Yamaha - Thermostat Assembly Engine Cooling System YF1-55317-30-10. Overview. Thermostats are one of those parts you don't think about until the engine starts running too hot or never quite warms up right. I've swapped plenty of them on Yamahas where the issue wasn't obvious until you looked at temperature behavior over time. The Yamaha Thermostat Assembly YF1-55317-30-10 keeps your engine running in that ideal temperature range so everything performs like it should. It's a small component that