1968 327 300HP Intake Manifold Insulation

[Keith McVicker]

I have a 1968 327 300HP Corvette. It is mostly original with a few bolt-on modifications. I've gone back and forth in my mind about restoring it to 100% original condition. It looks good and runs good so currently I’m satisfied with it, but I realize that it would be worth more as a collectible if it were completely original.

One of the modifications is that the original cast-iron intake manifold has been replaced by an aluminum Edelbrok manifold. I have the original manifold, but it has a problem. The insulating material underneath the shield below the carburetor has begun to decompose. It is breaking up and little chunks of it are falling out of the crack between the shield and the manifold. I certainly would not want to put the manifold on the engine in this condition.

My questions are:

1. What was this insulation material? (Being a 1968 car, I wouldn’t be surprised if it was asbestos.)
2. What insulating material could I use as a replacement?

I’m wondering if anyone here has had this same problem when restoring an old Corvette. I have several restoration books and none of them address this.

[Mike McKown]

It's probably not insulation, just baked motor oil *NM*

[Tom Hendricks (23758)]

Re: 1968 327 300HP Intake Manifold Insulation

Couldn't be an old ancient mouse nest could it ?? I'd take it to a local machine shop and have it tanked and cleaned. Should be good as new after a nice repaint.

[Duke Williams (22045)]

Re: 1968 327 300HP Intake Manifold Insulation

It's not clear to me what part you are refering to, but maybe it's just the old gasket.

I would think it's just a matter of replacing the deteriorated part. Your AIM should show the part with the original part number, and you go from there.

Duke

[Jeff Bukowski]

Re: 1968 327 300HP Intake Manifold Insulation

I agree with the baked motor oil theory. The exhaust crossover is under that cover. Oil that splashes up there gets baked and leaves the deposits that you see. I was able to bend the edges of the cover back and remove the stuff or you could remove the cover, do some cleaning, then tap the rivet holes and screw the cover back on. I thought the archives had some info on how to re-rivet it.

[John Hinckley (29964)]

Re: 1968 327 300HP Intake Manifold Insulation

Keith -

If you're talking about the BOTTOM of the manifold, pop the rivets out (you can pry them out with a sharp chisel), clean both the oil shield and the bottom of the intake thoroughly, and re-install it. GM still has the drive rivets (now called "Pin, Engine Air Ht Shld" - P/N 10229247, ten in a bag), which have MUCH sharper splines on them than the ones you'll remove.

[Keith McVicker]

Re: 1968 327 300HP Intake Manifold Insulation

Yes, I suppose it could be some 40 yr. old solidified motor oil sludge! The pieces coming off do have a coal-like appearance. I just assumed that because there is a good sized chunk of it filling the cavity between the shield and the manifold, it was some sort of material intentionally put there. I'm afraid I'll have to remove the shield to get it all out. A few spot welds could be used to put it back on or a machine shop might be able to re-rivet it. (I'd hate to have a screw vibrate loose and end up in the engine.) I may just keep the original manifold in a box for anyone who wants it if and when I ever sell it. Thanks for the advice.

[Kurt Bunting (26406)]

Re: 1968 327 300HP Intake Manifold Insulation

Keith,
I experienced the same thing on my 68 327 L79 manifold.
It was baked on oil that felt like pieces of carbon, but I just took it to a local machine shop and they hot tanked it and it came out fine and is now back on my motor.
Kurt

[Kent Keech (6201)]

Take John Hinkley's advice above

Keith,

Once you've removed the shield and cleande everything, the new rivets fit snugly and won't come out.

Regards, Kent Keech

[Keith McVicker]

Re: Take John Hinkley's advice above

I think that's a good solution.
Thanks, Keith

[Joe Lucia (12484)]

Re: Take John Hinkley's advice above

Keith-----

The coked oil deposits will be found on ANY small or big block Chevrolet V-8 which uses an oil shield (most do) and which has any significant number of miles "under its belt". The only way to prevent it is to block the heat crossover passage in the intake manifold. Doing that will render the choke inoperable on any 66-74 Corvette and also requires the removal or blocking open of the heat riser valve.