Unified inch · UNC
3/8-16 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
P
0.3230" · 64% thread
Or 8.1 mm for 69%
Form tap
S
0.3480" · 64% thread
Or 8.8 mm for 67%
Pick your material for its % of thread, tapping RPM and the rigid-tap code.
Thread dimensions
- Major Ø
- 0.3750" (9.525 mm)
- Threads per inch
- 16 (pitch 0.0625")
- Pitch Ø (basic)
- 0.3344" (8.494 mm)
- Minor Ø (internal, basic)
- 0.3073" (7.806 mm)
- Thread depth (internal)
- 0.0338" (0.859 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 |
|---|---|---|---|
| 7.8 mm | 0.3071 | 7.80 | 84% |
| 7.9 mm | 0.3110 | 7.90 | 79% |
| 5/16 | 0.3125 | 7.94 | 77% |
| 8 mm | 0.3150 | 8.00 | 74% |
| O | 0.3160 | 8.03 | 73% |
| 8.1 mmsuggested | 0.3189 | 8.10 | 69% |
| 8.2 mm | 0.3228 | 8.20 | 64% |
| P | 0.3230 | 8.20 | 64% |
| 8.3 mm | 0.3268 | 8.30 | 59% |
| 21/64 | 0.3281 | 8.33 | 58% |
| 8.4 mm | 0.3307 | 8.40 | 55% |
| Q | 0.3320 | 8.43 | 53% |
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 |
|---|---|---|---|
| R | 0.3390 | 8.61 | 85% |
| 8.7 mm | 0.3425 | 8.70 | 76% |
| 11/32 | 0.3438 | 8.73 | 74% |
| 8.8 mmsuggested | 0.3465 | 8.80 | 67% |
| S | 0.3480 | 8.84 | 64% |
| 8.9 mm | 0.3504 | 8.90 | 58% |
3-wire measurement
- Best wire size
- 0.0361" (0.917 mm)
- Over wires, at basic pitch Ø
- 0.3885" (9.869 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