ISO metric · fine
M10 × 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
8.9 mm
0.3504" · 68% thread
Form tap
9.4 mm
0.3701" · 71% thread
Pick your material for its % of thread, tapping RPM and the rigid-tap code.
Thread dimensions
- Major Ø
- 10.000 mm (0.3937")
- Pitch
- 1.25 mm
- Pitch Ø (basic)
- 9.188 mm (0.3617")
- Minor Ø (internal, basic)
- 8.647 mm (0.3404")
- 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 |
|---|---|---|---|
| 8.7 mm | 0.3425 | 8.70 | 80% |
| 11/32 | 0.3438 | 8.73 | 78% |
| 8.8 mm | 0.3465 | 8.80 | 74% |
| S | 0.3480 | 8.84 | 71% |
| 8.9 mmsuggested | 0.3504 | 8.90 | 68% |
| 9 mm | 0.3543 | 9.00 | 62% |
| T | 0.3580 | 9.09 | 56% |
| 9.1 mm | 0.3583 | 9.10 | 55% |
| 23/64 | 0.3594 | 9.13 | 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 |
|---|---|---|---|
| 9.3 mm | 0.3661 | 9.30 | 82% |
| U | 0.3680 | 9.35 | 77% |
| 9.4 mm | 0.3701 | 9.40 | 71% |
| 9.5 mm | 0.3740 | 9.50 | 59% |
| 3/8 | 0.3750 | 9.53 | 56% |
3-wire measurement
- Best wire size
- 0.722 mm (0.0284")
- Over wires, at basic pitch Ø
- 10.271 mm (0.4044")
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