Park Tool CC-4.2 Chain Checker Tool Description: The Park Tool Chain Checker is one of the easiest, yet most effective tools for keeping your bike running smoothly. It simply measures the stretch of your chain to take the guesswork out of determining when chain replacement is necessary. Catching a chain before it's completely worn out can save you time, money, and the headache of replacing the pricier cassette and chainring components. Features: Three points of contact with the chain produce the most accurate reading possible Quick and easy to use, a must-have for any cycling enthusiast Indicates when a chain reaches .5%, .75%, and 1.0% wear points Compatible with chains 5 - 13 speed including SRAM T-Type, SRAM Flattop, Shimano 12-speed, and Campagnolo Ekar 13-speed chains Also compatible with single-speed chains General replacement guidelines: 11-to-13 speed chains: Replace at or just before 0.5% 5-to-10 speed chains: Replace at or just before 0.75% Single speed chains: Replace at or just before 1.0% Note: Many drivetrain manufacturers have unique recommendations specific to their components. Always double-check to be sure.