Mercury - Serpentine Belt Idler Pulley MerCruiser Engine Pulley System Thermal Resin Construction 8M6500024. Overview. Every seasoned boater knows that reliability isn't built on the big, flashy components - it's the quiet parts, the ones that simply do their job day after day, that make all the difference. The serpentine belt idler pulley is one of those unsung heroes under the cowling. When it's right, the engine feels smooth, steady, and confident. When it's worn, you can hear the strain. The