4-40 Tap Drill Size and Drill Bit Guide

Use a #43 drill bit

4-40 · 0.0890 in / 2.26 mm · ~75% thread

4-40 is the smallest common UNC machine screw thread, used in electronics, hobby RC and small enclosure work. A #43 bit is the standard tap drill for a 75% thread.

At a glance

Tap drill	#43
Inches	0.0890 in
Millimetres	2.26 mm
Major diameter	0.1120 in
Pitch	0.0250 in
Threads per inch	40

Why this size

4-40 tap drill: drillØ = major − (pitch × 0.75) = 0.1120 − (0.0250 × 0.75) = 0.0933 in → nearest standard bit = #43 (0.0890 in)

At this size a sharp bit and light tapping pressure matter — small taps break easily. The 4-40 fine variant is 4-48, which uses a #42 bit (0.0935 in).