GM must have changed there flange seal. I put a set of shorties on my Trans am (aluminum 5.7 liter variant of our 6.0 liter motor) and they utilized the gasket I mentioned. I guess they must have switched up to o-ring donut seal along the way.
By the way what type of gasket was on the exhaust manifold to cylinder head? On the LS1, I have typically seen a 2-layer metal gasket used...pretty good stuff...especially coming from the factory.
Tom
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