Park Tool CC-2 Chain Checker
To alleviate poor shifting and uneven chain wear, most manufacturers recommend chain replacement at or before 1-percent stretch.
Now you can quickly and accurately determine the wear and stretch of any chain with the Park Tool Chain Checker. Simply insert the Chain Checker's pins into two links, press the swing arm gauge tight, and then check the gauge window for an accurate reading. Reads wear from 0.25% to 1%