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                                                                     EXHIBIT 5.1

[Summa Industries Letterhead]


December 14, 1998

Securities and Exchange Commission
450 Fifth Street, N.W.
Washington, D.C.  20549

        Re:  Registration Statement on Form S-8

Ladies and Gentlemen:

        I am the General Counsel of Summa Industries, a Delaware corporation
(the "Company"), and have acted as counsel for the Company in connection with
the preparation of a Registration Statement on Form S-8 (the "Registration
Statement") filed with the Securities and Exchange Commission under the
Securities Act of 1933 pertaining to the issuance and sale of up to an aggregate
of 100,000 shares of the Company's Common Stock, par value $.001 per share (the
"Shares") issuable under the Company's 1995 Stock Option Plan (the "Plan").

        For the purpose of rendering this opinion, I have examined and relied
upon such records, documents, certificates and other instruments and made such
legal and factual examinations and inquiries as I have deemed necessary or
appropriate. On the basis of such examinations and inquiries, and relying
thereon, I am of the opinion that the Shares are duly authorized and, when
issued and sold upon exercise of options granted under the Plan on the terms and
conditions set forth therein, will be validly issued, fully paid and
nonassessable under the laws of the State of Delaware.

        I hereby consent to the filing of this opinion as an exhibit to the
Registration Statement and to the reference to my name under Item 5 of Part II
of the Registration Statement.

                                            Very truly yours,

                                            /s/  Trygve M. Thoresen
                                            ------------------------------------
                                            Trygve M. Thoresen, Esq.
                                            General Counsel