dated March 7, 2007 (as the same may
be amended from time to time, the "STATEMENT"), and the related Consent Form
pursuant to which the Company has solicited consents to the adoption of
amendments to the Indenture as set forth in Article I hereof (the "AMENDMENTS");

            WHEREAS, Section 9.02 permits the Amendments with the consent of the
Requisite Holders;

            WHEREAS, the Company has received the written consents of the
Requisite Holders to the Amendments;

            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 9.02 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. Article Four is hereby
amended by inserting the following:

Section 4.23 TB Wood's Acquisition. Notwithstanding anything contained elsewhere
in this Indenture, in the event that the Company, directly or indirectly,
acquires more than 50%, but less than 100%, of the outstanding capital stock of
TB Wood's Corporation, a Delaware corporation ("TB Wood's") pursuant to the
tender offer (the "Tender Offer") contemplated by the Agreement and Plan of
Merger, dated February 17, 2007, as amended, by and among TB Wood's, Altra
Holdings, Inc. and Forest Acquisition Corporation ("FAC") (the "TB Wood's Merger
Agreement"), then notwithstanding that TB Wood's and its Subsidiaries shall
thereupon become Restricted Subsidiaries: (i) the requirement that TB Wood's and
its Domestic Restricted Subsidiaries guarantee the Notes shall be deferred until
10 days after the Company has acquired 100% of the issued and outstanding equity
interests in TB Wood's; (ii) the Company shall have the ability to incur,
subject to the Company's Consolidated Fixed Charge Coverage Ratio being at least
2.0 to 1.0, the Acquired Indebtedness of TB Wood's and its Subsidiaries that
otherwise could be incurred but for the fact that TB Wood's and its Domestic
Restricted Subsidiaries will not be Guarantors at such time; (iii) Section 4.09
is hereby modified to enable Altra to acquire shares of TB Wood's common stock
in the Tender Offer and the back-end merger (the "Back-end Merger") pursuant to
the TB Wood's Merger Agreement and comply with the obligations of the TB Wood's
Merger Agreement, in each case without reducing amounts otherwise available for
Restricted Payments; and (iv) Section 4.14 is hereby modified to waive the
requirement that the Company obtain a fairness opinion for the Back-end Merger.

                                   ARTICLE II
                                  EFFECTIVENESS

            Section 2.01.     EFFECTIVENESS. Subject to Section 2.02, this
Supplemental Indenture shall become effective and binding on the Company, the
Trustee and the Holders upon and execution and delivery of this Supplemental
Indenture by the parties hereto.

            Section 2.02.     SUPPLEMENTAL INDENTURE TO BECOME OPERATIVE. This
Supplemental Indenture shall become operative upon delivery to the Trustee of an
Officers' Certificate stating that the Company (A) has accepted and paid for
shares of TB Wood's common stock in the Tender Offer, funded in part by the
issuance of approximately $92.5 million aggregate principal amount of additional
9% Senior Secured Notes due 2011 (the "TACK-ON NOTE OFFERING"), and (B) has not
completed the Back-end Merger on the same date (and, accordingly, TB Wood's does
not become a wholly owned subsidiary of the Company on such date).


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                                  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, the Guarantors
and the New Guarantor.

            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 Guarantor 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/ David Wall
                                        ----------------------------------------
                                    Name:   David Wall
                                    Title:  Chief Financial Officer


                                    THE BANK OF NEW YORK, as Trustee

                                    By:   /s/ Luis Perez
                                        ----------------------------------------
                                    Name:    Luis Perez
                                    Title:   Assistant 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 Wall
                                        ----------------------------------------
                                    Name:   David Wall
                                    Title:  Chief Financial Officer








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