What drill for a 8-32 roll (form) tap?

About 0.1502 in; the nearest standard drill is #25. Check your tap maker's table.

What drill for a 8-32 Helicoil?

Drill #18 (0.1695 in), then tap with the 8-32 STI tap.

8-32 tap drill size

Use a #29 drill (0.1360 in) for a 8-32 cutting tap at 75% thread.

Drills for 8-32 (UNC)
Tap type	Drill	Decimal (in)
Cutting tap, 75% thread	#29	0.1360
Cutting tap, 50% thread (hard material)	#27	0.1440
Roll / form tap, 65% thread	#25	0.1495
Helicoil / STI insert (#8-32×1D)	#18	0.1695

Pitch: 0.7938 mm (32 TPI)
Major Ø: 0.1640
Pitch Ø: 0.1437
Minor Ø: 0.1257

Open the tap drill calculator Roll tap chart Helicoil chart

Common questions

What size drill for a 8-32 tap?: Use a #29 drill (0.1360 in) for a 8-32 cutting tap at 75% thread.
What drill for a 8-32 roll (form) tap?: About 0.1502 in; the nearest standard drill is #25. Check your tap maker's table.
What drill for a 8-32 Helicoil?: Drill #18 (0.1695 in), then tap with the 8-32 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.