Unified inch · UNC

#4-40 UNC tap drill size

For general work, OSG suggests 65–70% of full thread. Harder materials and deep holes want less; the calculator sets it from your material.

Cut tap

#43

0.0890" · 71% thread

Or 2.3 mm for 66%

Form tap

#38

0.1015" · 62% thread

Open #4-40 UNC in the calculator

Pick your material for its % of thread, tapping RPM and the rigid-tap code.

Thread dimensions

Major Ø: 0.1120" (2.845 mm)
Threads per inch: 40 (pitch 0.0250")
Pitch Ø (basic): 0.0958" (2.432 mm)
Minor Ø (internal, basic): 0.0849" (2.157 mm)
Thread depth (internal): 0.0135" (0.344 mm)

Basic sizes of the 60° form. Tolerance classes (2B/3B) set the limits around them.

Cut tap drills

For a cutting (fluted or spiral) tap. Every standard drill from 50% to 85% thread; suggested rows are in the 65–70% band.

Cut tap drills for #4-40 UNC
Drill	Ø in	Ø mm	Thread
#44	0.0860	2.18	80%
2.2 mm	0.0866	2.20	78%
#43	0.0890	2.26	71%
2.3 mmsuggested	0.0906	2.30	66%
#42	0.0935	2.37	57%
3/32	0.0938	2.38	56%
2.4 mm	0.0945	2.40	54%

Form tap drills

A form (roll) tap displaces metal instead of cutting it, so it needs a bigger hole. Every standard drill from 50% to 85% thread; suggested rows are in the 65–70% band.

Form tap drills for #4-40 UNC
Drill	Ø in	Ø mm	Thread
#40	0.0980	2.49	82%
2.5 mm	0.0984	2.50	80%
#39	0.0995	2.53	74%
#38	0.1015	2.58	62%
2.6 mm	0.1024	2.60	57%

3-wire measurement

Best wire size: 0.0144" (0.367 mm)
Over wires, at basic pitch Ø: 0.1174" (2.982 mm)

For the external thread (the screw). The best wire touches the flanks at the pitch diameter.

Sources

ASME B1.1 — Unified Inch Screw Threads (UN and UNR thread form)
OSG — Tap drill size and pitch diameter limit chart (formula, suggested % of full thread)
Cutting Tool Engineering — "Threads will roll": form tap hole size = D − 0.0068 × % / TPI
ASME B94.11M — Twist drills (fractional, number, letter and metric sizes)
NIST (NBS) Letter Circular 23 and Pratt & Whitney — three-wire thread measurement, best wire size