TAP DRILL SIZES AND MINOR DIAMETER CONSTANTS, Continued . . .
It is a well known fact that drills generally produce a larger diameter hole than the drill diameter, depending upon the condition of the drill, accuracy of the sharpening, etc., and also the material which is to be drilled.

When tapping some materials, especially with fine pitch threads, a spin-up occurs at the crest of the thread so that the actual minor diameter of the thread after tapping is smaller than the size of the drilled hole. For this reason, the minor diameter of the tapped hole should be checked to make certain that it is within the limits specified. In other words, it may be necessary to experiment with various drill sizes to find the exact size that is satisfactory.