What drill for a 1-64 roll (form) tap?

About 0.0661 in; the nearest standard drill is #51. Check your tap maker's table.

What drill for a 1-64 Helicoil?

Drill #43 (0.0890 in), then tap with the 1-64 STI tap.

1-64 tap drill size

Use a #53 drill (0.0595 in) for a 1-64 cutting tap at 75% thread.

Drills for 1-64 (UNC)
Tap type	Drill	Decimal (in)
Cutting tap, 75% thread	#53	0.0595
Cutting tap, 50% thread (hard material)	#52	0.0635
Roll / form tap, 65% thread	#51	0.0670
Helicoil / STI insert (#1-64×1D)	#43	0.0890

Pitch: 0.3969 mm (64 TPI)
Major Ø: 0.0730
Pitch Ø: 0.0629
Minor Ø: 0.0538

Open the tap drill calculator Roll tap chart Helicoil chart

Common questions

What size drill for a 1-64 tap?: Use a #53 drill (0.0595 in) for a 1-64 cutting tap at 75% thread.
What drill for a 1-64 roll (form) tap?: About 0.0661 in; the nearest standard drill is #51. Check your tap maker's table.
What drill for a 1-64 Helicoil?: Drill #43 (0.0890 in), then tap with the 1-64 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.