It depends on whether it is a u-tube or a stock Straitube. The u-tube gaskets are based on the nominal diameter of the heat exchanger and not the diameter of the gasket. For example, a 12" u-tube uses a 12" heat exchanger gasket. This gasket is the same diameter as the raised face of an ANSI flange. A Straitube gasket is model specific and should be chosen based on the actual model of the heat exchanger. For example, a model 320 Straitube uses model 260, 320, and 380 replacement gaskets. Straitubes also have front and rear-specific gaskets. Be sure to specify if it is for the front or the return head.