Unified inch · UNF
7/16-20 UNF 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
25/64
0.3906" · 72% thread
Or 10 mm for 67%
Form tap
Z
0.4130" · 72% thread
Pick your material for its % of thread, tapping RPM and the rigid-tap code.
Thread dimensions
- Major Ø
- 0.4375" (11.113 mm)
- Threads per inch
- 20 (pitch 0.0500")
- Pitch Ø (basic)
- 0.4050" (10.288 mm)
- Minor Ø (internal, basic)
- 0.3834" (9.738 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 |
|---|---|---|---|
| 9.8 mm | 0.3858 | 9.80 | 80% |
| W | 0.3860 | 9.80 | 79% |
| 9.9 mm | 0.3898 | 9.90 | 73% |
| 25/64 | 0.3906 | 9.92 | 72% |
| 10 mmsuggested | 0.3937 | 10.00 | 67% |
| X | 0.3970 | 10.08 | 62% |
| 10.1 mm | 0.3976 | 10.10 | 61% |
| 10.2 mm | 0.4016 | 10.20 | 55% |
| Y | 0.4040 | 10.26 | 52% |
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 |
|---|---|---|---|
| 10.4 mm | 0.4094 | 10.40 | 83% |
| Z | 0.4130 | 10.49 | 72% |
| 10.5 mm | 0.4134 | 10.50 | 71% |
| 10.6 mm | 0.4173 | 10.60 | 59% |
3-wire measurement
- Best wire size
- 0.0289" (0.733 mm)
- Over wires, at basic pitch Ø
- 0.4483" (11.387 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