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
7/8-9 tapping speeds and feeds
Feed = pitch (0.1111 in per rev). Starting points for a general-purpose HSS cut tap:
- Aluminum 6061 / 7075: 310 RPM · 34.4 IPM
- Free-machining / mild steel: 90 RPM · 10.0 IPM
- 304 / 316 stainless: 40 RPM · 4.44 IPM
7/8-9 tap speeds and feeds: all materials, form taps and G84 code · 49/64" drill size and speeds
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.
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