Park Tool CC-4.2Chain Wear IndicatorPARK TOOL CC 4 DROP IN CHAIN CHECKER The CC 4 is a drop in style gauge that measures bicycle chain wear. The laser cut CC 4 accurately measures a long section of chain and strategically comes in contact with the chain in three places, quickly and easily taking the guesswork out of knowing when chain replacement is necessary. A worn chain will cause poor shifting as well as accelerate drivetrain wear. The tool works on any 5 to 12 speed derailleur