Exhaust Manifold French Locks

[Joe L.]

As most are aware, exhaust manifold french locks were used on Chevrolet small blocks for virtually all year model engines through 1979. Many later engines used them, too, but many Corvettes went to tubular manifolds beginning with the 1980 305 engine and these manifolds do not, of course, use french locks. When used, french locks were used only on the end manifold positions.

Big block engines, Corvette and otherwise, used french locks on the exhaust manifolds for 1965, 1966, and all or part of 1967. Like small blocks, they were used only for the end positions but were, obviously, a different configuration than those used for small blocks.

Now, for the "technical trivia question": does anyone know why the french locks were deleted from big block engines? If no one comes up with it after a reasonable amount of time, I'll post the answer.

[Mike McKown]

I'm sure this is the wrong answer but,

Anytime I enountered an engine with these things on them, they went in the garbage and weren't replaced with either french locks or split lock washers. I NEVER, EVER had an exhaust manifold bolt back off. So, what's the purpose?

What's the real answer?

[lyndon sharpton]

Re: Exhaust Manifold French Locks

they found that they could save 14 cents per engine, by not using them.
I give, what is the answer?

[Joe R.]

Re: Exhaust Manifold French Locks

Lyndon,

That's a real good answer and it might be correct. I was going to say that they were discontinued and GM couldn't find anyone in Tiawan to reproduce them. AND they didn't want to use Paragons cause they were stainless and too expensive. OK, I give! What's the answer Joey?

JR

[Joe L.]

Technical Trivia Answer

Well, I guess with 225+ views, no one out there has the answer to this one.

First of all, I don't think that the deletion of the big block french locks had anything to do with cost reduction. If that was GM's intent, they would have eliminated them from small blocks, too, and saved themselves some REAL money. There were a whole lot more small blocks built from 1967-onward than there were big blocks.

Here's the deal as I've been able to dig up: as background, if you check out the exhaust manifolds on a big block Corvette (and other big blocks, too), you will note that the bolt holes for the END ports are machined to a bolt depth of about 19/32". The holes for the 2 center ports are machined to a depth of about 11/16". The difference for the end holes is so that, considering the thickness of the french locks, the same length bolts could be used for all positions. This machining was the same regardless of year.

For 1965, 1966, and all or part of 1967, the french locks were used. The bolts used were GM #3736035. These are a conventional hex head, trimmed (i.e. non-indented head) 3/8"-16 X 1-19/64" bolts. They carry a "double circle"
material grade headmark (SAE grade 8; GM 300M) and various manufacturer's ID marks. They are black oxide or black phosphate-finished.

Sometime during 1967, but certainly by the start of production for 1968, the bolts used for the end port flanges changed. A very special new bolt was used. This bolt was GM #3909821. This bolt was also 3/8"-16 thread size but it was of 1-3/16" length. The reduced length was to make up of the fact that it would be used with the shorter-machined, end flange bolt holes but WITHOUT a french lock. This bolt was a FLANGED hex head, untrimmed (i.e. indented head) bolt that was finished in black oxide or black phosphate. However, it was very special in another way. This bolt uses a special LOCKING THREAD design. In this design, the minor thread dimension (i.e. the bottom of the thread "grooves") is increased by a small amount, thereby making the bolt an interference fit. The first 2 threads of the bolt are conventional, though, so that the bolt can be started in the bolt hole without difficulty. Thereafter, significant torque is required to run the bolt in. This prevents the subsequent loosening of the bolt much in the same way that a "deformed thread" or Nylock locknut prevents loosening of a fastened assembly.

This type of bolt is really quite unusual, especially in automotive applications. The above thread locking system is a proprietary system originally patented by Accurate Automatic Parts. I presume that the bolts were/are manufactured by them or a licensee to them (or, perhaps, the patent has run out by now).

So, 1965-66 Corvette big blocks should have french locks at the end port flanges and have "double circle", trimmed hex bolts at all positions (except where a stud is used for N-40 or C-60 applications). For 1967 it's a question mark. I believe that the french locks were phased out about mid-production, but I'm not sure. One thing is fairly certain, though: the french locks were not used in conjunction with the 3909821 bolts. As I mentioned, the 3909821 bolts are FLANGED HEAD bolts and these do not work properly with an underlying french lock.

1968-74 should have the flanged head, 3909821 bolts at the end port flanges and the 3736035 at the other flange bolt positions (except where a stud is used for N-40 or C-60 applications). I won't rule out the possibility that the 3909821 bolts might even have been used at all positions in some cases. It wasn't supposed to have been done that way, but it may have been done that way.

And, that's the story of why french locks were deleted from big blocks ABOUT 1967, or so. Actually, I'm surprised that GM didn't do the same for small blocks. In fact, they may have but if they did, it didn't happen until the 90s. I'm still researching that point.

[lyndon sharpton]

Re: Technical Trivia Answer

so your saying that after 67 no BB corvette came with french locks? I thought all 72 BB had the french locks? an that no 70 BB had them an that they were intermittent use in 71?

[Joe L.]

Re: Technical Trivia Answer

lyndon----

What you're saying sounds more like the situation with spark plug barrel shields than french locks. As far as I know, the french locks were not used after 1967. However, I suppose that it's possible that for some period they ran out of the special bolts and went back to french locks for awhile.

[lyndon sharpton]

Re: Technical Trivia Answer

well I have a old JM an it has that some years have the locks. I know that some catalogs had them listed, but now I dont see the BB for the 70-72 just the small b cars.

[Terry F.]

I have the answer!

Hey, what do I get if I get this right? The answer is this....and I have not looked at any other posts yet....The bolts for the exhaust manifolds on the big blocks starting in 68 were different. The threads were cut in such a way that they were only about half there normal depth. They also seemed a little rounded. In any case, they went in tight. As to how the thread in the cylinder head were I am not certain. They may have been cut with a different thread dimension also. But, in any case, if you study the bolts that come out of exhaust manifold you will notice the threads are distinctly different.

Ok, where's my prize?
Terry

[Terry F.]

Re: Technical Trivia Answer

I wish I would have read your post earlier. Later, Terry

[Joe L.]

Addendum

By the way, does anyone have a 1968-74 Corvette with big block that they believe to be original that does have exhaust manifold french locks?

Also, what about 1967 big blocks? Does anyone have what they believe to be an original installation with the flange head bolts at the end runner positions and no french locks?

[Mike McKown]

So, they used torque prevailing bolts.

That means to me that the installation torque spec should be higher on these bolts to get the same clamping force between the head and manifold. Right? Wrong?

[Terry F.]

Re: So, they used torque prevailing bolts.

You are probably asking Joe. But, I don't recall any special torque on the bolts based on what the overhaul manual showed.

Terry

[Joe L.]

Re: So, they used torque prevailing bolts.

Mike-----

I regard any torque spec on these bolts to ne "meaningless". However, the "official" torque spec for ALL big block exhaust manifold bolts is 20 lb/ft. That's significantly lower than the "standard" torque for a grade 8 3/8-16 bolt.