Unified inch · UNEF
1-3/8-18 UNEF 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
1-21/64
1.3281" · 65% thread
Form tap
1-11/32
1.3438" · 83% 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
- 18 (pitch 0.0556")
- Pitch Ø (basic)
- 1.3389" (34.008 mm)
- Minor Ø (internal, basic)
- 1.3149" (33.397 mm)
- Thread depth (internal)
- 0.0301" (0.764 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 |
|---|---|---|---|
| 33.5 mm | 1.3189 | 33.50 | 78% |
| 1-21/64suggested | 1.3281 | 33.73 | 65% |
| 34 mm | 1.3386 | 34.00 | 50% |
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 |
|---|---|---|---|
| 1-11/32 | 1.3438 | 34.13 | 83% |
3-wire measurement
- Best wire size
- 0.0321" (0.815 mm)
- Over wires, at basic pitch Ø
- 1.3870" (35.231 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