Unified inch · UNF

#4-48 UNF 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

3/32

0.0938" · 67% thread

Form tap

#38

0.1015" · 74% thread

Or 2.6 mm for 68%

Open #4-48 UNF 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: 48 (pitch 0.0208")
Pitch Ø (basic): 0.0985" (2.501 mm)
Minor Ø (internal, basic): 0.0894" (2.272 mm)
Thread depth (internal): 0.0113" (0.286 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 #4-48 UNF
Drill	Ø in	Ø mm	Thread
#43	0.0890	2.26	85%
2.3 mm	0.0906	2.30	79%
#42suggested	0.0935	2.37	68%
3/32suggested	0.0938	2.38	67%
2.4 mmsuggested	0.0945	2.40	65%
#41	0.0960	2.44	59%
#40	0.0980	2.49	52%
2.5 mm	0.0984	2.50	50%

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 #4-48 UNF
Drill	Ø in	Ø mm	Thread
#38	0.1015	2.58	74%
2.6 mmsuggested	0.1024	2.60	68%
#37	0.1040	2.64	56%

3-wire measurement

Best wire size: 0.0120" (0.306 mm)
Over wires, at basic pitch Ø: 0.1165" (2.959 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