Mercury - Thermostat Gasket FourStroke / SeaPro / Verado / Racing Series Heat-Resistant O-Ring 887978. Overview. There's a certain comfort in knowing your engine is running at the exact temperature it should - not too hot, not too cool, just right in that sweet spot where everything feels smooth and dependable. Anyone who spends long mornings trolling or full afternoons cruising knows how quickly peace of mind disappears when something in the cooling system starts acting up. The Mercury Marine p