<PAGE>   1
                                                                  EXHIBIT 4.5(f)

                        SIXTH AMENDMENT TO THIRD AMENDED
                          AND RESTATED CREDIT AGREEMENT

         This SIXTH AMENDMENT (this "Amendment") is entered into as of August
18, 2000, among IDEX Corporation, a Delaware corporation (the "Company"), the
several financial institutions from time to time party to the Credit Agreement
(as defined herein) (collectively, the "Banks"; individually, a "Bank"), and
Bank of America, N.A. (formerly known as Bank of America National Trust and
Savings Association), as agent for the Banks (in such capacity, the "Agent").

                                    RECITALS:

         WHEREAS, the Company, the Banks and the Agent have entered into that
certain Third Amended and Restated Credit Agreement dated as of July 17, 1996
(as heretofore amended and as the same may be further amended or modified from
time to time, the "Credit Agreement") and the Loan Documents referred to in the
Credit Agreement;

         WHEREAS, the Company, the Banks and the Agent have determined that the
Credit Agreement should be amended in certain respects and to make certain other
changes agreed to by the parties.

         NOW, THEREFORE, the parties hereto hereby agree as follows:

     1.   Definitions. Capitalized terms used but not otherwise defined herein
shall have the meanings ascribed to such terms in the Credit Agreement.

     2.   Amendment to Credit Agreement. Schedule 2.01 to the Credit Agreement
is hereby amended, effective on the date this Amendment becomes effective in
accordance with Section 3 hereof, by deleting it in its entirety and
substituting in its place Schedule 2.01 attached hereto and made a part hereof:

     3.   Conditions to Effectiveness of this Amendment. This Amendment shall
become effective upon the satisfaction of the following conditions (the
"Effective Date"):


     3.1  Executed Amendment. Receipt by the Agent of duly executed counterparts
of this Amendment from the Company and all of the Banks;


     3.2  Miscellaneous. Receipt by the Agent of such other documents,
certificates, instruments or opinions as may reasonably be requested by it.

     4.   Certain Representations and Warranties by the Company. In order to
induce the Banks and the Agent to enter into this Amendment, the Company
represents and warrants to the Banks and the Agent that:

     4.1  Authority. The Company has the right, power and capacity and has been
duly authorized and empowered by all requisite corporate and shareholder action
to enter into, execute, deliver and perform this Amendment and the Credit
Agreement as amended hereby.
<PAGE>   2

     4.2  Validity. This Amendment and the Credit Agreement as amended hereby
have each been duly and validly executed and delivered by the Company and
constitutes its legal, valid and binding obligations, enforceable against the
Company in accordance with its respective terms, except as enforcement thereof
may be subject to the effect of any applicable bankruptcy, insolvency,
reorganization, moratorium or similar laws affecting creditors' rights generally
and general principles of equity (regardless of whether such enforcement is
sought in a proceeding in equity or at law or otherwise).

     4.3  No Conflicts. The Company's execution, delivery and performance of
this Amendment and the Credit Agreement as amended hereby does not and will not
violate its Certificates or Articles of Incorporation or Bylaws, any law, rule,
regulation, order, writ, judgment, decree or award applicable to the Company or
any contractual provision to which the Company is party or to which the Company
or any of its Subsidiaries are subject.

     4.4  Approvals. No authorization or approval or other action by, and no
notice to or filing or registration with, any Governmental Authority or
regulatory body (other than those which have been obtained and are in force and
effect) is required in connection with the Company's execution, delivery and
performance of this Amendment and the Credit Agreement as amended hereby.

     4.5  Incorporated Representations and Warranties. All representations and
warranties contained in the Loan Documents are true and correct in all material
respects with the same effect as though such representations and warranties had
been made on and as of the date hereof and the effective date hereof, except as
to any representations or warranties which expressly relate to an earlier date,
in which event, such representations and warranties are true as of such date.

     4.6  No Defaults. No Default or Event of Default exists as of the date
hereof or will exist after giving effect to this Amendment.

     5.   Assignment and Assumption. Pursuant to the terms of this Amendment,
the Commitments of Bank of America, N.A. ("BofA"), PNC Bank, National
Association ("PNC"), Harris Trust and Savings Bank ("Harris") and National City
Bank ("NCB") will be increased in an aggregate amount not to exceed $25,000,000,
while the Commitments of Union Bank of California, N.A. ("Union Bank") and
U.S. Bank National Association ("US Bank") will not be increased. As a result
thereof, the Pro Rata Share of BofA will be increased to the amount set forth on
Schedule 2.01, and the Pro Rata Shares of Union Bank and US Bank will be
decreased to the amounts set forth on Schedule 2.01. In connection with the
changes in Pro Rata Shares, it is necessary for Union Bank and US Bank to assign
to BofA and for BofA to assume certain of the outstanding Committed Loans of
Union Bank and US Bank necessary to provide that the outstanding Committed Loans
of each Bank will be equal to such Bank's Pro Rata Share of all Committed Loans.

     5.1  Assignment. On the Effective Date and upon receipt of the payments
provided for in Section 5.3 hereof, each of Union Bank and US Bank (each, an
"Assignor") hereby assigns and transfers to BofA, without recourse,
representation or warranty of any kind, express or implied (except as provided
in Sections 5.5(a) and (b) hereof), and subject to Section 5.4 hereof,

                                      -2-
<PAGE>   3

all of such Assignor's rights, title and interest arising under the Credit
Agreement and the other Loan Documents relating to all rights and obligations
with respect to such Assignor's portion of the Committed Loans as set forth on
Annex 1 attached hereto and made a part hereof (the "Assigned Loans").

     5.2  Assumption. Effective on the Effective Date, BofA hereby irrevocably
purchases, assumes and takes from each Assignor, and each Assignor is hereby
expressly and absolutely released from, all of such Assignor's obligations
arising under the Credit Agreement and the other Loan Documents relating to the
Assigned Loans.

     5.3  Payment. In consideration of the assignment by each Assignor to BofA
as set forth above, (a) BofA agrees to pay to each Assignor the principal amount
of the Assigned Loans to be transferred by such Assignor to BofA hereunder, in
immediately available funds, at the Effective Date, and (b) the Company agrees
to pay to Assignors the accrued interest and any accrued commitment fees under
the Credit Agreement to the Effective Date on the Assigned Loans, in immediately
available funds, at the Effective Date. The Company hereby acknowledges and
agrees that pursuant to the provisions of Section 4.04 of the Credit Agreement
it will compensate each Assignor for any losses, expenses and liabilities of the
type described in Section 4.04 of the Credit Agreement resulting from the
transactions contemplated hereby. Amounts payable under the first two sentences
of this Section 5.3 shall be paid to the Agent for distribution to the
Assignors.

     5.4  Effectiveness. This Agreement shall become effective on the Effective
Date. No party hereto shall have any obligation hereunder prior to the Effective
Date. BofA recognizes and agrees that notwithstanding anything to the contrary
in this Agreement, Assignors shall retain all of their rights under the Credit
Agreement and the other Loan Documents for periods prior to the Effective Date.
The Company, by its execution hereof, acknowledges the assignments and
assumptions described above.

     5.5  Miscellaneous.


         (a)   Each Assignor and BofA represents and warrants to the other
parties hereto as follows:

              (i)   it has full power and authority, and has taken all action
     necessary, to execute and deliver this Amendment and to fulfill the
     obligations hereunder, and to consummate the transactions contemplated
     hereby;

              (ii)  the making and performance by it of this Amendment and all
     documents required to be executed and delivered by it hereunder do not and
     will not violate any law or regulation of the jurisdiction of its
     incorporation or organization or any other law or regulation applicable to
     it;

              (iii) this Amendment has been duly executed and delivered by it
     and constitutes its legal, valid and binding obligation, enforceable in
     accordance with the respective terms hereunder; and


                                      -3-
<PAGE>   4
              (iv)  all approvals, authorizations, or other actions by, or
     filings with, any governmental authority necessary for the validity or
     enforceability of its obligations under this Amendment have been obtained.

         (b)   Each Assignor represents and warrants to BofA that its portion of
Assigned Loans is not subject to any liens or security interests created by such
Assignor; provided, that such Assignor may have sold participating interests to
Participants in all or any portion of its portion of Assigned Loans pursuant to
Section 11.08 of the Credit Agreement.

         (c)   Except as set forth in Sections 5.5(a) and (b) hereof, Assignors
make no representations or warranties, express or implied, to BofA and shall not
be responsible to BofA for (i) the execution, effectiveness, genuineness,
legality, validity, enforceability, collectibility, regulatory status or
sufficiency of the Credit Agreement or any of the other Loan Documents, (ii) the
perfection, priority, value or adequacy of any collateral security or guaranty,
(iii) the taking of any action, or the failure to take any action, with respect
to any of the Loan Documents, (iv) any representations, warranties, recitals or
statements made in any of the Loan Documents or in any written or oral financial
or other statements, instruments, reports, certificates or documents made or
furnished by Assignors to BofA or by or on behalf of the Company or any of its
Affiliates to Assignors or BofA in connection with the Loan Documents and the
transactions contemplated thereby, (v) the financial or other condition of the
Company or any other Person, or (vi) any other matter having any relation to any
of the foregoing. Assignors shall not be required to ascertain or inquire as to
the performance or observance of any of the terms, conditions, provisions,
covenants or agreements contained in any of the Loan Documents or as to the use
of the proceeds of the Loans or the existence or possible existence of any
Default or Event of Default. Additionally, Assignors shall not have any duty or
responsibility either initially or on a continuing basis to make any
investigation or any appraisal on BofA's behalf or to provide BofA with any
credit or other information with respect thereto, whether coming into Assignors'
possession before the execution of the Credit Agreement or at any time or times
thereafter. Assignors shall have no responsibility with respect to the accuracy
of, or the completeness of, any information provided to BofA, whether by
Assignors or by or on behalf of the Company or any other Person obligated under
the Credit Agreement or any related instrument or document.

         (d)   BofA represents and warrants that it has made its own independent
investigation of each of the foregoing matters, including, without limitation,
the financial condition and affairs of the Company and its Affiliates, in
connection with the making of the Loans and the execution of this Agreement
(including, but not limited to, the solvency of the Company and its Affiliates,
the ability of the Company and its Affiliates to pay their respective debts as
they mature and the capital of the Company and its Affiliates remaining after
the closing under the Credit Agreement and the other Loan Documents and the
consummation of the transactions contemplated thereby) and has made and shall
continue to make its own appraisal of the creditworthiness of the Company and
its Affiliates. BofA (i) confirms that it has received copies of the Loan
Documents together with copies of such other closing documents delivered in
connection with the Credit Agreement, such financial statements and such other
documents and information as it has requested or deemed appropriate to make its
own credit analysis and decision to enter into this Agreement and (ii) agrees
that it will, independently and without reliance upon the Agent or Assignors and
based on such documents and information as it shall


                                      -4-
<PAGE>   5

deem appropriate at the time, continue to make its own credit decisions in
taking or not taking action under the Loan Documents.

         (e)   To the extent necessary, Section 11.08 of the Credit Agreement is
hereby amended to permit the transactions contemplated hereby.

     6.   Miscellaneous. The parties hereto hereby further agree as follows:


     6.1  Further Assurances. Each of the parties hereto hereby agrees to do
such further acts and things and to execute, deliver and acknowledge such
additional agreements, powers and instruments as any other party hereto may
reasonably require to carry into effect the purposes of this Amendment and the
Credit Agreement as amended hereby.

     6.2  Counterparts. This Amendment may be executed in one or more
counterparts, each of which, when executed and delivered, shall be deemed to be
an original and all of which counterparts, taken together, shall constitute but
one and the same document with the same force and effect as if the signatures of
all of the parties were on a single counterpart, and it shall not be necessary
in making proof of this Amendment to produce more than one such counterpart.

     6.3  Headings. Headings used in this Amendment are for convenience of
reference only and shall not affect the construction of this Amendment.

     6.4  Integration. This Amendment and the Loan Documents constitute the
entire agreement among the parties hereto with respect to the subject matter
hereof and thereof.

     6.5  Governing Law. THIS AMENDMENT SHALL BE DEEMED TO BE A CONTRACT MADE
UNDER THE LAWS OF THE STATE OF ILLINOIS, AND FOR ALL PURPOSES SHALL BE GOVERNED
BY AND CONSTRUED AND ENFORCED IN ACCORDANCE WITH THE INTERNAL LAWS AND DECISIONS
OF SAID STATE, WITHOUT REGARD TO PRINCIPLES OF CONFLICTS OF LAWS.

     6.6  Binding Effect. This Amendment shall be binding upon and inure to the
benefit of and be enforceable by the parties hereto and their respective
successors and assigns; provided, however, that the Company may not assign or
transfer its rights, interests or obligations hereunder without the prior
written consent of the Agent and all of the Banks. Except as expressly set forth
to the contrary herein, this Amendment shall not be construed so as to confer
any right or benefit upon any Person other than the parties to this Amendment
and their respective successors and permitted assigns.

     6.7  Amendment; Waiver; Reaffirmation of Loan Documents. The parties hereto
agree and acknowledge that nothing contained in this Amendment in any manner or
respect limits or terminates any of the provisions of the Credit Agreement or
the other Loan Documents other than as expressly set forth herein and further
agree and acknowledge that the Credit Agreement and each of the other Loan
Documents remain and continue in full force and effect and are hereby ratified
and reaffirmed in all respects. No delay on the part of any Bank or the Agent in
exercising any of their respective rights, remedies, powers and privileges under
the Credit Agreement or any of the other Loan Documents or partial or single
exercise thereof, shall constitute a waiver thereof. None of the terms and
conditions of this Amendment may be


                                      -5-
<PAGE>   6

changed, waived, modified or varied in any manner, whatsoever, except in
accordance with Section 11.01 of the Credit Agreement.

     6.8  Reference to and Effect on the Credit Agreement and the other Loan
Documents. Upon the effectiveness hereof, each reference in the Credit Agreement
to "this Agreement," "hereunder," "hereof," "herein," or words of like import
referring to the Credit Agreement and each reference in the other Loan Documents
to the "Credit Agreement," "thereunder," "thereof," or words of like import
referring to the Credit Agreement shall mean and be a reference to the Credit
Agreement as amended by this Amendment. The Credit Agreement shall be deemed to
be amended wherever and as necessary to reflect the foregoing amendments.

                            [signature page follows]

















                                      -6-
<PAGE>   7



         IN WITNESS WHEREOF, this Amendment has been duly executed and delivered
as of the date first above written.


                 IDEX CORPORATION


                 By:      /S/      DOUGLAS C. LENNOX
                      ----------------------------------------------------------
                 Title:            Vice President-Treasurer
                        --------------------------------------------------------


                 BANK OF AMERICA, N.A., as Agent


                 By:      /S/      GRETCHEN SPOO
                      ----------------------------------------------------------
                 Title:            Vice President
                        --------------------------------------------------------


                 BANK OF AMERICA N.A., as a Bank


                 By:      /S/      GRETCHEN SPOO
                      ----------------------------------------------------------
                 Title:            Vice President
                        --------------------------------------------------------




                                       S-1
                              [TO SIXTH AMENDMENT]
<PAGE>   8


                 NATIONAL CITY BANK


                 By:      /S/      JENNIFER L. KOFOD
                      ----------------------------------------------------------
                 Title:            Account Officer
                        --------------------------------------------------------




                                      S-2


                              [TO SIXTH AMENDMENT]
<PAGE>   9


                 PNC BANK, NATIONAL ASSOCIATION


                 By:      /S/      BOB KRUSNON
                      ---------------------------------------------------------
                 Title:            Sr. Vice President
                        -------------------------------------------------------







                                      S-3
                              [TO SIXTH AMENDMENT]
<PAGE>   10


                 UNION BANK OF CALIFORNIA, N.A.,
                 (Successor in Interest to Union Bank)


                 By:      /S/      PATRICIA C. ROHLING
                      ---------------------------------------------------------
                 Title:            Senior Vice President
                        -------------------------------------------------------








                                      S-4
                              [TO SIXTH AMENDMENT]
<PAGE>   11


                 U.S. BANK NATIONAL ASSOCIATION,
                 (Successor in Interest to United States National
                 Bank of Oregon)


                 By:      /S/      SARAH L. HEMMER
                      ----------------------------------------------------------
                 Title:            Vice President
                        --------------------------------------------------------








                                      S-5
                              [TO SIXTH AMENDMENT]
<PAGE>   12


                  HARRIS TRUST AND SAVINGS BANK


                  By:      /S/      M. JAMES BARRY, III
                       ---------------------------------------------------------
                  Title:            Vice President
                         -------------------------------------------------------








                                      S-6
                              [TO SIXTH AMENDMENT]
<PAGE>   13





                                  SCHEDULE 2.01


                         COMMITMENTS AND PRO RATA SHARES


<TABLE>
<CAPTION>


                  Lender                                    Commitment                              Pro Rata Share
-------------------------------------------- ----------------------------------------- -----------------------------------------
<S>                                                      <C>                                        <C>

Bank of America, N.A.                                     $73,021,276.60                            31.072883660%
PNC Bank, National Association                            $40,000,000.00                            17.021276596%
Harris Trust and Savings Bank                             $33,000,000.00                            14.042553191%
National City Bank                                        $30,000,000.00                            12.765957447%
Union Bank of California, N.A.                            $29,489,361.70                            12.548664553%
U.S. Bank National Association                            $29,489,361.70                            12.548664553%
         TOTAL                                           $235,000,000.00                                 100%
                                                         ===============                                 ====

</TABLE>




<PAGE>   14




                                     ANNEX 1

                                 ASSIGNED LOANS

<TABLE>
<CAPTION>


                 Assignor                                      BofA                          Percentage Interest Assigned
-------------------------------------------- ----------------------------------------- -----------------------------------------
<S>                                          <C>                                               <C>

Union Bank of California, N.A.               Bank of America, N.A.                             1.493888637%
U.S. Bank National Association               Bank of America, N.A.                             1.493888637%

</TABLE>