Mercury - Serpentine Belt Engine Drive System Black Composite 865635Q02. Overview. If you spend enough time on the water, you start to appreciate the parts that quietly carry the load. A good serpentine belt is one of those pieces you almost forget about… until the day it fails. That's why having a belt you can trust feels so reassuring. With this Mercury Marine part number 865635Q02, you get that easy confidence - the kind that lets you focus on the horizon instead of worrying about what's spin