ISO metric · fine
M42 × 4 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
38.5 mm
1.5157" · 67% thread
Form tap
40 mm
1.5748" · 74% thread
Or 1-37/64 (1.5781") for 70%
Pick your material for its % of thread, tapping RPM and the rigid-tap code.
Thread dimensions
- Major Ø
- 42.000 mm (1.6535")
- Pitch
- 4 mm
- Pitch Ø (basic)
- 39.402 mm (1.5513")
- Minor Ø (internal, basic)
- 37.670 mm (1.4831")
- Thread depth (internal)
- 2.165 mm (0.0852")
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 |
|---|---|---|---|
| 1-31/64 | 1.4844 | 37.70 | 83% |
| 38 mm | 1.4961 | 38.00 | 77% |
| 1-1/2 | 1.5000 | 38.10 | 75% |
| 1-33/64suggested | 1.5156 | 38.50 | 67% |
| 38.5 mmsuggested | 1.5157 | 38.50 | 67% |
| 1-17/32 | 1.5313 | 38.89 | 60% |
| 39 mm | 1.5354 | 39.00 | 58% |
| 1-35/64 | 1.5469 | 39.29 | 52% |
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 |
|---|---|---|---|
| 40 mm | 1.5748 | 40.00 | 74% |
| 1-37/64suggested | 1.5781 | 40.08 | 70% |
| 1-19/32 | 1.5938 | 40.48 | 56% |
| 40.5 mm | 1.5945 | 40.50 | 55% |
3-wire measurement
- Best wire size
- 2.309 mm (0.0909")
- Over wires, at basic pitch Ø
- 42.866 mm (1.6876")
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