What thickness is best for a gasket?

In general, Phelps recommends the use of thinner gaskets wherever possible. This increases the performance of the joint by lowering emissions, lowering product loss, and increasing blowout resistance. On the downside, thinner gaskets will not seal as many flange irregularities as thicker gaskets, and require flatter flanges. The most commonly used thicknesses for Standard Flange gaskets are 1/16" and 1/8".