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General Motors Head Bolts

Guest Ravensmiles

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Guest Ravensmiles

Here is a question that has become an issue for me and I believe the answer could help a lot of people restoring GM cars and trucks. First I'll givea little back ground on the question.

I have a 1940 Oldsmobile that I am restoring and not knowing much about pre-war cars I thought I had the original 257 ci engine. I started looking for specs for the missing head cover bolts. A good guy in California told me the specs were 7/16-14 by 2-7/8" and specified an undercut on the diameter of the bolts about 1/4" below the bolt head. I Couldn't find the original bolts anywere so I went to Lowes in desperation. They sold me a grade 8 bolt that matched the specs but does not match the undercut under the head and has threads longer than the originals. The bolt fits perfectly and tightens down flat against the head cover. Then I discovered that the engine I have is actually a 1938 pontiac 249 ci.

I was surprised that the bolt even fit at that point to say the least. So now I'm wondering, did all GM products of roughly the same time period use identical head cover bolts and Is the undercut below the bolt head critical or will the engine work ok without it? The desk person at O'reily was shocked that the bolt seated properly when I showed him video of putting one in.

I guess the core question is If you can't find the original head cover bolts for a car from the 30's or 40's can you substitute similar bolts and if so what dimensions are critical and which ones can you fudge without destroying your engine?

Honestly I'm just learning how to work on cars and I appologize if this is a dumb question.

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