Mr Gasket Water Outlet Gasket - Edge Moulded Rubber 738G MG738G Suit Small Block Chevy 1955-88 & Big Block Chevy 1965-90 THERMOSTAT HOUSING GASKET These high performance thermostats are designed to resist large variations in coolant pressures that occur at high RPM's where coolant temperature and coolant pressure fight for control of the thermostat keeping it from opening at it's designated temperature. The balanced HP Thermostat features all brass and copper construction, and opens at the right temperature regardless of engine RPM.