Mercury - Serpentine Belt Engine Drive System Rubber Composite 865615Q08. Overview. There's a certain comfort in hearing your engine hum the way it's supposed to. When you ease the throttle forward and everything responds smoothly-no shudder, no strain-it reminds you why you trust the gear under your hatch. On older Bravo-series V6 and V8 engines, the serpentine belt plays a bigger role than most boaters realize. It ties the whole system together, keeping the alternator steady, the cooling flowi