Mercury - Exhaust Elbow Gasket (Full Flow) Cooling & Exhaust System Marine Composite 863726. Overview. There are parts you install and forget about, and then there are parts you trust because you know exactly what they do for your engine. A full flow exhaust elbow gasket is one of those unsung heroes. When you are heading out at first light or easing back toward the marina after a long, windy run, you want the exhaust and cooling system working together smoothly. This Mercury Marine part brings