The Park Tool CC-2 Chain Checker is used to measure the chain stretch. It works on 7-Speed to 12-Speed chains. Mine is a 9-Speed chain.
The CC-2 measures 5.35" long, 0.40" wide (body) and 0.40 tall (body). The mass is 42g.
The chain wear (stretch) is important to measure to ensure that a worn chain does not degrade the cogs and chainrings. The measurement permits determination of the chain condition so that the replacement timing is appropriate.
The CC-2 has a set of pins and a moveable plate w/ a measurement scale. An instruction sheet is included.
Making the measurement is easy. Set the scale on 0%, insert the pins into the chain links, lightly push the arc plate until the pins are just snug, then read the measurement scale.
The included instructions state that 5,6,7,8 and 9-Speed chains should be replaced when the measurement is 0.75%. The 10, 11 and 12-Speed chains should be replaced when the measurement is 0.50%.
When I made the measurement, I performed this at multiple locations along the length. There was a small variability depending upon the location.
I would not hesitate to recommend this tool.
