Park Tool CC-4.2 Chain Checker Tool. A worn bicycle chain can cause poor shifting and expensive drivetrain damage. The Park Tool CC-4.2 Chain Checker is a drop-in gauge that measures chain wear accurately. It contacts the chain in three places eliminating guesswork. Compatible with 5 to 13-speed and single-speed chains it indicates wear at 0.5% 0.75% and 1.0% matching manufacturer replacement recommendations.