Unified inch · UNC
#10-24 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
#23
0.1540" · 67% thread
Form tap
11/64
0.1719" · 64% thread
Pick your material for its % of thread, tapping RPM and the rigid-tap code.
Thread dimensions
- Major Ø
- 0.1900" (4.826 mm)
- Threads per inch
- 24 (pitch 0.0417")
- Pitch Ø (basic)
- 0.1629" (4.139 mm)
- Minor Ø (internal, basic)
- 0.1449" (3.680 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 |
|---|---|---|---|
| #27 | 0.1440 | 3.66 | 85% |
| 3.7 mm | 0.1457 | 3.70 | 82% |
| #26 | 0.1470 | 3.73 | 79% |
| #25 | 0.1495 | 3.80 | 75% |
| 3.8 mm | 0.1496 | 3.80 | 75% |
| #24suggested | 0.1520 | 3.86 | 70% |
| 3.9 mmsuggested | 0.1535 | 3.90 | 67% |
| #23suggested | 0.1540 | 3.91 | 67% |
| 5/32 | 0.1563 | 3.97 | 62% |
| #22 | 0.1570 | 3.99 | 61% |
| 4 mm | 0.1575 | 4.00 | 60% |
| #21 | 0.1590 | 4.04 | 57% |
| #20 | 0.1610 | 4.09 | 54% |
| 4.1 mm | 0.1614 | 4.10 | 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 |
|---|---|---|---|
| #19 | 0.1660 | 4.22 | 85% |
| 4.3 mm | 0.1693 | 4.30 | 73% |
| #18 | 0.1695 | 4.31 | 72% |
| 11/64 | 0.1719 | 4.37 | 64% |
| #17 | 0.1730 | 4.39 | 60% |
| 4.4 mm | 0.1732 | 4.40 | 59% |
3-wire measurement
- Best wire size
- 0.0241" (0.611 mm)
- Over wires, at basic pitch Ø
- 0.1990" (5.055 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