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                                                                    EXHIBIT 4.13


               INDENTURE SUPPLEMENT TO ADD A SUBSIDIARY GUARANTOR

                  This Supplemental Indenture, dated as of May 15, 2001 (this
"Supplemental Indenture" or "Guarantee"), among Generac Portable Products, Inc.*
(the "New Guarantor"), Briggs & Stratton Corporation (together with its
successors and assigns, the "Company"), and Bank One, N.A., as Trustee under the
Indenture referred to below.



                              W I T N E S S E T H:

                  WHEREAS, the Company and the Trustee have heretofore executed
and delivered an Indenture, dated as of June 4, 1997, as previously amended by a
First Supplemental Indenture dated as of May 14, 2001 (as amended, supplemented,
waived or otherwise modified, the "Indenture"), providing for the issuance of an
aggregate principal amount of $100 million of 7 1/4% Senior Notes due September
15, 2007 of the Company (the "Securities");

                  WHEREAS, Section 10.10 of the Indenture provides that the
Company may cause a Subsidiary (which may include for the purposes of such
Section 10.10 only, a limited liability company or limited liability partnership
in which at least a majority of the ownership interests therein is owned
directly or indirectly in the Company) to execute and deliver to the Trustee a
Supplemental Indenture pursuant to which such Subsidiary will unconditionally
guarantee, on a joint and several basis with any other Subsidiary who has
previously guaranteed the Securities or may concurrently or thereafter guaranty
the Securities, the full and prompt payment of the principal of, premium, if
any, and interest on the Securities; and

                  WHEREAS, pursuant to Section 9.1(2) of the Indenture, the
Trustee and the Company are authorized to execute and deliver this Supplemental
Indenture without the consent of any Holder;

                  NOW, THEREFORE, in consideration of the foregoing and for
other good and valuable consideration, the receipt of which is hereby
acknowledged, the New Guarantor, the Company, the other Subsidiary Guarantors
and the Trustee mutually covenant and agree for the equal and ratable benefit of
the Holders of the Securities as follows:

                                   ARTICLE I

                                   Definitions

                  SECTION 1.1. Defined Terms. As used in this Supplemental
Indenture, terms defined in the Indenture or in the preamble or recital hereto
are used herein as therein defined, except that the term "Holders" in this
Guarantee shall refer to the term "Holders" as defined in the Indenture and the
Trustee acting on behalf or for the benefit of such Holders. The words "herein,"
"hereof" and "hereby" and other words of similar import used in this
Supplemental Indenture refer to this Supplemental Indenture as a whole and not
to any particular section hereof.


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                                   ARTICLE II

                                    Guarantee

                  SECTION 2.1. Guarantee. The New Guarantor hereby fully and
unconditionally guarantees, as primary obligor and not merely as surety, jointly
and severally with each other Subsidiary Guarantor, to each Holder of the
Securities and the Trustee, the full and punctual payment when due, whether at
maturity, upon redemption or repurchase, by declaration of acceleration or
otherwise, of the Securities on a senior basis and subject to the terms and
conditions of the Indenture.

                                  ARTICLE III

                                  Miscellaneous

                  SECTION 3.1. Notices. All notices and other communications to
the New Guarantor shall be given as provided in the Indenture to the New
Guarantor, at its address set forth below, with a copy to the Company as
provided in the Indenture for notices to the Company.

                  SECTION 3.2. Parties. Nothing expressed or mentioned herein is
intended or shall be construed to give any Person, firm or corporation, other
than the Holders and the Trustee, any legal or equitable right, remedy or claim
under or in respect of this Supplemental Indenture or the Indenture or any
provision herein or therein contained.

                  SECTION 3.3. Governing Law. This Supplemental Indenture shall
be governed by the laws of the State of New York.

                  SECTION 3.4. Severability Clause. In case any provision 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 and such provision shall be ineffective
only to the extent of such invalidity, illegality or unenforceability.

                  SECTION 3.5. Ratification of Indenture; Supplemental
Indentures Part of Indenture; Trustee's Disclaimer. Except as expressly amended
hereby, the Indenture is in all respects ratified and confirmed and all the
terms, conditions and provisions thereof shall remain in full force and effect.
This Supplemental Indenture shall form a part of the Indenture for all purposes,
and every Holder of Securities heretofore or hereafter authenticated and
delivered shall be bound hereby. The Trustee makes no representation or warranty
as to the validity or sufficiency of this Supplemental Indenture.

                  SECTION 3.6. Counterparts. The parties hereto may sign one or
more copies of this Supplemental Indenture in counterparts, all of which
together shall constitute one and the same agreement.

                  SECTION 3.7. Headings. The headings of the Articles and the
sections in this Guarantee are for convenience of reference only and shall not
be deemed to alter or affect the meaning or interpretation of any provisions
hereof.


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

                                     GENERAC PORTABLE PRODUCTS, INC.



                                     By: /s/ John S. Shiely
                                         ---------------------------------------
                                         Name:  John S. Shiely
                                         Title: President

                                     Address:       12301 West Wirth Street
                                                    Wauwatosa, WI  53222


                                     BRIGGS & STRATTON CORPORATION



                                     By: /s/ Carita Twinem
                                         ---------------------------------------
                                         Name:  Carita R. Twinem
                                         Title: Treasurer


                                     BANK ONE N.A., as Trustee



                                     By: /s/ Marla S. Roth
                                         ---------------------------------------
                                         Name:  Marla S. Roth
                                         Title: Assistant Vice President

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*  GPPD, Inc., GPPW, Inc. and Generac Portable Products, LLC each executed and
   delivered separate Supplemental Indentures in this form as the "New
   Guarantor."


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