Yamaha Thermostat Assembly (7NF5) YF1-55317-30-12. Overview. The Yamaha YF1-55317-30-12 Thermostat Assembly (7NF5) is a genuine original equipment manufacturer component designed to regulate engine temperature by controlling coolant flow within the cooling system. The thermostat opens and closes based on engine temperature, allowing coolant to circulate when needed to maintain proper operating conditions.. Maintaining a stable engine temperature is critical for efficient performance and long ter