Let me interject one thing here before this procedes any further. When you switch from a very solid, ridged, cast iron exhaust manifold, which, when newly assembled to the head at the factory, -----WITHOUT ANY GASKET------, to a thinwall, steel tube which flexes-------A LOT, you're going to have leaks. Period. Obviously, the quest is to find a suitable gasket which will do the best job of sealing for the longest period of time. ONLY a very few of my cars have headers, the rest have stock type exhaust manifolds. There is no question that a good set of headers allows the engine to breathe better and produce more power. Period. Plus, headers (nice coated ones) just look good, manifolds don't (except on a 100% correct restoration). Personally, I'd give anything if the old 2 1/2in Rams Horn manifolds would clear on a Chevelle, and other cars. That's all that I would have.
Also, header flange-to-head fit is just like fingerprints. They're ALL different. Very probably you could take 2 identical pairs of heads and 2 sets of headers made at the same time, and use the same brand/type of gaskets, bolt them all together, and one may leak tomorrow and one may never leak for 5yrs.
No question about it, sealing headers for a long term is a real challange.
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Tom Parsons
