Unified inch · UNEF
7/8-20 UNEF 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
53/64
0.8281" · 72% thread
Form tap
55/64
0.8594" · 46% thread
Pick your material for its % of thread, tapping RPM and the rigid-tap code.
Thread dimensions
- Major Ø
- 0.8750" (22.225 mm)
- Threads per inch
- 20 (pitch 0.0500")
- Pitch Ø (basic)
- 0.8425" (21.400 mm)
- Minor Ø (internal, basic)
- 0.8209" (20.850 mm)
- Thread depth (internal)
- 0.0271" (0.687 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 |
|---|---|---|---|
| 21 mm | 0.8268 | 21.00 | 74% |
| 53/64 | 0.8281 | 21.03 | 72% |
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 |
|---|---|---|---|
| 21.5 mm | 0.8465 | 21.50 | 84% |
3-wire measurement
- Best wire size
- 0.0289" (0.733 mm)
- Over wires, at basic pitch Ø
- 0.8858" (22.500 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