Mercury - Cylinder Head Gasket Yamaha 4.2L Series Marine-Grade Composite 8M6010321. Overview. There's a certain confidence that settles in when your engine fires up clean and strong-the kind of confidence you only get when every critical seal in your motor is doing its job. On a big 4.2-liter Yamaha outboard, nothing is more important than keeping combustion where it belongs. That's why boaters who run hard and expect reliability turn to a gasket they can trust. The Quicksilver 8M6010321 Cylinde