Mercury - Thermostat Yamaha 4-Stroke Cooling System Marine-Grade Construction 8M6010297. Overview. Any boater who's spent long days offshore knows there's a certain comfort in hearing a big four-stroke come to life and settle into a smooth, confident idle. That steady heartbeat tells you everything is working the way it should. The Quicksilver 8M6010297 Thermostat helps keep Yamaha F250 outboards in that sweet spot-warming quickly, maintaining the right operating temperature, and delivering reli