What Is a Drill Bit for a 1/4 -18 NPT Tap? | eHow
2021-7-3 · For sizes less than 1/4″, fractional measurements are not used. Instead, a numbering system has been created and ranges from 0-12. “0” of course is the smallest with “12” being the size just before you get to 1/4″. Sizes 1/4″ and Larger: For sizes greater than 1/4″, fractional sizing based on the actual diameter of the screw is used. For the sake of our tap drill chart, we’ll see this range go from 1/4″ up to 1″ to correlate over to our tap drill …
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