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Why do chain wear indicators have an indication for 0.5% wear and one for 0.75% wear? Are both measurements included because there is some personal preference from mechanic to mechanic, or rather are there separate uses for each?

I find myself asking this because right now I have a chain that measures at least .5% but less than .75% and I want to replace it before it negatively impacts the cassette teeth. For what it's worth, it's an 11 speed chain.

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Its up to personal preference at the end of the day, but a general recommendation is to replace at (or before) the 0.5% point for 11 speed and 0.75% for 10 speed or lower. You can also see if theres something in the chain manual or chain wear gauge's manual.

The multiple indicators are there because some people will want to change at 0.5%, others at 0.75% and so on. For a more expensive drivetrain, it makes sense to change earlier given the insane costs associated with all the other parts (whereas with a cheap drivetrain, you can run longer).

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Every new 9 spd chain I have bought came out of the box with 0.5% wear according to the Park CC2 chain checker. I replace my chains when they start to show any wear (i.e. 0.75 %), because chains are cheap and the rest of the drive train is not.

You have to be aware that just because a measuring tool has different measurements, it does not mean the tool has that accuracy or that repeatability of measurements.

Every measuring tool takes some practice and skill to get accurate measurements.

There's also the fact of variation in the manufacturing process. We are talking about very small measurements. Given all the potential sources of error, I think it's a mistake to focus too much on any specific number.

In the case of chains, what you really care about is the difference between new and worn. Whatever tool you are using, measure the chain when it's new and replace it when it starts to show signs of wear. This is why I use the CC2 model, because it's easy to see the difference between new and worn long before it's worn enough to affect the cassette.

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