Mercury - Serpentine Belt Engine Drive System Rubber Composite 865635Q01. Overview. There is a certain confidence that comes from hearing a big MerCruiser engine settle into its familiar rhythm. When the belt is right, everything feels balanced-the alternator hums, the cooling system flows smoothly, and the engine responds with that steady, reassuring pull you count on. Boats powered by the 496 MAG are built for moments like that: strong acceleration, long ranges, and effortless cruising. This M