Unified inch · UNC
2-4.5 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
1-13/16
1.8125" · 65% thread
Form tap
1-57/64
1.8906" · 72% thread
Pick your material for its % of thread, tapping RPM and the rigid-tap code.
Thread dimensions
- Major Ø
- 2.0000" (50.800 mm)
- Threads per inch
- 4.5 (pitch 0.2222")
- Pitch Ø (basic)
- 1.8557" (47.134 mm)
- Minor Ø (internal, basic)
- 1.7594" (44.690 mm)
- Thread depth (internal)
- 0.1203" (3.055 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 |
|---|---|---|---|
| 1-49/64 | 1.7656 | 44.85 | 81% |
| 45 mm | 1.7717 | 45.00 | 79% |
| 1-25/32 | 1.7813 | 45.24 | 76% |
| 45.5 mm | 1.7913 | 45.50 | 72% |
| 1-51/64suggested | 1.7969 | 45.64 | 70% |
| 46 mmsuggested | 1.8110 | 46.00 | 65% |
| 1-13/16suggested | 1.8125 | 46.04 | 65% |
| 1-53/64 | 1.8281 | 46.43 | 60% |
| 46.5 mm | 1.8307 | 46.50 | 59% |
| 1-27/32 | 1.8438 | 46.83 | 54% |
| 47 mm | 1.8504 | 47.00 | 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 |
|---|---|---|---|
| 1-7/8 | 1.8750 | 47.63 | 83% |
| 48 mm | 1.8898 | 48.00 | 73% |
| 1-57/64 | 1.8906 | 48.02 | 72% |
| 1-29/32 | 1.9063 | 48.42 | 62% |
| 48.5 mm | 1.9094 | 48.50 | 60% |
| 1-59/64 | 1.9219 | 48.82 | 52% |
3-wire measurement
- Best wire size
- 0.1283" (3.259 mm)
- Over wires, at basic pitch Ø
- 2.0481" (52.022 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