Mercury - Timing Belt 9.9-20hp EFI Series Marine-Grade Polymer 8M0135591. Overview. Anyone who spends time on the water knows that the smallest parts can make the biggest difference. A timing belt isn't something you admire or talk about at the dock, but you definitely feel the confidence it gives when it's doing its job right. With the Mercury Marine part 8M0135591 Timing Belt, you get that quiet reassurance every time the engine fires up. Whether you're easing out from the slip at dawn or push