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It's a lot of money when you think about it, a small piece of metal devoid of technology or moving parts, but it's sturdy it don't bend and it does it's job, you won't need a degree to work it out, and I guess if you do actually use it and don't relegate it to to the bowls of your cycle toolbox it'll save you a lot of money in replacing parts eye wateringly more expensive than your worn out chain has created because you never cleaned it or checked how it was wearing.
Previous to purchasing this I was checking chain wear with a ruler - 12 complete (inner and outer) links should measure 12 inches pin to pin (at 1/10th inch over the chain is worn and should be replaced). This tool is so much easier to use, and seems accurate - my chain is only very slightly over the 12 ins measurement, and this tool doesn't 'drop in' yet! So I'd say it's a good indicator of chain wear... It does it's job.
This chain wear indicator is made of a good strong material, and it easily tells you when the chain is 0.5% stretched and 0.75% stretched, this is when one or both end "arms" fit into the chain links. The recommended point to change the chain is 0.8% because after this amount of stretch it is more likely to damage the front or rear sprockets, which are considerably more expensive to replace than the chain. I have read that you should be able to make the rear sprockets last for about 4 chain replacements then change both the chain & the rear sprockets.
A simple tool and cheaper are available but being Park Tools you know it will be accurate. Its an easy way of keeping on top of chain wear and saving yourself having to change expensive cassettes and chain rings from premature wear.
Very easy to use and pointed out it was time to change the chain. Shifting was noticeably better after. Not the cheepest for a thin piece of metal but I guess its the precision engineering you pay for as its so minor the eye can't really see.
OK, it's pricey for a simple die-stamped piece of flat metal and there are cheaper alternatives out there but I've always been able to trust the 'Park Tool' name and, as an extremely quick and simple way of determining when I should consider replacing the chain and save on sprocket wear, it does the job brilliantly.