Unified inch · UNEF
7/16-28 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
13/32
0.4063" · 67% thread
Form tap
27/64
0.4219" · 64% thread
Or 10.7 mm for 67%
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
- 28 (pitch 0.0357")
- Pitch Ø (basic)
- 0.4143" (10.523 mm)
- Minor Ø (internal, basic)
- 0.3988" (10.130 mm)
- Thread depth (internal)
- 0.0193" (0.491 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 |
|---|---|---|---|
| 10.2 mm | 0.4016 | 10.20 | 77% |
| Y | 0.4040 | 10.26 | 72% |
| 10.3 mmsuggested | 0.4055 | 10.30 | 69% |
| 13/32suggested | 0.4063 | 10.32 | 67% |
| 10.4 mm | 0.4094 | 10.40 | 60% |
| Z | 0.4130 | 10.49 | 53% |
| 10.5 mm | 0.4134 | 10.50 | 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 |
|---|---|---|---|
| 10.6 mm | 0.4173 | 10.60 | 83% |
| 10.7 mmsuggested | 0.4213 | 10.70 | 67% |
| 27/64 | 0.4219 | 10.72 | 64% |
| 10.8 mm | 0.4252 | 10.80 | 51% |
3-wire measurement
- Best wire size
- 0.0206" (0.524 mm)
- Over wires, at basic pitch Ø
- 0.4452" (11.309 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