Mercury - Plug for Flush Valves on FourStroke Outboards Engine Cooling & Maintenance Black Polymer Finish 8M0215105. Overview. Every boater knows that peace of mind comes from the little things - the parts you barely notice because they just work. That's exactly how the Mercury Plug for Flush Valves feels once it's in place. You push off from the dock knowing your cooling system is sealed the way it should be, and that quiet confidence settles in before you even throttle up. It's a small compone