A worn bicycle chain causes poor shifting performance and and 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 when chain replacement is necessary. The tool works on any 5 to 13-speed derailleur chain (including SRAM AXS and Shimano XTR 12-speed chains) and is designed to accurately indicate when a chain reaches 0.5%, 0.75% and 1.0% wear, the points at which most chain manufacturers suggest replacement. The CC-4.2 provides measurements at the two most common wear intervals recommended for chain replacement on multi-speed bicycles: <0.5% Wear - acceptable on all derailleur and single-speed drivetrains. ≥0.5% Wear - 11-to-12-speed chains should be replaced at or before this measurement ≥0.75% Wear - 5-to-10-speed chains should be replaced at or before this measurement ≥1.0% Wear - chains on single-speed and two-sprocket systems should be replaced at or before this measurement
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This is one of Park's few remaining tools that's still made in the US. It's easy to use and said to be more accurate than other chain checkers Park sells because it isolates roller wear from pin wear (it's pin wear you care about when checking for chain stretch). It's solidly made and should easily last for decades.
Really, ... there is only one word worthy of mention here, .... accuracy. In a word, this chain guide is accurate to 1/1000 of an inch. Provided you use it as intended, this guide will accurately tell you if your chain is stretched or not. Five stars.