A lot depends on the quality of the header flange itself...as in how thick it is. Some flanges are so thin that after you tighten them the first time, they are bent/warped, and no matter what you do, they just won't seal.
One old timer trick I have is to use a normal paper/asbestos(like the white Mr. Gasket gaskets) header gaskets, and soak them in water for a few minutes before installing. This softens the material, and allows you to tighten the headers down a little more than usual. Once the moisture dries out in the gaskets, they seal up really nice. The softening of the gasket material makes it easier for it to conform to small irregularities in the flange and the head itself. This works pretty well on headers that have bent/warped flanges, and it helps to keep from bending the flanges on new headers, too.
Make sure whatever gasket you use is the same shape and size as your header tubes/flanges. If the hole in the gasket is round, and your tubes are square, or visa-versa, your gonna eventually get a leak.
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Joel Adams
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