Mercury - Replacement Thermostat 170 Degree Cooling System Marine-Grade Steel 864493. Overview. Out on the water, there's something reassuring about an engine that finds its temperature, settles in, and just runs the way it's supposed to. You can feel it in the throttle, hear it in the steady hum behind you, and trust it when the day takes you farther from shore than planned. A dependable thermostat plays a bigger role in that confidence than most folks realize. The Mercury Marine 864493 Thermos