Unified inch · UNC
1-3/8-6 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
1-15/64
1.2344" · 65% thread
Form tap
1-19/64
1.2969" · 69% thread
Pick your material for its % of thread, tapping RPM and the rigid-tap code.
Thread dimensions
- Major Ø
- 1.3750" (34.925 mm)
- Threads per inch
- 6 (pitch 0.1667")
- Pitch Ø (basic)
- 1.2667" (32.175 mm)
- Minor Ø (internal, basic)
- 1.1946" (30.342 mm)
- Thread depth (internal)
- 0.0902" (2.291 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 |
|---|---|---|---|
| 30.5 mm | 1.2008 | 30.50 | 80% |
| 1-13/64 | 1.2031 | 30.56 | 79% |
| 1-7/32 | 1.2188 | 30.96 | 72% |
| 31 mm | 1.2205 | 31.00 | 71% |
| 1-15/64suggested | 1.2344 | 31.35 | 65% |
| 31.5 mm | 1.2402 | 31.50 | 62% |
| 1-1/4 | 1.2500 | 31.75 | 58% |
| 32 mm | 1.2598 | 32.00 | 53% |
| 1-17/64 | 1.2656 | 32.15 | 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 |
|---|---|---|---|
| 32.5 mm | 1.2795 | 32.50 | 84% |
| 1-9/32 | 1.2813 | 32.54 | 83% |
| 1-19/64suggested | 1.2969 | 32.94 | 69% |
| 33 mmsuggested | 1.2992 | 33.00 | 67% |
| 1-5/16 | 1.3125 | 33.34 | 55% |
3-wire measurement
- Best wire size
- 0.0962" (2.444 mm)
- Over wires, at basic pitch Ø
- 1.4111" (35.842 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