What drill for a 4-40 roll (form) tap?

About 0.1010 in; the nearest standard drill is #38. Check your tap maker's table.

What drill for a 4-40 Helicoil?

Drill #31 (0.1200 in), then tap with the 4-40 STI tap.

4-40 tap drill size

Use a #43 drill (0.0890 in) for a 4-40 cutting tap at 75% thread.

Drills for 4-40 (UNC)
Tap type	Drill	Decimal (in)
Cutting tap, 75% thread	#43	0.0890
Cutting tap, 50% thread (hard material)	#41	0.0960
Roll / form tap, 65% thread	#38	0.1015
Helicoil / STI insert (#4-40×1D)	#31	0.1200

Pitch: 0.635 mm (40 TPI)
Major Ø: 0.1120
Pitch Ø: 0.0958
Minor Ø: 0.0813

Open the tap drill calculator Roll tap chart Helicoil chart

Common questions

What size drill for a 4-40 tap?: Use a #43 drill (0.0890 in) for a 4-40 cutting tap at 75% thread.
What drill for a 4-40 roll (form) tap?: About 0.1010 in; the nearest standard drill is #38. Check your tap maker's table.
What drill for a 4-40 Helicoil?: Drill #31 (0.1200 in), then tap with the 4-40 STI tap.

Other UNC 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.