7/8-20 tap drill size
Use a 53/64 drill (0.8281 in) for a 7/8-20 cutting tap at 75% thread.
| Tap type | Drill | Decimal (in) |
|---|---|---|
| Cutting tap, 75% thread | 53/64 | 0.8281 |
| Cutting tap, 50% thread (hard material) | 27/32 | 0.8438 |
| Roll / form tap, 65% thread | 55/64 | 0.8594 |
- Pitch
- 1.27 mm (20 TPI)
- Major Ø
- 0.8750
- Pitch Ø
- 0.8425
- Minor Ø
- 0.8209
Common questions
- What size drill for a 7/8-20 tap?
- Use a 53/64 drill (0.8281 in) for a 7/8-20 cutting tap at 75% thread.
- What drill for a 7/8-20 roll (form) tap?
- About 0.8529 in; the nearest standard drill is 55/64. Check your tap maker's table.
Other UNEF 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.
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