Too thick of gaskets can be the cause of a leak, especially on a 2-bolt flange. As you tighten the bolts it can crush the gasket right at the bolts and the flanges can bow and not apply the same pressure across the whole flange. If the flanges are perfectly flat, have a clean surface texture, and have ample surface area contact... sometimes I dont even use a gasket, I just apply a thin coating of copper sealant. This can keep the contact pressure even and prevent the flanges from warping when torqued.
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