Park Tool CC-2 Chain Checker
A worn bicycle chain can cause poor shifting and in time can even cause excessive wear to the derailleur and rear cogs, causing a much more expensive problem. That's why bike component manufacturers recommend chain replacement at regular wear intervals. Now you can quickly and accurately determine the wear (or "stretch") of nearly any bike chain with the CC-2 Chain Checker.
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 12-speed chains (not SRAM AXS road chains).