Unified inch · UNC
#8-32 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
#29
0.1360" · 69% thread
Form tap
#25
0.1495" · 68% thread
Pick your material for its % of thread, tapping RPM and the rigid-tap code.
Thread dimensions
- Major Ø
- 0.1640" (4.166 mm)
- Threads per inch
- 32 (pitch 0.0313")
- Pitch Ø (basic)
- 0.1437" (3.650 mm)
- Minor Ø (internal, basic)
- 0.1302" (3.306 mm)
- Thread depth (internal)
- 0.0169" (0.430 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 |
|---|---|---|---|
| 3.3 mm | 0.1299 | 3.30 | 84% |
| 3.4 mm | 0.1339 | 3.40 | 74% |
| #29suggested | 0.1360 | 3.45 | 69% |
| 3.5 mmsuggested | 0.1378 | 3.50 | 65% |
| #28 | 0.1405 | 3.57 | 58% |
| 9/64 | 0.1406 | 3.57 | 58% |
| 3.6 mm | 0.1417 | 3.60 | 55% |
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 |
|---|---|---|---|
| #26 | 0.1470 | 3.73 | 80% |
| #25suggested | 0.1495 | 3.80 | 68% |
| 3.8 mmsuggested | 0.1496 | 3.80 | 68% |
| #24 | 0.1520 | 3.86 | 56% |
3-wire measurement
- Best wire size
- 0.0180" (0.458 mm)
- Over wires, at basic pitch Ø
- 0.1708" (4.337 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