To alleviate poor shifting and uneven drive train wear, manufacturers recommend chain replacement before rear cog replacement. Now you can quickly and accurately determine the wear and stretch of any chain with the Park Tool Chain Checker. Simply insert the Chain Checkers 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. For 9 and 10-speed chains, replace chain at or just before the 0.75% reading. For 11 and 12-speed chain, replace at or just before the 0.5% reading. For single-speed chains, replace chain at or just before the 1% reading. Check with manufacturer for specific replacement instructions. Use the CC-4 for SRAM® AXS 12 speed road chains.