For 3 3/4 inch bore, .040 inches thick. For all Big Twin Evolution motors 1984-1999. Multilayer steel gaskets are the latest development in head gasket technology. Two NBR coated metal gaskets sandwich a middle layer that offers superior sealing over Teflon or graphite head gaskets. Three-piece head gaskets are more tolerant of thermal expansion and contraction. These are a must when running higher compression motors.