Dirt Tricks chain wear gauge is a very simple tool to determine if you chain has reached its wear limit and should be replaced. Dirt Tricks is asked all the time "How do I know when my chain is worn out?" The best way to check this is to measure it! Regina Chain's replacement wear spec for 520 chain is 10.099" between 16 pins - when you hit 10.099" of length between 16 pins, its time for a new chain. Most people do not own 12" calipers to take this measurement, so this tool is a great way to check. How is works: Put your bike in gear and roll it backwards so the top chain gets tight. Hook a roller with the hook end of the tool and drop the indicator tip down into the chain. If the closest roller touches the backside of the tool, its time for a new chain.