Mercury – 140° Thermostat | Cooling System Series | Stainless Steel Construction | 8M0162139Mercury 140 Thermostat Cooling System Series Stainless Steel Construction 8M0162139 Overview If youve run big Mercury outboards long enough, you start to appreciate the simple parts that quietly keep everything in balance. The thermostat is one of those unsung heroes. It doesnt roar, it doesnt shine, and youll probably never brag about it at the dock but you feel its importance every time the engine settles into that smooth, steady rhythm you trust.