Definition
A drill bit used to make a hole of the correct diameter to be threaded by a tap. The tap drill size is smaller than the finished thread diameter, leaving enough material for the tap to cut threads into the wall of the hole.
Plain English
A drill bit sized to make the starter hole that a thread-cutting tool will then turn into a threaded hole. The hole has to be slightly smaller than the final screw size so there is metal left for the threads to be cut into.
Context Anchor
Seen in aircraft maintenance when a mechanic is making, repairing, or checking a threaded hole for a screw or bolt.
Derivation
A 'tap' is the cutting tool that forms internal threads inside a hole. A 'tap drill' is simply the drill that prepares the hole for that tap. Naming the drill after the tool it serves makes the relationship clear: drill first, then tap.
Why Pilots Care
Correct hole size ensures strong, reliable threads that hold critical fasteners under flight loads and vibration.
Intuition Check
Do not read “tap drill” as a drill used for tapping lightly. Here, “tap” is the tool that cuts inside threads, and the tap drill makes the hole ready for that tool.
Example Sentence 1
Before threading the bracket for the new antenna mount, the mechanic looked up the correct tap drill size for a 10-32 screw.
Example Sentence 2
Using the wrong tap drill size caused the tap to bind and damaged the threads in the aluminum spar.