This gasket seals the thermostat housing to the engine block, preventing coolant leakage at the joint. It sits between the thermostat flange and the machined surface of the engine, where it must withstand thermal cycling and pressure from the cooling loop. A degraded gasket allows coolant to escape, causing gradual pressure loss and potential overheating. The gasket matches the original profile so the thermostat seats at the correct depth, maintaining the designed opening temperature. On reinstallation, clean both mating surfaces and tighten the housing bolts to the specified torque.