C3 Exhaust Manifold

[Jay L. Flinders]

Here I go again with another question. I have a 1968 427/390 chasis build date of M03 August 3d. The cylinder heads part number are 3931063 dtd 7/10/68. The exhaust manifolds are 3880827 dtd 3/4/68 and 3880828 dtd 12/11/67.

My question is in regards to the mounting holes in the exhaust manifold.

Is their any reason why the holes on the outside of both ends of the exhaust manifold larger in diameter than the 4 center holes?

The bolts for all 16 are the same diameter.

Yes I know the date of the 3880828 is probably to far out from the build date but these manifolds are getting a little hard to find and the orginal one I have has a hole in it.
Thanks Jay

[Dick W.]

Re: C3 Exhaust Manifold

Jay, I would not worry about the dates. They are not judged. As for the holes, someone else will have to step up on this.

[Chuck S.]

Re: C3 Exhaust Manifold

If the manifolds are original, and they seem to be, why worry about the mounting hole diameters? GM had engineers figure out how to design those manifolds. If you have an academic interest, then it probably has to do with the greatest linear expansion being at the ends of the manifold, but that's only a guess...bigger hole = more sloppiness available for movement.

If your manifolds have cracks, they can be repaired by people that do cast iron welding. Do an archive search on cast iron welding; this has been covered in the last few months, but I don't remember the particulars.

[Jay L. Flinders]

Re: C3 Exhaust Manifold

Thanks chuck for the information. It doesn't appear to be linear expansion. They were cast that way. I have been told by a GM parts guy that his best guess would be that there was some play for mounting purposes. I just thought it was odd and thought those of you who had done this a lot longer than me had come across this irregularity. Or maybe there was a sleave used.

I did find a welder here who had done cast iron welding. He tried welding the crack using heating, drilling at the end of the crack, cleaning the crack, and using the correct rod. The result was the hole just got bigger and bigger were now you can drive a car through it. He said that some times it works and others it doesn't.

[Chuck S.]

Re: C3 Exhaust Manifold

Ouch...Your welder must have quoted Dustin Hoffman's character in Little Big Man on how effective the Indians bone chest armor was at stopping the calvary troopers' bullets...Sometimes the magic works; sometimes it doesn't.

Try another welder with "stronger" magic...this one may have stole your chance.

[Terry M.]

Re: C3 Exhaust Manifold

Jay,
The cast iron exhaust manifolds on Gen II LT1s are made the same way -- larger holes on the end positions. For reasons not related to Corvettes I had occasion to examine this, and some other features of the Gen II exhaust manifolds, with a powertrain engineer who supposedly got the information from the release engineer for those (Gen II) exhaust manifolds.

The center fasteners are intended to hold the manifold securely to the head, and the expansion and contraction due to heating cycles is supposed to create a sliding action between the head and exhaust manifold nearer the ends. The larger diameter holes in the manifold allow the end fasteners to continue their clamping force regardless the expansion and contraction.

One fly in this explanation as it relates to Gen I engines is that the Gen II engines used a gasket at the manifold to head mating surface and Gen 1 did not. You may take this explanation for what it is worth.

[Joe L.]

Re: C3 Exhaust Manifold

Jay-----

I believe that the reason is exactly as Terry described for the small block application.

Also, I can tell you this, for sure: every big block manifold that you're going to find, Corvette or otherwise, will have the same situation that you described as far as manifold bolt hole sizes are concerned.

[Jay L. Flinders]

Re: C3 Exhaust Manifold

Thanks to all. I knew that Terry would have an answer. I should have just asked you in the first place.

[Terry M.]

Re: C3 Exhaust Manifold

Thanks Jay, but it is good to publicize the information here so everyone will know.