Chain wear indicator - CC-4.2 - from Park Tool USA
Some people forget that a chain is a part of the bicycle which needs to be replaced sometimes due to wear and tear and that a worn chain shifts poorly and wears sprockets / cogs at an accelerated rate.
The CC-4.2 is a "go, no go" gauge designed to accurately indicate when a chain reaches 0.5% and 0.75% wear (NOT 1.0 as pictured), the points at which most chain manufacturers suggest replacement.
The CC-4.2 works on any 5 to 12-speed derailleur chain and flat top chains.
Made from precision, laser cut steel.
Park Tool Reference: CC-4.2 (replaces CC-4)