Unified inch · UNC
1-3/4-5 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
1-37/64
1.5781" · 66% thread
Form tap
1-21/32
1.6563" · 69% thread
Pick your material for its % of thread, tapping RPM and the rigid-tap code.
Thread dimensions
- Major Ø
- 1.7500" (44.450 mm)
- Threads per inch
- 5 (pitch 0.2000")
- Pitch Ø (basic)
- 1.6201" (41.150 mm)
- Minor Ø (internal, basic)
- 1.5335" (38.951 mm)
- Thread depth (internal)
- 0.1083" (2.750 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 |
|---|---|---|---|
| 1-17/32 | 1.5313 | 38.89 | 84% |
| 39 mm | 1.5354 | 39.00 | 83% |
| 1-35/64 | 1.5469 | 39.29 | 78% |
| 39.5 mm | 1.5551 | 39.50 | 75% |
| 1-9/16 | 1.5625 | 39.69 | 72% |
| 40 mmsuggested | 1.5748 | 40.00 | 67% |
| 1-37/64suggested | 1.5781 | 40.08 | 66% |
| 1-19/32 | 1.5938 | 40.48 | 60% |
| 40.5 mm | 1.5945 | 40.50 | 60% |
| 1-39/64 | 1.6094 | 40.88 | 54% |
| 41 mm | 1.6142 | 41.00 | 52% |
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 |
|---|---|---|---|
| 1-41/64 | 1.6406 | 41.67 | 80% |
| 42 mm | 1.6535 | 42.00 | 71% |
| 1-21/32suggested | 1.6563 | 42.07 | 69% |
| 1-43/64 | 1.6719 | 42.47 | 57% |
| 42.5 mm | 1.6732 | 42.50 | 56% |
3-wire measurement
- Best wire size
- 0.1155" (2.933 mm)
- Over wires, at basic pitch Ø
- 1.7933" (45.550 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