Mercury - Serpentine Belt Engine Drive System Rubber Composite 865615Q02. Overview. When you're running a boat that carries the weight and grunt of an 8.1S or 496 Magnum engine, you can feel every part doing its job-especially the serpentine belt. It's one of those pieces you rarely think about when everything's working right, but you absolutely notice when it isn't. That's why having a reliable, factory-matched belt gives you a level of comfort you can't put a price on. Whether you're cruising