1"-8 tap drill size
Use a 7/8" drill (0.8750 in) for a 1"-8 cutting tap at 75% thread.
| Tap type | Drill | Decimal (in) |
|---|---|---|
| Cutting tap, 75% thread | 7/8" | 0.8750 |
| Cutting tap, 50% thread (hard material) | 59/64" | 0.9219 |
| Roll / form tap, 65% thread | 15/16 | 0.9375 |
| Helicoil / STI insert (1"-8×1D) | 1-1/64" | 1.0156 |
- Pitch
- 3.175 mm (8 TPI)
- Major Ø
- 1.0000
- Pitch Ø
- 0.9188
- Minor Ø
- 0.8466
Common questions
- What size drill for a 1"-8 tap?
- Use a 7/8" drill (0.8750 in) for a 1"-8 cutting tap at 75% thread.
- What drill for a 1"-8 roll (form) tap?
- About 0.9448 in; the nearest standard drill is 15/16. Check your tap maker's table.
- What drill for a 1"-8 Helicoil?
- Drill 1-1/64" (1.0156 in), then tap with the 1"-8 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.