1/2-20 tap drill size
Use a 29/64" drill (0.4531 in) for a 1/2-20 cutting tap at 75% thread.
| Tap type | Drill | Decimal (in) |
|---|---|---|
| Cutting tap, 75% thread | 29/64" | 0.4531 |
| Cutting tap, 50% thread (hard material) | 15/32" | 0.4688 |
| Roll / form tap, 65% thread | 31/64 | 0.4844 |
| Helicoil / STI insert (1/2-20×1D) | 35/64" | 0.5469 |
- Pitch
- 1.27 mm (20 TPI)
- Major Ø
- 0.5000
- Pitch Ø
- 0.4675
- Minor Ø
- 0.4387
Common questions
- What size drill for a 1/2-20 tap?
- Use a 29/64" drill (0.4531 in) for a 1/2-20 cutting tap at 75% thread.
- What drill for a 1/2-20 roll (form) tap?
- About 0.4779 in; the nearest standard drill is 31/64. Check your tap maker's table.
- What drill for a 1/2-20 Helicoil?
- Drill 35/64" (0.5469 in), then tap with the 1/2-20 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.