Yamaha - Thermostat Engine Cooling 6GR-12411-00-00. Overview. On a big offshore motor like the Yamaha XTO, proper cooling system control is critical. The Yamaha 6GR-12411-00-00 Thermostat regulates coolant flow through the engine so it warms up correctly and maintains stable operating temperatures under heavy load. When thermostats begin to wear or stick, engines can run too hot or fail to reach proper operating temperature. Installing a fresh OEM thermostat during cooling system service helps k