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^ When I calibrate a tach, it is within about a 1% error. Removing the needle is a requirement to get it right.
I've found many cracked needle shafts, the plastic cracks over time from the tight fit on the shaft. You should glue the split first.
And, Turbo tachs and N/A tachs require different gain resistors. I don't use a pot, I put two resistors in parallel when necessary to get the value as close as possible. This is actually more stable in the long run, but it takes a lot more time to get it right.
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