ISO metric · fine
M8 × 0.75 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
7.3 mm
0.2874" · 72% thread
Or L (0.2900") for 65%
Form tap
7.7 mm
0.3031" · 59% thread
Or N (0.3020") for 65%
Pick your material for its % of thread, tapping RPM and the rigid-tap code.
Thread dimensions
- Major Ø
- 8.000 mm (0.3150")
- Pitch
- 0.75 mm
- Pitch Ø (basic)
- 7.513 mm (0.2958")
- Minor Ø (internal, basic)
- 7.188 mm (0.2830")
- Thread depth (internal)
- 0.406 mm (0.0160")
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 |
|---|---|---|---|
| 7.2 mm | 0.2835 | 7.20 | 82% |
| 7.3 mm | 0.2874 | 7.30 | 72% |
| Lsuggested | 0.2900 | 7.37 | 65% |
| 7.4 mm | 0.2913 | 7.40 | 62% |
| M | 0.2950 | 7.49 | 52% |
| 7.5 mm | 0.2953 | 7.50 | 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 |
|---|---|---|---|
| 7.6 mm | 0.2992 | 7.60 | 78% |
| Nsuggested | 0.3020 | 7.67 | 65% |
| 7.7 mm | 0.3031 | 7.70 | 59% |
3-wire measurement
- Best wire size
- 0.433 mm (0.0170")
- Over wires, at basic pitch Ø
- 8.162 mm (0.3214")
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