What drill for a 3/8-24 roll (form) tap?

About 0.3566 in; the nearest standard drill is T. Check your tap maker's table.

What drill for a 3/8-24 Helicoil?

Drill 27/64" (0.4219 in), then tap with the 3/8-24 STI tap.

3/8-24 tap drill size

Use a Q drill (0.3320 in) for a 3/8-24 cutting tap at 75% thread.

Drills for 3/8-24 (UNF)
Tap type	Drill	Decimal (in)
Cutting tap, 75% thread	Q	0.3320
Cutting tap, 50% thread (hard material)	S	0.3480
Roll / form tap, 65% thread	T	0.3580
Helicoil / STI insert (3/8-24×1D)	27/64"	0.4219

Pitch: 1.0583 mm (24 TPI)
Major Ø: 0.3750
Pitch Ø: 0.3479
Minor Ø: 0.3239

Open the tap drill calculator Roll tap chart Helicoil chart

Common questions

What size drill for a 3/8-24 tap?: Use a Q drill (0.3320 in) for a 3/8-24 cutting tap at 75% thread.
What drill for a 3/8-24 roll (form) tap?: About 0.3566 in; the nearest standard drill is T. Check your tap maker's table.
What drill for a 3/8-24 Helicoil?: Drill 27/64" (0.4219 in), then tap with the 3/8-24 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.