Max Torque "SS" (Six Shoe) Clutch for Box Stock and Clone EnginesThis is a near bullet-proof centrifugal shoe-type drum clutch set up specifically for an engine running the Box Stock class-#35 Chain-3/4" Bore-Engagement at 3100 RPM's (Black Spring)-Built in Clutch KeyThe Max Torque "SS" Series is the clutch of choice among fun kart manufacturers. Since it was introduced in 1970, it has been the most popular style clutch used by manufacturers because of its versatility and durability. Its spring, made of 302 stainless steel, can take more than twice the heat of music wire commonly used in competitors "look alike" clutches. It can take on this level of heat without ruining the garter spring.