Document:

<PAGE>   1
                                                                     EXHIBIT 4-1

                           CERTIFICATE OF ASSUMPTION

THIS CERTIFICATE OF ASSUMPTION is made and entered into effective as of the
Effective Time specified below, and is by DTE ENTERPRISES, INC., a Michigan
corporation having an address of 2000 Second Ave., Detroit, Michigan 48226
("Enterprises").

                              W I T N E S S E T H:

WHEREAS, DTE Energy Company, a Michigan corporation and parent company of
Enterprises ("DTE"), MCN Energy Group Inc., a Michigan corporation ("MCN"), and
Enterprises are parties to an Agreement and Plan of Merger, dated as of October
4, 1999, as amended as of November 12, 1999, and as further amended as of
February 28, 2001 (as amended from time to time, the "Merger Agreement"); and

WHEREAS, pursuant to the Merger Agreement, MCN will merge with and into
Enterprises, with Enterprises being the surviving company; and

WHEREAS, by operation of law, upon the Effective Time, Enterprises will succeed
to and assume all of the rights, duties and obligations of MCN; and

WHEREAS, certain agreements of MCN require that in the event MCN merges with
another entity, the surviving entity must expressly assume MCN's obligations.

NOW THEREFORE, in consideration of the promises contained in the Merger
Agreement, and other good and valuable consideration, Enterprises agrees as
follows:

1.   DEFINITION. As used herein, "Effective Time" shall be that time that the
Certificate of Merger executed by Enterprises and MCN has been duly endorsed by
the Department of Consumer & Industry Services - Bureau of Commercial Services
of the State of Michigan.

2.   ASSUMPTION. Enterprises hereby assumes, as of the Effective Time, all
rights, duties and obligations of MCN, of any kind or nature whatsoever, and
whenever accrued or incurred. This assumption is intended to be broadly
construed, and includes, without limitation, the assumption by Enterprises of
the following obligations of MCN.

     a.   Guarantee Agreement, dated March 13, 2001, between MCN Energy Group
Inc. and MCN Energy Enterprises Inc.;

     b.   Support Agreement, dated July 15, 1995, between MCN Corporation (now
named MCN Energy Group Inc.) and MCN Investment Corporation (now named MCN
Energy Enterprises, Inc.);

     c.   Support Agreement, dated September 1, 1995, between MCN Corporation
(now named MCN Energy Group Inc.) and MCN Investment Corporation (now named MCN
Energy Enterprises, Inc.);

     d.   Guaranty, dated January 1, 1995, by MCN Corporation (now named MCN
Energy Group Inc.) in favor of Phibro Division of Salomon;
<PAGE>   2
     e.   Performance of every covenant of MCN Corporation (now named MCN Energy
Group Inc.) under the Indenture, dated as of September 1, 1995 for Senior Debt
Securities of MCN Investment Corporation (now named MCN Energy Enterprises,
Inc.) as Issuer, in favor of NBD Bank (now Bank One Company, National
Association), as Trustee; and

     f.   Payment and Guarantee Agreement, dated as of October 26, 1994, by MCN
Corporation for the benefit of the Holders (as defined in such Payment and
Guarantee Agreement) from time to time of the Series A Preferred Securities of
MCN Michigan Limited Partnership, a Michigan Limited Partnership.

3.   GOVERNING LAW. This Agreement shall be governed by, and construed in
accordance with, the State of Michigan.

     Executed this 31st day of May, 2001, effective as of the Effective Time.

                                           DTE ENTERPRISES, INC.

                                           By: /s/ Thomas A. Hughes
                                               -----------------------------
                                               Name:  Thomas A. Hughes
                                               Title: Associate General Counsel

                                       2<PAGE>   1

                                                                    EXHIBIT 4(t)

                                AMENDMENT NO. 11

                     ELEVENTH AMENDMENT TO THIRD AMENDED AND
                 RESTATED REVOLVING CREDIT AGREEMENT AND CONSENT

         THIS ELEVENTH AMENDMENT, dated as of the 29th day of June, 2001, by and
between Newcor, Inc., a Delaware corporation, of Bloomfield Hills, Michigan
(herein called "Company") and Comerica Bank, a Michigan banking corporation, of
Detroit, Michigan (herein called "Bank");

                                   WITNESSETH:

         WHEREAS, Company and Bank desire to amend that certain Third Amended
and Restated Revolving Credit Agreement dated as of January 15, 1998, entered
into by and between Company and Bank, which was amended by ten amendments
(herein called "Agreement");

         NOW, THEREFORE, it is agreed that the Agreement is amended as follows:

         1.  Section 6.4 of the Agreement is amended to read in its
entirety as follows:

             "Maintain as of the end of each fiscal quarter EBITDA of not
less than the following amounts:

             Fiscal Quarter Ending                     Amount
             ----------------------                    ------

             June 30, 2001                             $10,000,000
             September 30, 2001                        $ 7,500,000
             December 31, 2001                         $13,500,000
             March 31, 2002 and each                   $14,000,000
             fiscal quarter thereafter"

         2.  Section 6.5 of the Agreement is amended to read in its
entirety as follows:

             "6.5 Maintain as of the end of each fiscal quarter a ratio of
         current assets of Company and its Consolidated Subsidiaries to current
         liabilities of Company and its consolidated Subsidiaries (excluding
         from current liabilities any portion of the revolving credit under this
         Agreement included in current liabilities in accordance with GAAP) of
         not less than 1.25 to 1.00, all as determined in accordance with GAAP."

         3.  The  following  subclause (e) is hereby added to Section 6.1 and
the period at the end of subclause (d) is changed to be a semi-colon:

             "(e) within ten (10) days after and as of the 21st day and
         last day of each month (beginning with the period ended June 30, 2001),
         the aging of Company's and its consolidated Subsidiaries accounts, each
         in form acceptable to Bank."

         4.  Section 6.8 of the Agreement is amended to read in its entirety as
follows:

             "Permit, and cause its Subsidiaries to permit, Bank, through
         its authorized attorneys, accountants, and representatives, to examine
         Company's and its Subsidiaries' books, accounts, records, ledgers and
         assets of every kind and description at all reasonable times upon oral
         or written request of Bank, including collateral audits at the cost and
         expense of Company."

<PAGE>   2

         5.  The following Section 6.17 is hereby added to the Agreement:

             "6.17 Provide to Bank appraisals of not less than half of the
         machinery and equipment of Company and its Subsidiaries on or before
         September 30, 2001 and provide appraisals of the remainder of such
         machinery and equipment on or before such date as is required by Bank
         (but not earlier than December 31, 2001), in each case on a forced sale
         liquidation basis by an appraiser who is acceptable to Bank.

         6.  Schedule 2.11 of the Agreement is deleted and attached Schedule
2.11 is substituted  therefor and the initial margins shall be those under Level
V.

         7.  Company, the Guarantors and Bank agree that the Company's
indebtedness to Bank shall be on a remittance basis and the remittance basis
provisions of the Security Agreements executed by Company and the Guarantors
shall apply.

         8.  Company hereby represents and warrants that, after giving effect to
the amendments and waivers contained herein, (a) execution, delivery and
performance of this Amendment and any other documents and instruments required
under this Amendment or the Agreement are within Company's corporate powers,
have been duly authorized, are not in contravention of law or the terms of
Company's Certificate of Incorporation or Bylaws, and do not require the consent
or approval of any governmental body, agency, or authority; and this Amendment
and any other documents and instruments required under this Amendment or the
Agreement, will be valid and binding in accordance with their terms; (b) the
continuing representations and warranties of Company set forth in Sections 5.1
through 5.7 and 5.9 through 5.15 of the Agreement are true and correct on and as
of the date hereof with the same force and effect as made on and as of the date
hereof; (c) the continuing representations and warranties of Company set forth
in Section 5.8 of the Agreement are true and correct as of the date hereof with
respect to the most recent financial statements furnished to the Bank by Company
in accordance with Section 6.1 of the Agreement; and (d) no event of default, or
condition or event which, with the giving of notice or the running of time, or
both, would constitute an event of default under the Agreement, has occurred and
is continuing as of the date hereof.

         9.  This Amendment shall be effective upon (a) execution of this
Amendment by Company and Bank (b) execution by the Guarantors of the attached
Acknowledgment, and (c) payment by Company to Bank of a non-refundable amendment
fee in the amount of $15,000.

         10. Except as modified hereby, all of the terms and conditions of the
Agreement shall remain in full force and effect.

<PAGE>   3

     WITNESS the due execution hereof on the day and year first above written.

COMERICA BANK                          NEWCOR, INC.

By:   /s/ Preeti S. Sarnaik            By:   /s/ James J. Connor
   ----------------------------           -------------------------------------

Its: Assistant Vice President          Its:  President & CEO
                                           ------------------------------------

                                       By:   /s/ Thomas D. Parker
                                          -------------------------------------

                                       Its:  V.P. Human Resources & Secretary
                                           ------------------------------------

                                 ACKNOWLEDGMENT

         The undersigned accept and agree to the Amendment No. 11 to the Third
Amended and Restated Revolving Credit Agreement, agree to the continued
effectiveness of the Guaranties originally executed and delivered to Comerica
Bank by the undersigned guarantying all obligations of Newcor, Inc. to the Bank
and acknowledge that the remittance basis provisions of the security agreements
executed and delivered by the undersigned to the Bank shall apply.

                                       ROCHESTER GEAR, INC.

                                       By:   /s/ James J. Connor
                                          -------------------------------------

                                       Its:  President & CEO
                                           ------------------------------------

                                       By:   /s/ Thomas D. Parker
                                          -------------------------------------

                                       Its:  V.P. Human Resources & Secretary
                                           ------------------------------------

<PAGE>   4

<TABLE>
<S><C>
                                                         ENC CORP.

                                                         By:   /s/ James J. Connor
                                                            -------------------------------------

                                                         Its:  President & CEO
                                                             ------------------------------------

                                                         By:   /s/ Thomas D. Parker
                                                            -------------------------------------

                                                         Its:  V.P. Human Resources & Secretary
                                                             ------------------------------------
DECO TECHNOLOGIES, INC.                                  PLASTRONICS PLUS, INC.

By:   /s/ James J. Connor                                By:   /s/ James J. Connor
    ------------------------------------                     ------------------------------------

Its:  President & CEO                                    Its:  President & CEO
   -------------------------------------                     ------------------------------------

By:   /s/ Thomas D. Parker                               By:   /s/ Thomas D. Parker
  --------------------------------------                     ------------------------------------

Its:  V.P. Human Resources & Secretary                   Its:  V.P. Human Resources & Secretary
   -------------------------------------                     ------------------------------------

DECO INTERNATIONAL, INC.                                 NEWCOR M-T-L, INC.

By:   /s/ James J. Connor                                By:   /s/ James J. Connor
    ------------------------------------                     ------------------------------------

Its:  President & CEO                                    Its:  President & CEO
   -------------------------------------                      -----------------------------------

By:   /s/ Thomas D. Parker                               By:  /s/ Thomas D. Parker
  --------------------------------------                     ------------------------------------

Its:  V.P. Human Resources & Secretary                   Its: V.P. Human Resources & Secretary
   -------------------------------------                     ------------------------------------

TURN-MATIC, INC.                                         GRAND MACHINING COMPANY

By:   /s/ James J. Connor                                By:  /s/ James J. Connor
    ------------------------------------                     ------------------------------------

Its:  President & CEO                                    Its: President & CEO
   -------------------------------------                     ------------------------------------

By:   /s/ Thomas D. Parker                               By: /s/ Thomas D. Parker
  --------------------------------------                    -------------------------------------

Its:  V.P. Human Resources & Secretary                   Its:  V.P. Human Resources & Secretary
   -------------------------------------                     ------------------------------------

</TABLE>

<PAGE>   5

Schedule 2.11

<TABLE>
<CAPTION>

============================================================================================================

                            LEVEL I                 LEVEL II     LEVEL III        LEVEL IV      LEVEL V

------------------------------------------------------------------------------------------------------------

<S>                   <C>                         <C>          <C>             <C>            <C>
                     (less than/equal to)        (greater than) (greater than) (greater than) (greater than)
Funded Debt to           4.0 to 1.0               4.0 to 1.0 &   4.5 to 1.0 &   5.0 to 1.0 &   9.0 to 1.0
                                                 (less than/    (less than/    (less than/
                                                  equal to)      equal to)      equal to)
EBITDA Ratio                                      4.5 to 1.0     5.0 to 1.0     9.0 to 1.0
------------------------------------------------------------------------------------------------------------

Applicable                    1%                   1 3/4%          2 1/2%         3 1/4%          3 3/4%
Margin (Eurodollar-based
Advances)
------------------------------------------------------------------------------------------------------------

Applicable                  1/4%                     3/8%            3/8%           3/8%           3/8%
Commitment Fee
Percentage                                                                                     -------------

Applicable                    0%                       0%              0%           1/2%             1%
Margin (Prime-
based Advances)
============================================================================================================
</TABLE>

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