chain (must be compatible with your bicycle for example 9-speed for a 9-speed cassette).Park Tool Chain Checker Tool or a ruler.
If the distance measures 12 1/8 inches or more, your chain is worn out and should be replaced. On a new chain you'll be able to measure exactly 12 inches between 2 chain pins. You can purchase the Park Tool Chain Checker and use it, or you can measure the chain for wear. For example, as long as you carry a chain tool, it's a fairly simple matter to repair something as serious as a broken chain, which would otherwise leave you stranded. The techniques used to replace a chain will also help you repair one should you or a riding companion have a problem on the road or trail. Bicycles with derailleurs use a chain that normally cannot be removed unless you separate one link with a chain tool (photo, left), a special bicycle tool that lets you push a pin out of the chain (more about this procedure later). Here we explain how to replace a chain on a derailleur-equipped bicycle so that you can install a new chain when yours wears out.
