Unified inch · UNC
1-1/8-7 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
1.0000" · 67% thread
Form tap
1-1/16
1.0625" · 64% thread
Pick your material for its % of thread, tapping RPM and the rigid-tap code.
Thread dimensions
- Major Ø
- 1.1250" (28.575 mm)
- Threads per inch
- 7 (pitch 0.1429")
- Pitch Ø (basic)
- 1.0322" (26.218 mm)
- Minor Ø (internal, basic)
- 0.9704" (24.647 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 |
|---|---|---|---|
| 31/32 | 0.9688 | 24.61 | 84% |
| 25 mm | 0.9843 | 25.00 | 76% |
| 63/64 | 0.9844 | 25.00 | 76% |
| 1suggested | 1.0000 | 25.40 | 67% |
| 25.5 mmsuggested | 1.0039 | 25.50 | 65% |
| 1-1/64 | 1.0156 | 25.80 | 59% |
| 26 mm | 1.0236 | 26.00 | 55% |
| 1-1/32 | 1.0313 | 26.19 | 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 |
|---|---|---|---|
| 26.5 mm | 1.0433 | 26.50 | 84% |
| 1-3/64 | 1.0469 | 26.59 | 80% |
| 1-1/16 | 1.0625 | 26.99 | 64% |
| 27 mm | 1.0630 | 27.00 | 64% |
3-wire measurement
- Best wire size
- 0.0825" (2.095 mm)
- Over wires, at basic pitch Ø
- 1.1559" (29.361 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