Old 05-20-10, 05:40 AM
gone ride'n
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Not sure whats going on here but this is what I would do. Measure the tool with the ruler and see what you get.
I suspect the tool is correct and there is too much variability in your use of the ruler. Since both the tool and the ruler measure cumulative stretch across multiple links you will see both link stretch and roller wear.

Three is a process called a gauge r&r used to determine the variability in a measurement system but baring the full use of statistical methods I would do as I described above.
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