3/8-16 tap drill size
Use a 5/16" drill (0.3125 in) for a 3/8-16 cutting tap at 75% thread.
| Tap type | Drill | Decimal (in) |
|---|---|---|
| Cutting tap, 75% thread | 5/16" | 0.3125 |
| Cutting tap, 50% thread (hard material) | Q | 0.3320 |
| Roll / form tap, 65% thread | S | 0.3480 |
| Helicoil / STI insert (3/8-16×1D) | 25/64" | 0.3906 |
- Pitch
- 1.5875 mm (16 TPI)
- Major Ø
- 0.3750
- Pitch Ø
- 0.3344
- Minor Ø
- 0.2983
Common questions
- What size drill for a 3/8-16 tap?
- Use a 5/16" drill (0.3125 in) for a 3/8-16 cutting tap at 75% thread.
- What drill for a 3/8-16 roll (form) tap?
- About 0.3474 in; the nearest standard drill is S. Check your tap maker's table.
- What drill for a 3/8-16 Helicoil?
- Drill 25/64" (0.3906 in), then tap with the 3/8-16 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.