thermostat bolts

[John F.]

Would it be possible ('66 300hp) to have thermostat bolts with "TR" and not have three radiating lines??

[Scott S.]

Re: thermostat bolts

Would it be possible ('66 300hp) to have thermostat bolts with "TR" and not have three radiating lines??

John,

I would also like to know the answer to your question, I think the answer will apply to both 300hp and 350hp engines for 1966 & 1967.

I have a '67 with 350hp, my long thermostat bolt is Grade 5, with "CP" headmark, which is correct according to the 1966 JG (the older 1967 JG doesn't mention "CP", only "TR" and "RSC"). The short thermostat bolt on my car has NO grade lines, not even a headmark, just a raised "dot" in the center. It is a regular hex head, 3/8-16 x 1-1/4".

The picture in the 1966 JG (p. 109) shows a Grade 5 bolt for the long bolt, but I don't see any grade lines on the short bolt, just two letters which I can't make out. It may be that the 1966 JG reference to "three radiating lines" applies only to the long bolt.

[Rich P.]

Re: thermostat bolts

John,
your 66 300 HP thermostat should be the new for that year aluminum/pot metal 660 housing. For the long bolt it will be a 3/8-16 thread x 2" long grade 5. The short bolt will be the same except it is 1" long, again grade 5.

Scott,
your 67 L-79 will use the steel curved neck thermostat housing. The long bolt will be 3/8-16 thread x 3" long grade 5. The short bolt will be 3/8-16 thread x 1" long grade 2. The CP is what I would expect to see for the long bolt and the blank head short bolt with a dot is also era correct just not as widely seen. I would not change either one.

Rich

[Scott S.]

Re: thermostat bolts

I found a great picture of a survivor 1965 L79 thermostat housing, but it uses studs, not bolts. The best picture I can find of a '66 or '67 thermostat housing is this '67 L79 Top Flight car that was in the car corral at Carlisle last year. I can see the three grade lines on the long bolt (here and in other pictures), but I only see what looks like a letter near the perimeter of the short bolt. If there are grade lines on the short bolt, it must be turned just perfectly so as not to be visible at this angle.

The second picture is zoomed in 1 x.

[Scott S.]

Re: thermostat bolts

John,
your 66 300 HP thermostat should be the new for that year aluminum/pot metal 660 housing. For the long bolt it will be a 3/8-16 thread x 2" long grade 5. The short bolt will be the same except it is 1" long, again grade 5.

Scott,
your 67 L-79 will use the steel curved neck thermostat housing. The long bolt will be 3/8-16 thread x 3" long grade 5. The short bolt will be 3/8-16 thread x 1" long grade 2. The CP is what I would expect to see for the long bolt and the blank head short bolt with a dot is also era correct just not as widely seen. I would not change either one.

Rich

Thanks Rich, that's great news. I thought the short bolt with no headmark (except the raised dot) had to be a later replacement. It will now be going back on the car for sure.

I wonder why they used a Gr5 short bolt on the aluminum thermostat housing, but only a Gr2 short bolt on what the JG calls the "cast iron" L79 thermostat housing?

[John F.]

Re: thermostat bolts

Thanks for the replies. I have the correct housing with both long and short bolts marked "TR" no lines???

[Rich P.]

Re: thermostat bolts

Thanks for the replies. I have the correct housing with both long and short bolts marked "TR" no lines???

John,

sorry for not being clearer. Your bolts should have 3 lines each. That makes them grade 5. Your bolts now with no lines are grade 2, not typical factory production.

Rich

[Scott S.]

Re: thermostat bolts

Thanks for the replies. I have the correct housing with both long and short bolts marked "TR" no lines???

John,

Here is the quote from the 1966 JG for the 300hp engine: "Known original cars use graded attaching bolts with head marks of 'TR', 'M', 'CP' or 'NAT' with three radiating lines; others may exist."

Since you have the correct "TR" headmarks, there's at least a chance that yours came from the factory with Grade 2 bolts (no lines). You might want to contact the 1966 Team Leader Bill Calorico to find out if he has ever seen a known original 300hp '66 with no grade lines on either of the two thermostat housing bolts.