Have a 2" dual exhaust on my 71 Skylark 350. Since I bought the car I have blown the exhaust gaskets 2X so far, what gives?? Am I blowing the gaskets because the lead pipe is 2.5 and being downsized to 2"???????
The manifold gaskets? If so, pull the manifolds and have them machined flat. Then reinstall. I don't ever use gaskets on mine if I have them milled flat. The factory didn't use them, either. They'll probably hold up much better with a flat surface, though. So if you feem more comfortable using gaskets, go ahead. It won't hurt.
I suspect the 350 like the 455 never had these gaskets from the factory. Get the manifold trued flat, reinstall metal to metal and torque these in the correct sequence.
Ive heard that many times that no gaskets were used new.... and a friend of mine has a 71 Pontiac Lemans with stock exhaust manifolds and there are no gaskets on that. I still cant get my mind around it though. Even if milled flat I would think you'd get small leaks. I guess it doesnt hurt to try but I cant find anyone here in Orlando that will mill the exhaust manifolds.
If your surfaces aren't FLAT flat, the gaskets will burn through as soon as they find a weak spot...which is what's happening with Steve's setup. To aid in sealing, smear a SMALL bit of red high temp RTV on the surfaces. I had the same thoughts you did about the "metal to metal" seal, but I've done this on 4 cars now, and it makes me wonder why I ever tried to use gaskets in the first place:laugh:
Bought the car with exhaust manifold gaskets already on it and then blew them out. Had them replaced, used header gaskets and it happened again! Never knew that the motor did not come from the factory with exhaust manifold gaskets!! How would you stop an exhaust leak if one occured? Already snapped one manifold bolt the first time trying to loosen then snug things up. Car is in the shop now getting both sides redone with gaskets that will supposedly hold. Hope I just didn't throw good money after bad. I thought as the car had a somewhat new 2" dual exhaust on it mated to 2.5 manifold pipes that I was producing too much back pressure.
