Mercury - Seal Flush Plug Cooling & Maintenance Components Marine Rubber 8M0219168. Overview. Every boater knows that small parts can make a big difference, especially when it comes to keeping your engine's cooling system protected. The Mercury Seal Flush Plug 8M0219168 is one of those quiet essentials you don't think much about-until the day you really need it. It's a simple piece, sure, but it's also the kind of part that gives you peace of mind when you're rinsing the engine after a long day