Unified inch · UNC
5/8-11 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
35/64
0.5469" · 66% thread
Form tap
37/64
0.5781" · 76% thread
Pick your material for its % of thread, tapping RPM and the rigid-tap code.
Thread dimensions
- Major Ø
- 0.6250" (15.875 mm)
- Threads per inch
- 11 (pitch 0.0909")
- Pitch Ø (basic)
- 0.5660" (14.375 mm)
- Minor Ø (internal, basic)
- 0.5266" (13.375 mm)
- Thread depth (internal)
- 0.0492" (1.250 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 |
|---|---|---|---|
| 17/32 | 0.5313 | 13.49 | 79% |
| 13.5 mm | 0.5315 | 13.50 | 79% |
| 35/64suggested | 0.5469 | 13.89 | 66% |
| 14 mm | 0.5512 | 14.00 | 63% |
| 9/16 | 0.5625 | 14.29 | 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 |
|---|---|---|---|
| 37/64 | 0.5781 | 14.68 | 76% |
| 15 mm | 0.5906 | 15.00 | 56% |
| 19/32 | 0.5938 | 15.08 | 51% |
3-wire measurement
- Best wire size
- 0.0525" (1.333 mm)
- Over wires, at basic pitch Ø
- 0.6447" (16.375 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