Unified inch · UNC
5/16-18 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
17/64
0.2656" · 65% thread
Form tap
L
0.2900" · 60% thread
Or 7.3 mm for 66%
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
- 18 (pitch 0.0556")
- Pitch Ø (basic)
- 0.2764" (7.021 mm)
- Minor Ø (internal, basic)
- 0.2524" (6.410 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 |
|---|---|---|---|
| 6.4 mm | 0.2520 | 6.40 | 84% |
| 6.5 mm | 0.2559 | 6.50 | 78% |
| F | 0.2570 | 6.53 | 77% |
| 6.6 mm | 0.2598 | 6.60 | 73% |
| G | 0.2610 | 6.63 | 71% |
| 6.7 mmsuggested | 0.2638 | 6.70 | 68% |
| 17/64suggested | 0.2656 | 6.75 | 65% |
| H | 0.2660 | 6.76 | 64% |
| 6.8 mm | 0.2677 | 6.80 | 62% |
| 6.9 mm | 0.2717 | 6.90 | 57% |
| I | 0.2720 | 6.91 | 56% |
| 7 mm | 0.2756 | 7.00 | 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 |
|---|---|---|---|
| K | 0.2810 | 7.14 | 83% |
| 9/32 | 0.2813 | 7.14 | 83% |
| 7.2 mm | 0.2835 | 7.20 | 77% |
| 7.3 mmsuggested | 0.2874 | 7.30 | 66% |
| L | 0.2900 | 7.37 | 60% |
| 7.4 mm | 0.2913 | 7.40 | 56% |
3-wire measurement
- Best wire size
- 0.0321" (0.815 mm)
- Over wires, at basic pitch Ø
- 0.3245" (8.243 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