Unified inch · UNC
1-1/4-7 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
1-1/8
1.1250" · 67% thread
Form tap
1-3/16
1.1875" · 64% thread
Pick your material for its % of thread, tapping RPM and the rigid-tap code.
Thread dimensions
- Major Ø
- 1.2500" (31.750 mm)
- Threads per inch
- 7 (pitch 0.1429")
- Pitch Ø (basic)
- 1.1572" (29.393 mm)
- Minor Ø (internal, basic)
- 1.0954" (27.822 mm)
- Thread depth (internal)
- 0.0773" (1.964 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 |
|---|---|---|---|
| 1-3/32 | 1.0938 | 27.78 | 84% |
| 28 mm | 1.1024 | 28.00 | 80% |
| 1-7/64 | 1.1094 | 28.18 | 76% |
| 28.5 mmsuggested | 1.1220 | 28.50 | 69% |
| 1-1/8suggested | 1.1250 | 28.58 | 67% |
| 1-9/64 | 1.1406 | 28.97 | 59% |
| 29 mm | 1.1417 | 29.00 | 58% |
| 1-5/32 | 1.1563 | 29.37 | 51% |
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 |
|---|---|---|---|
| 1-11/64 | 1.1719 | 29.77 | 80% |
| 30 mm | 1.1811 | 30.00 | 71% |
| 1-3/16 | 1.1875 | 30.16 | 64% |
| 30.5 mm | 1.2008 | 30.50 | 51% |
3-wire measurement
- Best wire size
- 0.0825" (2.095 mm)
- Over wires, at basic pitch Ø
- 1.2809" (32.536 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