Mercury - Exhaust Elbow Gasket (Closed Cooling) Cooling & Exhaust System Marine Composite 8637251. OverviewWhen you run a MerCruiser with a closed cooling system, every seal matters just a little more. You rely on the engine to keep its temperatures steady whether you're cruising across calm morning water or pushing through a long afternoon chop. An exhaust elbow gasket may seem like a small detail, but when it's the right one, you feel the difference in the way the engine settles into its rhyth