Unified inch · UNF
5/16-24 UNF 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
J
0.2770" · 66% thread
Form tap
M
0.2950" · 62% thread
Pick your material for its % of thread, tapping RPM and the rigid-tap code.
Thread dimensions
- Major Ø
- 0.3125" (7.938 mm)
- Threads per inch
- 24 (pitch 0.0417")
- Pitch Ø (basic)
- 0.2854" (7.250 mm)
- Minor Ø (internal, basic)
- 0.2674" (6.792 mm)
- Thread depth (internal)
- 0.0226" (0.573 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 |
|---|---|---|---|
| 6.8 mm | 0.2677 | 6.80 | 83% |
| 6.9 mm | 0.2717 | 6.90 | 75% |
| I | 0.2720 | 6.91 | 75% |
| 7 mmsuggested | 0.2756 | 7.00 | 68% |
| Jsuggested | 0.2770 | 7.04 | 66% |
| 7.1 mm | 0.2795 | 7.10 | 61% |
| K | 0.2810 | 7.14 | 58% |
| 9/32 | 0.2813 | 7.14 | 58% |
| 7.2 mm | 0.2835 | 7.20 | 54% |
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 |
|---|---|---|---|
| L | 0.2900 | 7.37 | 79% |
| 7.4 mm | 0.2913 | 7.40 | 75% |
| M | 0.2950 | 7.49 | 62% |
| 7.5 mm | 0.2953 | 7.50 | 61% |
| 19/64 | 0.2969 | 7.54 | 55% |
3-wire measurement
- Best wire size
- 0.0241" (0.611 mm)
- Over wires, at basic pitch Ø
- 0.3215" (8.167 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