Mercury - Serpentine Belt Engine Drive System Rubber Composite 865615Q01. Overview. There's something calming about knowing the heart of your engine is turning exactly the way it should. When you're running a big block like the MerCruiser 496 Magnum, every accessory driven by that serpentine belt plays a part in keeping the day smooth-whether you're easing away from the dock at sunrise or trimming up for a long run across open water. A dependable belt isn't flashy, but you feel its importance ev