Unified inch · UNC
1-8 UNC tap drill size
For general work, OSG suggests 65–70% of full thread. Harder materials and deep holes want less; the calculator sets it from your material.
Cut tap
57/64
0.8906" · 67% thread
Form tap
15/16
0.9375" · 74% thread
Or 24 mm for 65%
Pick your material for its % of thread, tapping RPM and the rigid-tap code.
Thread dimensions
- Major Ø
- 1.0000" (25.400 mm)
- Threads per inch
- 8 (pitch 0.1250")
- Pitch Ø (basic)
- 0.9188" (23.338 mm)
- Minor Ø (internal, basic)
- 0.8647" (21.963 mm)
- Thread depth (internal)
- 0.0677" (1.719 mm)
Basic sizes of the 60° form. Tolerance classes (2B/3B) set the limits around them.
Cut tap drills
For a cutting (fluted or spiral) tap. Every standard drill from 50% to 85% thread; suggested rows are in the 65–70% band.
| Drill | Ø in | Ø mm | Thread |
|---|---|---|---|
| 22 mm | 0.8661 | 22.00 | 82% |
| 7/8 | 0.8750 | 22.23 | 77% |
| 22.5 mmsuggested | 0.8858 | 22.50 | 70% |
| 57/64suggested | 0.8906 | 22.62 | 67% |
| 23 mm | 0.9055 | 23.00 | 58% |
| 29/32 | 0.9063 | 23.02 | 58% |
Form tap drills
A form (roll) tap displaces metal instead of cutting it, so it needs a bigger hole. Every standard drill from 50% to 85% thread; suggested rows are in the 65–70% band.
| Drill | Ø in | Ø mm | Thread |
|---|---|---|---|
| 15/16 | 0.9375 | 23.81 | 74% |
| 24 mmsuggested | 0.9449 | 24.00 | 65% |
| 61/64 | 0.9531 | 24.21 | 55% |
3-wire measurement
- Best wire size
- 0.0722" (1.833 mm)
- Over wires, at basic pitch Ø
- 1.0271" (26.087 mm)
For the external thread (the screw). The best wire touches the flanks at the pitch diameter.
Sources
- ASME B1.1 — Unified Inch Screw Threads (UN and UNR thread form)
- OSG — Tap drill size and pitch diameter limit chart (formula, suggested % of full thread)
- Cutting Tool Engineering — "Threads will roll": form tap hole size = D − 0.0068 × % / TPI
- ASME B94.11M — Twist drills (fractional, number, letter and metric sizes)
- NIST (NBS) Letter Circular 23 and Pratt & Whitney — three-wire thread measurement, best wire size