The best answer to this question is that the proper gasket thickness is the thinnest possible gasket. This is because a thinner gasket has less surface area (thickness x ID) exposed to the medium. The minimum stress required to seal decreases with thickness; thinner gaskets have superior load bearing resistance, undergo less creep relaxation than thicker ones, and tend to be less expensive. If flange surfaces are of the proper surface finish, or have recently been refinished, a 1/16” or thinner gasket is recommended. A thicker gasket material may be necessary if a flange has been damaged or has imperfections which a thinner gasket cannot seal and the flanges cannot be resurfaced.