Mercury - Cylinder Bore Seal High-Performance Outboards Precision Composite 884391. Overview. There's a certain comfort that settles in when your engine fires up without a hiccup - that deep, steady confidence only boaters understand. When you're pushing a powerful outboard hard, whether you're chasing early-morning redfish or running upriver with the family, you want every internal seal pulling its weight. The Mercury Marine part number 884391 Cylinder Bore Seal feels like one of those quiet he