What drill for a 2-56 roll (form) tap?

About 0.0781 in; the nearest standard drill is 5/64. Check your tap maker's table.

What drill for a 2-56 Helicoil?

Drill #41 (0.0960 in), then tap with the 2-56 STI tap.

2-56 tap drill size

Use a #50 drill (0.0700 in) for a 2-56 cutting tap at 75% thread.

Drills for 2-56 (UNC)
Tap type	Drill	Decimal (in)
Cutting tap, 75% thread	#50	0.0700
Cutting tap, 50% thread (hard material)	#49	0.0730
Roll / form tap, 65% thread	5/64	0.0781
Helicoil / STI insert (#2-56×1D)	#41	0.0960

Pitch: 0.4536 mm (56 TPI)
Major Ø: 0.0860
Pitch Ø: 0.0744
Minor Ø: 0.0641

Open the tap drill calculator Roll tap chart Helicoil chart

Common questions

What size drill for a 2-56 tap?: Use a #50 drill (0.0700 in) for a 2-56 cutting tap at 75% thread.
What drill for a 2-56 roll (form) tap?: About 0.0781 in; the nearest standard drill is 5/64. Check your tap maker's table.
What drill for a 2-56 Helicoil?: Drill #41 (0.0960 in), then tap with the 2-56 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.