Mercury - Serpentine Belt Engine Drive System Rubber Composite 865615Q10. Overview. There's a familiar comfort that comes when your inboard engine settles into that smooth, steady rhythm you've come to rely on. When the belt is right, everything feels in sync-your alternator hums along, your cooling system stays consistent, and your engine responds without hesitation. On MerCruiser MIE engines, especially those with closed cooling, a dependable serpentine belt isn't just helpful… it's essential.