TRIMAS CORP — Major Shareholding Notification 2010

May 14, 2010

UNITED STATES
SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549
SCHEDULE 13D

*Under the Securities Exchange Act of 1934 (Amendment No. 4)**

*TriMas Corporation*

(Name of Issuer)

*Common Stock, par value $0.01 per share*

(Title of Class of Securities)

*896215209*

(CUSIP Number)

*Daniel P. Tredwell*

*Heartland Industrial Associates L.L.C.*

*177 Broad Street, 10th Floor*

*Stamford, Connecticut 06901*

*(203) 327-1202*

(Name, Address and Telephone Number of Person Authorized to Receive Notices and Communications)

*Copies to:*

*Brian J. Fahrney*

*Sidley Austin LLP*

*One South Dearborn*

*Chicago, Illinois*

*(312) 853-7000*

*May 12, 2010*

(Date of Event Which Requires Filing of this Statement)

If the filing person has previously filed a statement on Schedule 13G to report the acquisition that is the subject of this Schedule 13D, and is filing this schedule because of Rule 13d-1(e), 13d-1(f) or 13d-1(g), check the following box o .

*Note* : Schedules filed in paper format shall include a signed original and five copies of the schedule, including all exhibits. See §240.13d-7 for other parties to whom copies are to be sent.

• The remainder of this cover page shall be filled out for a reporting person's initial filing on this form with respect to the subject class of securities, and for any subsequent amendment containing information which would alter disclosures provided in a prior cover page.

The information required on the remainder of this cover page shall not be deemed to be "filed" for the purpose of Section 18 of the Securities Exchange Act of 1934 (the "Act") or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

SEQ.=1,FOLIO='',FILE='C:\JMS\105952\10-10265-1\task4118468\10265-1-ba.htm',USER='105952',CD='May 15 02:23 2010'

| 2 | Name of Reporting Persons Heartland Industrial Associates L.L.C. — Check the Appropriate Box
if a Member of a Group | |
| --- | --- | --- |
| | (a) | o |
| | (b) | x |
| 3 | SEC Use Only | |
| 4 | Source of Funds WC | |
| 5 | Check if Disclosure of Legal Proceedings Is
Required Pursuant to Item 2(d) or 2(e) o | |
| 6 | Citizenship or Place of Organization Delaware | |
| Number of Shares Beneficially Owned by Each Reporting Person With | 7 | Sole Voting Power 0 Shares of Common Stock |
| | 8 | Shared Voting Power 15,106,396 Shares of Common Stock |
| | 9 | Sole Dispositive Power 0 Shares of Common Stock |
| | 10 | Shared Dispositive Power 15,106,396 Shares of Common Stock |
| 11 | Aggregate Amount
Beneficially Owned by Each Reporting Person 15,106,396 Shares of Common Stock | |
| 12 | Check Box if the Aggregate
Amount in Row (11) Excludes Certain Shares o | |
| 13 | Percent of Class
Represented by Amount in Row (11) 44.4% | |
| 14 | Type of Reporting Person OO | |

2

SEQ.=1,FOLIO='2',FILE='C:\JMS\105952\10-10265-1\task4118468\10265-1-ba.htm',USER='105952',CD='May 15 02:23 2010'

| 2 | Name of Reporting Persons Heartland Industrial Partners, L.P. — Check the Appropriate Box
if a Member of a Group | |
| --- | --- | --- |
| | (a) | o |
| | (b) | x |
| 3 | SEC Use Only | |
| 4 | Source of Funds WC | |
| 5 | Check if Disclosure of Legal Proceedings Is
Required Pursuant to Item 2(d) or 2(e) o | |
| 6 | Citizenship or Place of Organization Delaware | |
| Number of Shares Beneficially Owned by Each Reporting Person With | 7 | Sole Voting Power 0 Shares of Common Stock |
| | 8 | Shared Voting Power 14,054,457 Shares of Common Stock |
| | 9 | Sole Dispositive Power 0 Shares of Common Stock |
| | 10 | Shared Dispositive Power 14,054,457 Shares of Common Stock |
| 11 | Aggregate Amount
Beneficially Owned by Each Reporting Person 14,054,457 Shares of Common Stock | |
| 12 | Check Box if the Aggregate
Amount in Row (11) Excludes Certain Shares o | |
| 13 | Percent of Class
Represented by Amount in Row (11) 41.3% | |
| 14 | Type of Reporting Person PN | |

3

SEQ.=1,FOLIO='3',FILE='C:\JMS\105952\10-10265-1\task4118468\10265-1-ba.htm',USER='105952',CD='May 15 02:23 2010'

| 2 | Name of Reporting Persons TriMas Investment Fund I, L.L.C. — Check the Appropriate Box
if a Member of a Group | |
| --- | --- | --- |
| | (a) | o |
| | (b) | x |
| 3 | SEC Use Only | |
| 4 | Source of Funds WC | |
| 5 | Check if Disclosure of Legal Proceedings Is
Required Pursuant to Item 2(d) or 2(e) o | |
| 6 | Citizenship or Place of Organization Delaware | |
| Number of Shares Beneficially Owned by Each Reporting Person With | 7 | Sole Voting Power 0 Shares of Common Stock |
| | 8 | Shared Voting Power 9,742,230 Shares of Common Stock |
| | 9 | Sole Dispositive Power 0 Shares of Common Stock |
| | 10 | Shared Dispositive Power 9,742,230 Shares of Common Stock |
| 11 | Aggregate Amount Beneficially
Owned by Each Reporting Person 9,742,230 Shares of Common Stock | |
| 12 | Check Box if the Aggregate
Amount in Row (11) Excludes Certain Shares o | |
| 13 | Percent of Class
Represented by Amount in Row (11) 28.6% | |
| 14 | Type of Reporting Person OO | |

4

SEQ.=1,FOLIO='4',FILE='C:\JMS\105952\10-10265-1\task4118468\10265-1-ba.htm',USER='105952',CD='May 15 02:23 2010'

| 2 | Name of Reporting Persons Metaldyne Investment Fund I, L.L.C. — Check the Appropriate Box
if a Member of a Group | |
| --- | --- | --- |
| | (a) | o |
| | (b) | x |
| 3 | SEC Use Only | |
| 4 | Source of Funds WC | |
| 5 | Check if Disclosure of Legal Proceedings Is
Required Pursuant to Item 2(d) or 2(e) o | |
| 6 | Citizenship or Place of Organization Delaware | |
| Number of Shares Beneficially Owned by Each Reporting Person With | 7 | Sole Voting Power 0 Shares of Common Stock |
| | 8 | Shared Voting Power 2,243,827 Shares of Common Stock |
| | 9 | Sole Dispositive Power 0 Shares of Common Stock |
| | 10 | Shared Dispositive Power 2,243,827 Shares of Common Stock |
| 11 | Aggregate Amount
Beneficially Owned by Each Reporting Person 2,243,827 Shares of Common Stock | |
| 12 | Check Box if the Aggregate
Amount in Row (11) Excludes Certain Shares o | |
| 13 | Percent of Class
Represented by Amount in Row (11) 6.6% | |
| 14 | Type of Reporting Person OO | |

5

SEQ.=1,FOLIO='5',FILE='C:\JMS\105952\10-10265-1\task4118468\10265-1-ba.htm',USER='105952',CD='May 15 02:23 2010'

| 2 | Name of Reporting Persons HIP Side-by-Side Partners, L.P. — Check the Appropriate Box
if a Member of a Group | |
| --- | --- | --- |
| | (a) | o |
| | (b) | x |
| 3 | SEC Use Only | |
| 4 | Source of Funds WC | |
| 5 | Check if Disclosure of Legal Proceedings Is
Required Pursuant to Item 2(d) or 2(e) o | |
| 6 | Citizenship or Place of Organization Delaware | |
| Number of Shares Beneficially Owned by Each Reporting Person With | 7 | Sole Voting Power 0 Shares of Common Stock |
| | 8 | Shared Voting Power 842,675 Shares of Common Stock |
| | 9 | Sole Dispositive Power 0 Shares of Common Stock |
| | 10 | Shared Dispositive Power 842,675 Shares of Common Stock |
| 11 | Aggregate Amount
Beneficially Owned by Each Reporting Person 842,675 Shares of Common Stock | |
| 12 | Check Box if the Aggregate
Amount in Row (11) Excludes Certain Shares o | |
| 13 | Percent of Class
Represented by Amount in Row (11) 2.5% | |
| 14 | Type of Reporting Person PN | |

6

SEQ.=1,FOLIO='6',FILE='C:\JMS\105952\10-10265-1\task4118468\10265-1-ba.htm',USER='105952',CD='May 15 02:23 2010'

| 2 | Name of Reporting Persons TriMas Investment Fund II, L.L.C. — Check the Appropriate Box
if a Member of a Group | |
| --- | --- | --- |
| | (a) | o |
| | (b) | x |
| 3 | SEC Use Only | |
| 4 | Source of Funds WC | |
| 5 | Check if Disclosure of Legal Proceedings Is
Required Pursuant to Item 2(d) or 2(e) o | |
| 6 | Citizenship or Place of Organization Delaware | |
| Number of Shares Beneficially Owned by Each Reporting Person With | 7 | Sole Voting Power 0 Shares of Common Stock |
| | 8 | Shared Voting Power 176,312 Shares of Common Stock |
| | 9 | Sole Dispositive Power 0 Shares of Common Stock |
| | 10 | Shared Dispositive Power 176,312 Shares of Common Stock |
| 11 | Aggregate Amount
Beneficially Owned by Each Reporting Person 176,312 Shares of Common Stock | |
| 12 | Check Box if the Aggregate
Amount in Row (11) Excludes Certain Shares o | |
| 13 | Percent of Class Represented
by Amount in Row (11) 0.5% | |
| 14 | Type of Reporting Person OO | |

7

SEQ.=1,FOLIO='7',FILE='C:\JMS\105952\10-10265-1\task4118468\10265-1-ba.htm',USER='105952',CD='May 15 02:23 2010'

| 2 | Name of Reporting Persons Metaldyne Investment Fund II, L.L.C. — Check the Appropriate Box
if a Member of a Group | |
| --- | --- | --- |
| | (a) | o |
| | (b) | x |
| 3 | SEC Use Only | |
| 4 | Source of Funds WC | |
| 5 | Check if Disclosure of Legal Proceedings Is
Required Pursuant to Item 2(d) or 2(e) o | |
| 6 | Citizenship or Place of Organization Delaware | |
| Number of Shares Beneficially Owned by Each Reporting Person With | 7 | Sole Voting Power 0 Shares of Common Stock |
| | 8 | Shared Voting Power 32,952 Shares of Common Stock |
| | 9 | Sole Dispositive Power 0 Shares of Common Stock |
| | 10 | Shared Dispositive Power 32,952 Shares of Common Stock |
| 11 | Aggregate Amount
Beneficially Owned by Each Reporting Person 32,952 Shares of Common Stock | |
| 12 | Check Box if the Aggregate
Amount in Row (11) Excludes Certain Shares o | |
| 13 | Percent of Class
Represented by Amount in Row (11) 0.1% | |
| 14 | Type of Reporting Person OO | |

8

SEQ.=1,FOLIO='8',FILE='C:\JMS\105952\10-10265-1\task4118468\10265-1-ba.htm',USER='105952',CD='May 15 02:23 2010'

| 2 | Name of Reporting Persons HIP Side-by-Side I-A, L.L.C. — Check the Appropriate Box
if a Member of a Group | |
| --- | --- | --- |
| | (a) | o |
| | (b) | x |
| 3 | SEC Use Only | |
| 4 | Source of Funds WC | |
| 5 | Check if Disclosure of Legal Proceedings Is
Required Pursuant to Item 2(d) or 2(e) o | |
| 6 | Citizenship or Place of Organization Delaware | |
| Number of Shares Beneficially Owned by Each Reporting Person With | 7 | Sole Voting Power 0 Shares of Common Stock |
| | 8 | Shared Voting Power 0 Shares of Common Stock |
| | 9 | Sole Dispositive Power 0 Shares of Common Stock |
| | 10 | Shared Dispositive Power 0 Shares of Common Stock |
| 11 | Aggregate Amount
Beneficially Owned by Each Reporting Person 0 Shares of Common Stock | |
| 12 | Check Box if the Aggregate
Amount in Row (11) Excludes Certain Shares o | |
| 13 | Percent of Class
Represented by Amount in Row (11) 0.0% | |
| 14 | Type of Reporting Person OO | |

9

SEQ.=1,FOLIO='9',FILE='C:\JMS\105952\10-10265-1\task4118468\10265-1-ba.htm',USER='105952',CD='May 15 02:23 2010'

| 2 | Name of Reporting Persons HIP Investment Holdings I, LLC — Check the Appropriate Box
if a Member of a Group | |
| --- | --- | --- |
| | (a) | o |
| | (b) | x |
| 3 | SEC Use Only | |
| 4 | Source of Funds WC | |
| 5 | Check if Disclosure of Legal Proceedings Is
Required Pursuant to Item 2(d) or 2(e) o | |
| 6 | Citizenship or Place of Organization Delaware | |
| Number of Shares Beneficially Owned by Each Reporting Person With | 7 | Sole Voting Power 0 Shares of Common Stock |
| | 8 | Shared Voting Power 2,068,400 Shares of Common Stock |
| | 9 | Sole Dispositive Power 0 Shares of Common Stock |
| | 10 | Shared Dispositive Power 2,068,400 Shares of Common Stock |
| 11 | Aggregate Amount
Beneficially Owned by Each Reporting Person 2,068,400 Shares of Common Stock | |
| 12 | Check Box if the Aggregate
Amount in Row (11) Excludes Certain Shares o | |
| 13 | Percent of Class
Represented by Amount in Row (11) 6.1% | |
| 14 | Type of Reporting Person OO | |

10

SEQ.=1,FOLIO='10',FILE='C:\JMS\105952\10-10265-1\task4118468\10265-1-ba.htm',USER='105952',CD='May 15 02:23 2010'

Amendment No. 4 to Statement on Schedule 13D

This Amendment (this “Amendment”) No. 4 to Statement on Schedule 13D (as heretofore amended, the “Statement”) relates to the beneficial ownership of common stock, par value $0.01 per share (the “Common Stock”), of TriMas Corporation, a Delaware corporation (the “Issuer”). This Amendment is being filed on behalf of the Reporting Persons (as defined below).

The Reporting Persons are filing this Amendment to amend the statements on Schedule 13D with respect to the Common Stock filed with the Securities and Exchange Commission on March 20, 2009 pursuant to Rule 13d-1(d).

This Amendment is being filed by the Reporting Persons to update the disclosure in Item 4 in connection with the sale of shares of Common Stock.

*Item 4. Purpose of the Transaction*

On May 12, 2010, HIP Investment Holdings I, LLC (“Holdings I”) sold 100,000 shares of Common Stock at a price of $10.38 per share, for net proceeds of approximately $1,034,000. On May 13, 2010, Holdings I sold an additional 31,600 shares of Common Stock at a price of $10.40 per share, for net proceeds of approximately $328,640. Subject to market conditions, the Reporting Persons may sell additional shares of common stock to yield proceeds of an additional $2—$5 million over the subsequent six month period. The purpose of these sales is to raise funds to meet Heartland Industrial Associates L.L.C.’s (“Heartland”) internal funding needs for the next twelve months and are not an indication of Heartland’s view of the business prospects of the Issuer.

Except as set forth above, although the Reporting Persons have no specific plan or proposal to acquire or dispose of Common Stock or other securities of the Issuer, consistent with their investment purpose, the Reporting Persons, at any time and from time to time, may acquire additional Common Stock or dispose of any or all of their Common Stock or other securities of the Issuer depending upon an ongoing evaluation of the investment in the Common Stock, prevailing market conditions, other investment opportunities, liquidity requirements of the Reporting Persons, and/or other investment considerations.

Also, consistent with the investment purpose, the Reporting Persons may engage in communications with one or more stockholders of the Issuer, one or more officers or employees of the Issuer, one or more members of the board of directors of the Issuer and/or one or more representatives of the Issuer regarding the Issuer, including but not limited to its operations, business, results, plans, prospects, ownership structure and management. The Reporting Persons may discuss ideas that, if effected, may result in acquisitions by persons of additional securities of the Issuer, an extraordinary corporate transaction involving the Issuer, changes in the Issuer’s capitalization and/or changes in the board of directors or management of the Issuer.

Except as set forth in this Statement, the Reporting Persons have no plans or proposals which relate to, or could result in, any of the matters referred to in paragraphs (a) through (j), inclusive, of the instructions to Item 4 of Schedule 13D. The Reporting Persons may, at any time and from time to time, review or reconsider their position and/or change their purpose and/or formulate plans or proposals with respect thereto.

11

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SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

Dated: May 14, 2010

HEARTLAND INDUSTRIAL ASSOCIATES L.L.C.

By:
Name:
Daniel P. Tredwell
Title: Managing Member

HEARTLAND INDUSTRIAL PARTNERS, L.P.

| By: | Heartland Industrial Associates L.L.C., its General
Partner |
| --- | --- |
| By: | /s/
Daniel P. Tredwell |
| | Name:
Daniel P. Tredwell |
| | Title: Managing Member |

TRIMAS INVESTMENT FUND I, L.L.C.

| By: | Heartland Industrial Associates L.L.C., the General
Partner of Heartland Industrial Partners, L.P., its Managing Member |
| --- | --- |
| By: | /s/
Daniel P. Tredwell |
| | Name:
Daniel P. Tredwell |
| | Title: Managing Member |

METALDYNE INVESTMENT FUND I, L.L.C.

| By: | Heartland Industrial Associates L.L.C., the General
Partner of Heartland Industrial Partners, L.P., its Managing Member |
| --- | --- |
| By: | /s/
Daniel P. Tredwell |
| | Name:
Daniel P. Tredwell |
| | Title: Managing Member |

HIP SIDE-BY-SIDE PARTNERS, L.P.

| By: | Heartland Industrial Associates L.L.C., its General
Partner |
| --- | --- |
| By: | /s/
Daniel P. Tredwell |
| | Name:
Daniel P. Tredwell |
| | Title: Managing Member |

TRIMAS INVESTMENT FUND II, L.L.C.

| By: | Heartland Industrial Associates L.L.C., its Managing
Member |
| --- | --- |
| By: | /s/
Daniel P. Tredwell |
| | Name:
Daniel P. Tredwell |
| | Title: Managing Member |

12

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METALDYNE INVESTMENT FUND II, L.L.C.

| By: | Heartland Industrial Associates L.L.C., its Managing
Member |
| --- | --- |
| By: | /s/
Daniel P. Tredwell |
| | Name:
Daniel P. Tredwell |
| | Title: Managing Member |

HIP SIDE-BY-SIDE I-A, L.L.C.

| By: | Heartland Industrial Associates L.L.C., its Managing
Member |
| --- | --- |
| By: | /s/
Daniel P. Tredwell |
| | Name:
Daniel P. Tredwell |
| | Title: Managing Member |

HIP INVESTMENT HOLDINGS I, LLC

| By: | Heartland Industrial Associates L.L.C., the General
Partner of Heartland Industrial Partners, L.P., its Member |
| --- | --- |
| By: | /s/
Daniel P. Tredwell |
| | Name:
Daniel P. Tredwell |
| | Title: Managing Member |

13

SEQ.=1,FOLIO='13',FILE='C:\JMS\105952\10-10265-1\task4118468\10265-1-ba.htm',USER='105952',CD='May 15 02:23 2010'