Mercury - Quicksilver 8M0204651 Thermostat Housing O-Ring MerCruiser V6 & V8 Series Rubber Seal 8M0204651. Overview. There's a certain comfort that comes with knowing your cooling system is sealed up the way it should be. You feel it on those long runs when the engine hums at a steady temperature, never wavering, never giving you a reason to second-guess what's happening under the cover. This Mercury Marine part plays a quiet but essential role in that peace of mind. It's the kind of simple comp