ISO metric · coarse
M20 × 2.5 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
18 mm
0.7087" · 62% thread
Or 45/64 (0.7031") for 66%
Form tap
19 mm
0.7480" · 59% thread
Pick your material for its % of thread, tapping RPM and the rigid-tap code.
Thread dimensions
- Major Ø
- 20.000 mm (0.7874")
- Pitch
- 2.5 mm
- Pitch Ø (basic)
- 18.376 mm (0.7235")
- Minor Ø (internal, basic)
- 17.294 mm (0.6809")
- Thread depth (internal)
- 1.353 mm (0.0533")
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 |
|---|---|---|---|
| 11/16 | 0.6875 | 17.46 | 78% |
| 17.5 mm | 0.6890 | 17.50 | 77% |
| 45/64suggested | 0.7031 | 17.86 | 66% |
| 18 mm | 0.7087 | 18.00 | 62% |
| 23/32 | 0.7188 | 18.26 | 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 |
|---|---|---|---|
| 47/64 | 0.7344 | 18.65 | 79% |
| 19 mm | 0.7480 | 19.00 | 59% |
| 3/4 | 0.7500 | 19.05 | 56% |
3-wire measurement
- Best wire size
- 1.443 mm (0.0568")
- Over wires, at basic pitch Ø
- 20.541 mm (0.8087")
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