ISO metric · coarse
M24 × 3 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
21.5 mm
0.8465" · 64% thread
Or 27/32 (0.8438") for 66%
Form tap
22.5 mm
0.8858" · 74% thread
Or 57/64 (0.8906") for 68%
Pick your material for its % of thread, tapping RPM and the rigid-tap code.
Thread dimensions
- Major Ø
- 24.000 mm (0.9449")
- Pitch
- 3 mm
- Pitch Ø (basic)
- 22.051 mm (0.8682")
- Minor Ø (internal, basic)
- 20.752 mm (0.8170")
- Thread depth (internal)
- 1.624 mm (0.0639")
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 |
|---|---|---|---|
| 21 mm | 0.8268 | 21.00 | 77% |
| 53/64 | 0.8281 | 21.03 | 76% |
| 27/32suggested | 0.8438 | 21.43 | 66% |
| 21.5 mm | 0.8465 | 21.50 | 64% |
| 55/64 | 0.8594 | 21.83 | 56% |
| 22 mm | 0.8661 | 22.00 | 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 |
|---|---|---|---|
| 22.5 mm | 0.8858 | 22.50 | 74% |
| 57/64suggested | 0.8906 | 22.62 | 68% |
3-wire measurement
- Best wire size
- 1.732 mm (0.0682")
- Over wires, at basic pitch Ø
- 24.650 mm (0.9705")
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