What drill for a 4-48 roll (form) tap?

About 0.1028 in; the nearest standard drill is #37. Check your tap maker's table.

What drill for a 4-48 Helicoil?

Drill #27 (0.1440 in), then tap with the 4-48 STI tap.

4-48 tap drill size

Use a #42 drill (0.0935 in) for a 4-48 cutting tap at 75% thread.

Drills for 4-48 (UNF)
Tap type	Drill	Decimal (in)
Cutting tap, 75% thread	#42	0.0935
Cutting tap, 50% thread (hard material)	#40	0.0980
Roll / form tap, 65% thread	#37	0.1040
Helicoil / STI insert (#4-48×1D)	#27	0.1440

Pitch: 0.5292 mm (48 TPI)
Major Ø: 0.1120
Pitch Ø: 0.0985
Minor Ø: 0.0864

Open the tap drill calculator Roll tap chart Helicoil chart

Common questions

What size drill for a 4-48 tap?: Use a #42 drill (0.0935 in) for a 4-48 cutting tap at 75% thread.
What drill for a 4-48 roll (form) tap?: About 0.1028 in; the nearest standard drill is #37. Check your tap maker's table.
What drill for a 4-48 Helicoil?: Drill #27 (0.1440 in), then tap with the 4-48 STI tap.

Other UNF sizes

Drill sizes from Machinery's Handbook and ASME B1.1/B1.13M; roll tap sizes from the form-tap formula. Tap and insert makers' tables are authoritative.