Shimano Alfine CT-S500 Chain Tensioner.CT-S500 Alfine Chain TensionerDo not use with a coaster brake or fixed gear20 tooth max rear sprocket16 tooth chainwrap capacity41.8mm chainlineUses washers on on the mounting bolt for 5mm of chainline adjustabilityItem SpecificationsColorBlackUnit of SaleEachFitsIntended UseSingle SpeedChain Compatibility3/32"