Mercury - Cylinder Block Exhaust Plate Gasket 2-Stroke Performance Series Composite Gasket Material 991764. Overview. There's a certain kind of confidence you feel when your outboard fires up clean and strong, the way it's supposed to. It comes from knowing every seal, every connection, every piece of your engine is working together the way the engineers intended. A good gasket may not be the most glamorous part on the motor, but when you're running twenty miles upriver or trimming down to punch