What drill for a 5-40 roll (form) tap?

About 0.1140 in; the nearest standard drill is #33. Check your tap maker's table.

What drill for a 5-40 Helicoil?

Drill #29 (0.1360 in), then tap with the 5-40 STI tap.

5-40 tap drill size

Use a #38 drill (0.1015 in) for a 5-40 cutting tap at 75% thread.

Drills for 5-40 (UNC)
Tap type	Drill	Decimal (in)
Cutting tap, 75% thread	#38	0.1015
Cutting tap, 50% thread (hard material)	#36	0.1065
Roll / form tap, 65% thread	#33	0.1130
Helicoil / STI insert (#5-40×1D)	#29	0.1360

Pitch: 0.635 mm (40 TPI)
Major Ø: 0.1250
Pitch Ø: 0.1088
Minor Ø: 0.0943

Open the tap drill calculator Roll tap chart Helicoil chart

Common questions

What size drill for a 5-40 tap?: Use a #38 drill (0.1015 in) for a 5-40 cutting tap at 75% thread.
What drill for a 5-40 roll (form) tap?: About 0.1140 in; the nearest standard drill is #33. Check your tap maker's table.
What drill for a 5-40 Helicoil?: Drill #29 (0.1360 in), then tap with the 5-40 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.