Mercury - Quicksilver Thermostat Gasket Cooling System Seal Marine Composite 41812. Overview. When you're working on your MerCruiser's thermostat, this little gasket is the part that makes the whole job count. The Mercury Marine 41812 Quicksilver Thermostat Gasket seals the housing to keep coolant where it belongs - inside the system, not dripping down your engine. Made for MerCruiser V6 and V8 engines, it's cut to exact factory specs and built from a durable marine-grade composite that stands u