PARK TOOL CC-3.2 CHAIN CHECKER
A worn chain shifts poorly and wears sprockets at an accelerated rate. The CC-3.2 is a go/no-go gauge designed to accurately indicate when a chain reaches .5% and .75% "stretch," the points at which most chain manufacturers suggest replacement. For 9 and 10-speed chains, replace chain just as the gauge fits the 0.75% side fits flat into the chain. For 11 and 12-speed chain, replace as the 0.5% side fits. The CC-3.2 is long, accurate and features permanent measurement markings.
The CC-3.2 works on most derailleur chains from 5 to 12-speed. Check with manufacturer for specific replacement instructions.