Yamaha - Thermostat Cover Gasket Cooling System 63P-12414-00-00. Overview. The Yamaha 63P-12414-00-00 Thermostat Cover Gasket is designed to create a tight seal between the thermostat housing and the engine. This gasket helps prevent coolant leaks while maintaining proper pressure and flow within the outboard's cooling system. Over time, heat cycles and exposure to water can cause gaskets to harden, compress, or deteriorate, which may lead to leaks around the thermostat housing.. Replacing the g