Unified inch · UNC

#5-40 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

#37

0.1040" · 65% thread

Form tap

#33

0.1130" · 71% thread

Open #5-40 UNC in the calculator

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

Thread dimensions

Major Ø: 0.1250" (3.175 mm)
Threads per inch: 40 (pitch 0.0250")
Pitch Ø (basic): 0.1088" (2.763 mm)
Minor Ø (internal, basic): 0.0979" (2.488 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 #5-40 UNC
Drill	Ø in	Ø mm	Thread
#40	0.0980	2.49	83%
2.5 mm	0.0984	2.50	82%
#39	0.0995	2.53	79%
#38	0.1015	2.58	72%
2.6 mmsuggested	0.1024	2.60	70%
#37suggested	0.1040	2.64	65%
2.7 mm	0.1063	2.70	58%
#36	0.1065	2.71	57%

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 #5-40 UNC
Drill	Ø in	Ø mm	Thread
#34	0.1110	2.82	82%
#33	0.1130	2.87	71%
2.9 mm	0.1142	2.90	64%
#32	0.1160	2.95	53%

3-wire measurement

Best wire size: 0.0144" (0.367 mm)
Over wires, at basic pitch Ø: 0.1304" (3.312 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