5- Threads, Taps, and Tapping - Tap Drill Sizes
For a thread to be formed by tapping, the hole must have sufficient material left over after drilling to form at least 75% of a full thread. The remaining 25% is formed by extrusion of the material inwards during the tapping process. This phenomenon can be checked by attempting to place the pilot drill back into the hole after tapping. It wont fit!

Lab tests have shown that 50 to 65% of a full thread holds as much as 75 to 100% of the load.

To determine a tap drill size, a formula or a table is used. Tables derived from mathematical calculations using the 75% factor are readily available and are widely used. Tap drill size other than 75 percent are also published. An example is printed to the right of this text.

Please note that blueprints will often specify a countersink or a bevel at the top of the hole to guide the tap and eliminate the possibility of getting a thread burr.