There are various tools available from manufacturers to measure chain wear. One of the most popular among them is the Parktool CC-3.2 model. It is very easy to use. It 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.
HERE is a good article on the subject of chain wear.
And HERE is another article on this.
Below is a video showing how to use it. Actually it shows the older CC-3 model but they are quite similar in the way they are used.
The images below show the older CC-3 model, It has been replaced by the CC-3.2 model which is longer and therefore more accurate. Also the CC-3.2 model is 0.5% and 0.75% instead of the .0.75% and 1.0% of the CC-3 model. That means it will measure the newer 10 speed (and up) chains. As a general rule of thumb, 9 speed chains and below should be replaced at 0.75% elongation, and 10 speed and above chains replaced at 0.5% elongation.
When the chain is relatively new and very little wear the .75 end will not go down inside the link as shown in image A. When the .75 end goes down inside the link as shown in image B it is time to replace the chain. When the 1.0 end goes all the way down as shown in image C it is badly worn and time to replace the chain and sprockets as the chain has probably worn the sprockets.
Please don’t be confused. Again, this is talking about the older CC-3 model which is okay to use if measuring 9 speed (or less) chain. However, if you have 10 speed (or more) chain you need to use the newer CC-3.2 model. With the newer CC-3.2 model if measuring a 10 speed (or higher) chain if the 0.5% drops down into the chain it is worn and should be replaced. When measuring a 9 speed (or less) chain if the 0.75% drops down into the chain the chain is worn and needs to be replaced.
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