Zodiac Rebuild Kit for Swimming Pool New Polaris R0543200 3900 Tune Up Kit Chain guard: Acts as a guide to ensure the drive chain is always aligned on the drive pulley Redesigned tensioner pulley & spring: Keeps chain engaged with wheel sprockets to maximize torque transfer Gear box upgrade: Bearing shield incorporated for superior cleaning, durability, extended lifespan, and improved longevity 8 point mechanical upgrade: Provides strength, durability, and ad hassle free experience.