ISO metric · coarse
M27 × 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
24.5 mm
0.9646" · 64% thread
Form tap
25.5 mm
1.0039" · 74% thread
Pick your material for its % of thread, tapping RPM and the rigid-tap code.
Thread dimensions
- Major Ø
- 27.000 mm (1.0630")
- Pitch
- 3 mm
- Pitch Ø (basic)
- 25.051 mm (0.9863")
- Minor Ø (internal, basic)
- 23.752 mm (0.9351")
- 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 |
|---|---|---|---|
| 15/16 | 0.9375 | 23.81 | 82% |
| 24 mm | 0.9449 | 24.00 | 77% |
| 61/64 | 0.9531 | 24.21 | 72% |
| 24.5 mm | 0.9646 | 24.50 | 64% |
| 31/32 | 0.9688 | 24.61 | 61% |
| 25 mm | 0.9843 | 25.00 | 51% |
| 63/64 | 0.9844 | 25.00 | 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 |
|---|---|---|---|
| 1 | 1.0000 | 25.40 | 78% |
| 25.5 mm | 1.0039 | 25.50 | 74% |
| 1-1/64 | 1.0156 | 25.80 | 59% |
3-wire measurement
- Best wire size
- 1.732 mm (0.0682")
- Over wires, at basic pitch Ø
- 27.650 mm (1.0886")
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