7/8-9 tap drill size
Use a 49/64" drill (0.7656 in) for a 7/8-9 cutting tap at 75% thread.
| Tap type | Drill | Decimal (in) |
|---|---|---|
| Cutting tap, 75% thread | 49/64" | 0.7656 |
| Cutting tap, 50% thread (hard material) | 51/64" | 0.7969 |
| Roll / form tap, 65% thread | 53/64 | 0.8281 |
| Helicoil / STI insert (7/8-9×1D) | 57/64" | 0.8906 |
- Pitch
- 2.8222 mm (9 TPI)
- Major Ø
- 0.8750
- Pitch Ø
- 0.8028
- Minor Ø
- 0.7387
Common questions
- What size drill for a 7/8-9 tap?
- Use a 49/64" drill (0.7656 in) for a 7/8-9 cutting tap at 75% thread.
- What drill for a 7/8-9 roll (form) tap?
- About 0.8259 in; the nearest standard drill is 53/64. Check your tap maker's table.
- What drill for a 7/8-9 Helicoil?
- Drill 57/64" (0.8906 in), then tap with the 7/8-9 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.