Chain Wear Indicator Park Tool CC-4.2Chain Checker Park Tool CC 4. 2 A worn bicycle chain causes poor shifting performance and can actually wear out drivetrain components, causing more expensive problems. The Park Tool CC 4. 2 Chain Checker is a drop in style gauge that measures bicycle chain wear. The laser cut CC 4. 2 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 determining