Bolt Head Markings

[Kent Lowry NCRS# 3053]

The "General Motors Engineering Standards" manuals have a section titled Bolts. One of the pages in this section is labeled "Bolt Steel Classification." This page specifies such things as Tensile Strength, Precedure for Testing, Head Marking, Hardness, etc. Prior to 1968 the paragraph specifying "Head Marking" states "Head marking identification where required by the specification shall consist of radial lines extending to the across flats top circle..... The manufacturer's trade mark if required by the purchaser shall be legible, in the center of the bolt head, and not over 1/3 of the across flats dimension". In October 1968 this paragraph was changed to read "Hex head bolts and screws shall be clearly marked with the grade identification symbol shown in Table 1, and with the manufacturer's symbol. Unless otherwise specified by the purchaser, markings shall be located on the top of the head, and may be either raised or depressed, at the option of the manufacturer." This means that prior to October 1968 vendor ID on bolt heads was optional. After October 1968 it was a requiremnt of the Engineering Standard.

[Jack H.]

A club service!

Personal thanks to you for sharing for sharing your resource(s)!!!!


An interesting observation. Perhaps a few more words as to your view of it's relavance would be appreciated. It's not often these days we see posts from 4-digit NCRS members!!!!


The act of leaving a manufacturer's trace audit trail on fasteners goes way back as a self-preservation act to mitigate consequential/incidental damage claims. Though GM's purchasing policy may have changed on this, the true issue remains what was the actual practice, by who, when....


If memory serves, one justification for the US entering a World War was the discovery of a 'K' (Krupp) bolt head observed in the damage area of the Lusitiania (sp?) that was attributed as evidence of a German torpedo. Interestingly enough, I believe, the house of Krupp who's screw machines served two German empires brought to their knees with war crime tribunals still exists and thrives. Point here, is his despite overwhelming liability threats to discontinue the industry practice of source identification, fastener manufactures continued to identify their products long before the GM '68 era....

[Chuck G.]

Re: Bolt Head Markings

I've seen the Engineering Standard book. What it doesn't say is who the hardware suppliers were. It does not give you the headmarkings that were available, i.e. TR, WB, A, M., etc. As long as the fastener met the standard, it could have been provided by any number of vendors. Chuck

[Kent Lowry #3053]

Re: Bolt Head Markings

Thanks for the above responses. I agree with both of them. Several years ago I did over a years research on every part number in the 63-67 AIM manuals. My primary source of reference were a old copies of the GM Engineering Standards manuals. These manuals had a number of sections depending on there age, and each page in the manual has a date indicating when that page became effctive. I also was able to establish contact with the "keeper" of these manuals at GM in Detroit. He was very helpful and provided me with considerable input. He sent me every page revision that has aver been issued by GM for the Bolt Classification page. This is where I got the date that the standards were changed. This data does not say when they were entered into production as we all know that depends on the many things that occur on the assembly line. He did give me the names of several vendors who provided bolts for GM, but I did not pursue those sources. He also provided me with a breakdown of P/N's as they were assigned to the different divisions of GM. I made a similiar contact with the "keeper" of these Standards at Cheverolet, but he was limited on what he coult share with me by Division policy. I spoke with Cheverolet Division lawyers, but got know where. Thanks for your interest and input. I hope this somewhat mellows and flattens the different practices of NCRS judges.

[Jack H.]

Re: Bolt Head Markings

You'll find a pretty dandy little section at the rear of M.F. Dobbins' '63-67 Corvette Fact Book that gives a reasonable survey of known GM fastener suppliers showing logo used and corresponding full name detail of the manufacturer. Also, I remember a pretty thick manual (don't now remember title) that acts as the fastener 'bible' giving similar info but covering the entire free world.

[Kent Lowry #3053]

Re: Bolt Head Markings

I agree with you. I have several editions of Dobbins 63-67 book and it is an excellant publication and belongs in every 63-67 owners library. It is interesting you mention this book for it played a part in my getting started on my 63-67 research. I have an original 66 corvette. While judging at chapter and regional meets, I would find minor differences in common hardware on the cars I was judging and my own. As I looked further into explaining these differences I used Dobbins books as well as other material. About this same time I gained access to early editions of the GM Engineering Standards. I also was given access to a data base of 25yrs of very detailed history of the restoration detail of solid axle and mid year corvettes. As I merged, sorted and cross referenced all of this data into a new data base, I came to a general conclusion which has guided my judging practices for several years. I don't believe there were two solid axle or mid year vettes that were ever manufactured identically. All of this info I learned tells how they were designed and engineered to be built, but not how they were actually built.