C3 Thermostat Bolts

[Kevin G.]

Need a little help, can anyone tell me the correct head marks on the thermostat bolts on 72sb vs 71bb? TIA

[Kevin G.]

Re: C3 Thermostat Bolts

Thermostat housing bolts, of course.

[Terry F.]

Re: C3 Thermostat Bolts

It is possible that several different head markings were used. At least 2, possibly 3 styles. You would probably be safe with TR head marks but I am sure someone that has an original car will chime in.

[Joe L.]

Re: C3 Thermostat Bolts

Kevin------

One known headmarking for the long bolt was a "C" with 3 lines (SAE grade 5). For many C3s, the "short bolt" was not a bolt, at all. It was a stud with 1/2" of 5/16-18 thread on the top and 1-1/8" of 3/8-16 thread on the bottom. The GM # was 3776391. The upper thread was not used for Corvette applications.

[Terry M.]

Re: C3 Thermostat Bolts

We usually see the stud Joe refers to on 1970-1972 small blocks.

[Joe L.]

Re: C3 Thermostat Bolts

Terry----

1969 small blocks had it, too. I'm not sure about 68. GM says that all 1974-82 small blocks had it, but I'm not sure if they actually did, or not.

[Kevin G.]

Re: C3 Thermostat Bolts

Thanks Terry, Joe, and Terry,
Lets see if I got it, on small blocks and big blocks the long bolt can be TR or C with three lines or grade five. The short bolt on the sb is a stud and nut. Now I'm guessing the short bolt on the bb should also be a matching TR or C grade five?

[Joe L.]

Re: C3 Thermostat Bolts

Kevin-----

There's no nut, at all, involved with the stud. It's a special stud with the dissimilarly threaded ends that I described and an integral wrenching surface. I don't think that big blocks used the stud, but I suppose it's possible they did.

The upper end of the stud had no functionality, whatsoever, with respect to the Corvette application. There must have been some other Chevrolet applications that used it for an anchor point for a bracket or something other reason. So, Flint engine "standardized" and installed the studs on all engines produced for some period of time.

I would definitely NOT assume that the bolts used for big block applications had the same manufacturer's headmarking. If the bolts were the same length, there would be a very good chance that the markings would be the same. However, the bolts were NOT the same length. So, it's VERY possible that they could have been supplied by different manufacturers. I don't know that they were, but it's very possible. Also, keep in mind that for most fastener applications, several manufacturers may have supplied the fasteners to PRODUCTION. Rarely, is there only one correct headmarking for any particular fastener. There are some that are "known to have been used". That does not equate to "the only ones to have been used". I believe that, generally, period correct headmarkings (i.e. common ones that are known to have been used for other fastener applications of the period) are accepted as correct for judging.

[Joe L.]

Addendum

Just to be clear, in my second paragraph I did not mean to imply that the bolts for small blocks and big block thermostat housings were of different lengths. They were the same length (except, of course, for the fact that the stud was used for the "short" position for small blocks. What I was intending to say was that the 2 bolts used for big blocks were of different length. That may or may not have been clear to some, so I herewith clarify.

[Terry M.]

Re: C3 Thermostat Bolts

Amen to Joe's posts of 1:49 and 1:53 PM.
Re: his post of 1:57 AM -- I also agree about 1969, and some 1968s. I have not seen enough original unrestored 1968s (and not that many restored either) to conclude much about the stud in those applications. I failed to mention the earlier application because this discussion was about 1972s and I wanted to stay close to there.
I recall a Chevelle or maybe a Caprice use for the stud -- I had way too many of each of those over the years to be sure -- the upper alternator bracket was mounted to that stud and to, I think, a stud on the water pump.

[John H.]

Re: C3 Thermostat Bolts

Joe -

The upper threaded portion of that special stud was used with a nut to attach the rear of the upper alternator bracket and the negative battery cable end on '69-up A-B-F-X passenger cars with the new long-water pump accessory drive system.

[Kevin G.]

Re: C3 Thermostat Bolts

Thanks for your help gentlemen!