Mercury - Plug Cooling & Drain Port Seal 97-8M0074531. Overview. If you've spent any time working on your own outboard, you know this plug plays a bigger role than its size suggests. When the old one starts getting worn, you'll see a slow drip or feel it tighten with that scratchy, "not right" kind of bite. A good plug should thread in smooth and land with a clean, firm stop-that's really all you're looking for. Once it's in, the cooling system stays sealed and you can get back to running the bo