Park's chain tools are suspect. They have two glaring faults:
1) Tool measures over a very small section of chain and thus amplifies the measuring error.
2) The tool uses the rollers as a point of reference. The rollers' resting position changes with clean and dirty chains. Given the above error, this then exaggerates the chain's wear.
Further, they don't really state what the 1% is a percentage of.
Use a standard inch ruler (try and fine one that goes slightly over 12 inches, ironically like Park's spoke rule that measures an inch or so more than 12 inches. Now file a notch at the zero point (get your notch lined up better than in this photo where I need to do a bit more filing) like this:
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And hold this point at a reference point (doesn't matter what point you use, as long as you use the same on the other side) on the slack run of your chain on the bike like here.
Now move your eye to the other side, keeping the reference point exactly in place. This is tricky because it doesn't hook or click in place. On a new chain, the 12 inch mark at the other end of the ruler will line up exactly with the chain. I don't have a photo for that but the next photo will explain what I mean.
On a worn chain that has started to elongate , the 12 link point is now growing past the 12 inch point like this:
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Here you can see that the reference point is between 12 inches and 12" and 1/16th of an inch. We are just lucky that this little 1 /116th inch mark over 12 inches is as near as can be to 0.5% of 12 inches. In this photo the chain is thus on its way to replacement but not quite there. As soon as it gets to 12 and 1/16th , it has elongated by 0.5% and has to be replaced. Once it has elongated to 12 1/8th, it has elongated by 1% and has ruined the cassette. We know a new chain will not mesh with the cassette and the cassette has to be replaced.
The distance between 12 1/16th and 12 1/8th is no man's land and you may be lucky, but you HAVE to test the new chain thoroughly with the old cassette to see if it doesn't skate on the rider's favourite gears.
It is ironic that Park has such a crapy chain tool but their ruler is ideally suited for the job because it has a few lines after 12 inches. You may come across other inch rulers with the same feature. I also have a carpenter's square with the same feature.
