Park Tool CC-3.2 Chain Wear Indicator
A worn chain shifts poorly and wears sprockets at an accelerated rate. The CC-3.2 is a “go-no go” gauge designed to accurately indicate when a chain reaches .5% and .75%, the points at which most chain manufacturers suggest replacement. The updated CC-3.2 is longer, more accurate and features permanent measurement markings.
This tool is really handy for indicating when it is time to change your chain especially if you have more than one bike and don't keep a check on the mileage. For such a low price it can save you the added expense of replacing cog sets where the chain was not replaced soon enough.