When cutting new threads in a blank hole, the hole size must be appropriate for the tap. This is size is called called the Tap Drill Size. The tap cuts and removes a certain amount of metal, leaving the internal thread. If the hole is too small, the tap will have difficulty removing material, and a great deal of force will be needed to turn the tap. The tap will bind in the hole and begin to gall and pull up material, leaving a poor internal thread. If the hole is too large, the tap will cut with little problem, but the internal thread will not be the correct size for the bolt/screw, and failure during torquing or use is very likely.
