M20 x 1.5 tap drill size
18.5 mm
- Inches
- 0.7283
- Inch bit
- 47/64"
- Thread
- 77%
Drill a 18.5 mm hole before cutting an M20 thread with 1.5 mm pitch. The rule for metric taps is diameter minus pitch.
No metric bits? A 47/64" bit (0.7344 in) gives about 69% thread.
M20 x 1.5 thread and hole sizes
| Thread | M20, 1.5 mm pitch, fine |
|---|---|
| Major diameter | 20 mm 0.7874 in |
| Tap drill | 18.5 mm 0.7283 in |
| Closest inch bit | 47/64" 0.7344 in |
| Clearance hole, close fit | 21 mm |
| Clearance hole, medium fit | 22 mm |
Why 18.5 mm
Subtract the pitch from the diameter: 20 minus 1.5 is 18.5 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 1.5 tap
- 18.5 mm drill bit
- Tap and drill set
- Drill bit size gauge
- Thread pitch gauge
- Cutting oil for tapping
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