Mercury – Replacement Thermostat | 170 Degree Cooling System | Marine-Grade Steel | 864493Mercury Replacement Thermostat 170 Degree Cooling System Marine Grade Steel 864493 Overview Out on the water, theres something reassuring about an engine that finds its temperature, settles in, and just runs the way its 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