Park Tool CC-2 Chain Wear Indicator. With the Park Tool CC-2 Chain Checker you can quickly and accurately determine the wear (or "stretch") of nearly any bike chain. Simply insert the Chain Checker’s pins into two links lightly press the swing arm gauge and then check the gauge window for an accurate reading. The CC-2 is compatible with most single-speed and derailleur chains including Shimano XTR 12-speed chains.
5-to-10 speed chains: replace chain at or just before 0.75%
11-to-12 speed chains*: replace at or just before 0.5%
Single-speed chains: replace at or just before 1%
Check with manufacturer for specific replacement instructions
*Use the CC-4 for SRAM Flattop and SRAM T-Type 12-speed chains