swensonm

Howdy! I am removing my headers and going back to OEM manifolds. But not sure about the bolt sizes. The holes in the manifolds are different diameters. Need a little help.....

derekderek

3/8ths coarse. AKA 3/8 16 NC. 1.5 or so inch length.take manifolds to lowes or home depot to get length right. I assume you want bolts that work, not correct orig bolts with correct stamping on head, etc?

gbvette62

The exhaust manifolds use four 3/8-16 x 2 1/4" bolts, and eight 3/8-16 x 1 1/4" bolts. If you have air conditioning, there is a special stud needed, that replaces the right front short bolt. This stud is double threaded with a hex head in the middle, to allow it to be properly tightened against the manifold. The original bolts have what is commonly referred to as a "double circle" head mark.

The two front and two rear bolts on each manifold, get a special thick washer to keep the bolts from bottoming out. The four longer center bolts, don't use the washers. The four outer pairs of bolts, also get french locks on them.

The correct double circle bolt sets are available from Corvette vendors for around $16 a set, and the bolt set with the stud for AC cars, runs about $28. The manifold washer sets and french locks are also available from most vendors.

swensonm

Thanks
Are the bolt different diameter for the larger holes in the manifold?

Mike

gbvette62

All of the bolts are the same diameter, only the length differs. The 2 1/4" long bolts go in the two center holes on each manifold, and the 1 1/4" long bolts go in the holes at the front and rear of each manifold.

leadfoot4

Just taking a SWAG (scientific wild assed guess) here, but I think the different diameter of the holes, is to allow linear expansion/contraction to naturally occur, as the manifolds heat and cool. The two center port holes are a snug fit, to positively locate the manifold, and the outer four holes are larger, allowing the manifold to "grow" with heat.

stingr69

Those exhaust manifolds have already been run with gaskets. The factory did not use gaskets but once a manifold has been run with them, you will likely need to keep using them.

You may find the new bolts will want to back off a few times before they stay put. The french locks will help but not in all the positions.

I would either use a crusty old set of the common factory bolts (preferred) or just buy the repro kit as a last resort. Factory stuff is of dependable quality in my opinion.

swensonm

Bolts

Thanks I have a line on some old OEM from a Forum member

TimAT

I'll add to put some anti-seize on them too. I agree that GM did not, but if you or anyone else wants to remove them for what ever reason, you'll appreciate not snapping one off.

leadfoot4

Let's be realistic here, GM wasn't worried about what happened to these cars, once they were out of the warranty, which at MOST, was 5 years....