M20 x 2.5 tap drill size
17.5 mm
- Inches
- 0.6890
- Inch bit
- 11/16"
- Thread
- 77%
Drill a 17.5 mm hole before cutting an M20 thread with 2.5 mm pitch. The rule for metric taps is diameter minus pitch.
No metric bits? A 11/16" bit (0.6875 in) gives about 78% thread.
M20 x 2.5 thread and hole sizes
| Thread | M20, 2.5 mm pitch, coarse |
|---|---|
| Major diameter | 20 mm 0.7874 in |
| Tap drill | 17.5 mm 0.6890 in |
| Closest inch bit | 11/16" 0.6875 in |
| Clearance hole, close fit | 21 mm |
| Clearance hole, medium fit | 22 mm |
Why 17.5 mm
Subtract the pitch from the diameter: 20 minus 2.5 is 17.5 mm. That leaves 77% thread, the usual target for a cut tap.
More thread than that adds almost no strength and makes the tap much easier to break. In hard or tough material, go 0.1 mm larger.
Tools for this job
- M20 x 2.5 tap
- 17.5 mm drill bit
- Tap and drill set
- Drill bit size gauge
- Thread pitch gauge
- Cutting oil for tapping
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