ISO metric · coarse
M8 × 1.25 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
6.9 mm
0.2717" · 68% thread
Form tap
7.4 mm
0.2913" · 71% thread
Pick your material for its % of thread, tapping RPM and the rigid-tap code.
Thread dimensions
- Major Ø
- 8.000 mm (0.3150")
- Pitch
- 1.25 mm
- Pitch Ø (basic)
- 7.188 mm (0.2830")
- Minor Ø (internal, basic)
- 6.647 mm (0.2617")
- Thread depth (internal)
- 0.677 mm (0.0266")
Basic sizes of the 60° form. Tolerance classes (6H) 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 |
|---|---|---|---|
| G | 0.2610 | 6.63 | 84% |
| 6.7 mm | 0.2638 | 6.70 | 80% |
| 17/64 | 0.2656 | 6.75 | 77% |
| H | 0.2660 | 6.76 | 77% |
| 6.8 mm | 0.2677 | 6.80 | 74% |
| 6.9 mmsuggested | 0.2717 | 6.90 | 68% |
| Isuggested | 0.2720 | 6.91 | 67% |
| 7 mm | 0.2756 | 7.00 | 62% |
| J | 0.2770 | 7.04 | 59% |
| 7.1 mm | 0.2795 | 7.10 | 55% |
| K | 0.2810 | 7.14 | 53% |
| 9/32 | 0.2813 | 7.14 | 53% |
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 |
|---|---|---|---|
| 7.3 mm | 0.2874 | 7.30 | 82% |
| L | 0.2900 | 7.37 | 75% |
| 7.4 mm | 0.2913 | 7.40 | 71% |
| M | 0.2950 | 7.49 | 60% |
| 7.5 mm | 0.2953 | 7.50 | 59% |
| 19/64 | 0.2969 | 7.54 | 54% |
3-wire measurement
- Best wire size
- 0.722 mm (0.0284")
- Over wires, at basic pitch Ø
- 8.271 mm (0.3256")
For the external thread (the screw). The best wire touches the flanks at the pitch diameter.
Sources
- ISO 261 / ISO 262 — ISO general purpose metric screw threads: general plan and selected sizes
- 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