Exhaust manifold torque procedures

[northerngmc]

The left cast exhaust manifold on my 396 in my 1968 C20 cracked/broke. I have replaced the manifold but I am wondering about the torque procedure.

I have tightened them all up, using a pattern to equally apply torque. Should they be tightened hot or cold or maybe both?

[68gmsee]

It's not a critical part. I've always just torqued the bolts down equally and double check them after I've run the engine to operating temperature. Specs say 25 ft lbs.

The important part is to make sure there aren't any high/low areas of the mating surface. The couple of times I've bought a new manifold I've had to have them resurfaced to make sure they were completely flat.

[northerngmc]

Thanks. I warmed it up and re-torqued today.

[68gmsee]

Forgot to mention that I don't use gaskets on the sbc stock cast iron manifolds, however, I do use one on my 250 six cylinder since the intake and exhaust are combined.

[northerngmc]

I did not use a gasket. I was surprised to learn that GM did not use gaskets.

Now I just need to find a plug that will fit the hole in the back of the manifold where smog equipment would go. 3/8" NPS plugs fit great in the holes on the top, but 3/8" is too small and 1/2" NPS is too large for the hole on the back. On my way to work tonight I picked up a #8 brass flare plug that I hope fits.......

[northerngmc]

So I think a combination of me not being familiar with pipe measurements and the bin at the parts store having the wrong size in it has caused me great pain over a small issue.

So I found plugs in the 3/8" pipe bin that fit the holes above each cylinder on the exhaust manifold. But these 3/8" plugs were too small for the hole near the exhaust dump and 1/2" was too large....... hummmmmmmm and they had nothing in between.

Turns out the plugs in the 3/8" bin were 1/4".......... and then when I went to a different store and bought a 3/8" brass pipe plug it fit the rear hole just fine.