What drill for a 6-40 roll (form) tap?

About 0.1270 in; the nearest standard drill is #30. Check your tap maker's table.

What drill for a 6-40 Helicoil?

Drill #19 (0.1660 in), then tap with the 6-40 STI tap.

6-40 tap drill size

Use a #33 drill (0.1130 in) for a 6-40 cutting tap at 75% thread.

Drills for 6-40 (UNF)
Tap type	Drill	Decimal (in)
Cutting tap, 75% thread	#33	0.1130
Cutting tap, 50% thread (hard material)	#31	0.1200
Roll / form tap, 65% thread	#30	0.1285
Helicoil / STI insert (#6-40×1D)	#19	0.1660

Pitch: 0.635 mm (40 TPI)
Major Ø: 0.1380
Pitch Ø: 0.1218
Minor Ø: 0.1073

Open the tap drill calculator Roll tap chart Helicoil chart

Common questions

What size drill for a 6-40 tap?: Use a #33 drill (0.1130 in) for a 6-40 cutting tap at 75% thread.
What drill for a 6-40 roll (form) tap?: About 0.1270 in; the nearest standard drill is #30. Check your tap maker's table.
What drill for a 6-40 Helicoil?: Drill #19 (0.1660 in), then tap with the 6-40 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.