Mercury - Quicksilver Thermostat Retaining Sleeve Cooling System Component Brass 47508T. Overview. If your MerCruiser runs hot or the thermostat doesn't seem to sit right, this little piece could be the missing link. The Mercury Marine 47508T Quicksilver Brass Retaining Sleeve holds the thermostat firmly in place inside the housing, keeping the cooling system sealed and working smoothly. It's made from solid marine brass and built to handle years of use, heat, and vibration. Designed for MerCrui