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
1"-8 tapping speeds and feeds
Feed = pitch (0.1250 in per rev). Starting points for a general-purpose HSS cut tap:
- Aluminum 6061 / 7075: 270 RPM · 33.8 IPM
- Free-machining / mild steel: 80 RPM · 10.0 IPM
- 304 / 316 stainless: 40 RPM · 5.00 IPM
1"-8 tap speeds and feeds: all materials, form taps and G84 code · 7/8" drill size and speeds
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.
Free for machinists & programmers
Put your skills on a free talent profile
Machines, controls, certs and saved calculations in one profile shops can find.
Create free profile For your shop
Bring shift handoffs to your shop
Stop losing job status between shifts. Every station's notes, counts and issues, handed off cleanly.
See shift handoffs