I quickly calculate proper tap-drill diameters using this simple formula. Tap-drill diameter equals nominal diameter minus thread pitch. For example, drill size for a 3/8-16 thread is 0.375 0.0625 (pitch) = 0.312 in. in diameter, and for a 10-32 thread, the drill diameter is 0.190 0.032 = 0.159 in. (a #21 drill). This formula works for all 60-degree threads, including metric ones. In addition, I recommend boring to tap-drill diameter for large fine threads.

Alan Ramsay
Rochester, N.Y.
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