The Park Tool Chain Wear Indicator makes it easier than ever to know when to replace your bike chain. This indicator has pre-made markings that indicate 0.5 and 0.75 percent wear. This shows how much your chain has stretched over time. It is recommended to replace a 5-10 speed chain at 0.75 percent wear and an 11-12 speed chain at 0.5 percent. The Park Tool Chain Checker is a great tool to have around to maintain your bikes performance.