Mercury - Quicksilver Water Shutter For Select MerCruiser Sterndrive Engines 807166A3. Overview. When the water gets rough, your engine needs protection from the backflow that can sneak up the exhaust system. The Mercury Marine 807166A3 Quicksilver Water Shutter-sometimes called a water flapper-does exactly that. Designed for various MerCruiser sterndrive engines, this simple but vital part closes automatically when the engine is shut off or idling to stop water from washing back into the cylind