ISO metric · fine
M4 × 0.5 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
3.6 mm
0.1417" · 62% thread
Or #28 (0.1405") for 66%
Form tap
3.8 mm
0.1496" · 59% thread
Pick your material for its % of thread, tapping RPM and the rigid-tap code.
Thread dimensions
- Major Ø
- 4.000 mm (0.1575")
- Pitch
- 0.5 mm
- Pitch Ø (basic)
- 3.675 mm (0.1447")
- Minor Ø (internal, basic)
- 3.459 mm (0.1362")
- Thread depth (internal)
- 0.271 mm (0.0107")
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 |
|---|---|---|---|
| #29 | 0.1360 | 3.45 | 84% |
| 3.5 mm | 0.1378 | 3.50 | 77% |
| #28suggested | 0.1405 | 3.57 | 66% |
| 9/64suggested | 0.1406 | 3.57 | 66% |
| 3.6 mm | 0.1417 | 3.60 | 62% |
| #27 | 0.1440 | 3.66 | 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 |
|---|---|---|---|
| #26 | 0.1470 | 3.73 | 78% |
| #25 | 0.1495 | 3.80 | 60% |
| 3.8 mm | 0.1496 | 3.80 | 59% |
3-wire measurement
- Best wire size
- 0.289 mm (0.0114")
- Over wires, at basic pitch Ø
- 4.108 mm (0.1617")
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