Mercury - Serpentine Belt Engine Drive System Rubber Composite 865615Q09. Overview. Every boater knows that the simplest parts often make the biggest difference. A serpentine belt isn't flashy, but when it's doing its job, you feel it in every smooth shift of the throttle and each steady hum from your alternator. Engines from this era-especially early Alpha-series V6 and V8 models-tend to tell you right away when something is off. That's why using a belt you trust matters. With this Mercury Mari