Unified inch · UNF
3/8-24 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
R
0.3390" · 67% thread
Form tap
T
0.3580" · 60% thread
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
- 24 (pitch 0.0417")
- Pitch Ø (basic)
- 0.3479" (8.838 mm)
- Minor Ø (internal, basic)
- 0.3299" (8.379 mm)
- Thread depth (internal)
- 0.0226" (0.573 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 |
|---|---|---|---|
| 8.4 mm | 0.3307 | 8.40 | 82% |
| Q | 0.3320 | 8.43 | 79% |
| 8.5 mm | 0.3346 | 8.50 | 75% |
| 8.6 mmsuggested | 0.3386 | 8.60 | 67% |
| Rsuggested | 0.3390 | 8.61 | 67% |
| 8.7 mm | 0.3425 | 8.70 | 60% |
| 11/32 | 0.3438 | 8.73 | 58% |
| 8.8 mm | 0.3465 | 8.80 | 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 |
|---|---|---|---|
| 9 mm | 0.3543 | 9.00 | 73% |
| T | 0.3580 | 9.09 | 60% |
| 9.1 mm | 0.3583 | 9.10 | 59% |
| 23/64 | 0.3594 | 9.13 | 55% |
3-wire measurement
- Best wire size
- 0.0241" (0.611 mm)
- Over wires, at basic pitch Ø
- 0.3840" (9.754 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