Unified inch · UNC
7/16-14 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
3/8
0.3750" · 67% thread
Form tap
Y
0.4040" · 69% 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
- 14 (pitch 0.0714")
- Pitch Ø (basic)
- 0.3911" (9.934 mm)
- Minor Ø (internal, basic)
- 0.3602" (9.148 mm)
- Thread depth (internal)
- 0.0387" (0.982 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 |
|---|---|---|---|
| 23/64 | 0.3594 | 9.13 | 84% |
| 9.2 mm | 0.3622 | 9.20 | 81% |
| 9.3 mm | 0.3661 | 9.30 | 77% |
| U | 0.3680 | 9.35 | 75% |
| 9.4 mm | 0.3701 | 9.40 | 73% |
| 9.5 mmsuggested | 0.3740 | 9.50 | 68% |
| 3/8suggested | 0.3750 | 9.53 | 67% |
| Vsuggested | 0.3770 | 9.58 | 65% |
| 9.6 mm | 0.3780 | 9.60 | 64% |
| 9.7 mm | 0.3819 | 9.70 | 60% |
| 9.8 mm | 0.3858 | 9.80 | 56% |
| W | 0.3860 | 9.80 | 56% |
| 9.9 mm | 0.3898 | 9.90 | 51% |
| 25/64 | 0.3906 | 9.92 | 51% |
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 |
|---|---|---|---|
| X | 0.3970 | 10.08 | 83% |
| 10.1 mm | 0.3976 | 10.10 | 82% |
| 10.2 mm | 0.4016 | 10.20 | 74% |
| Ysuggested | 0.4040 | 10.26 | 69% |
| 10.3 mmsuggested | 0.4055 | 10.30 | 66% |
| 13/32 | 0.4063 | 10.32 | 64% |
| 10.4 mm | 0.4094 | 10.40 | 58% |
| Z | 0.4130 | 10.49 | 50% |
3-wire measurement
- Best wire size
- 0.0412" (1.047 mm)
- Over wires, at basic pitch Ø
- 0.4530" (11.505 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