M18 x 2.5 tap drill size
15.5 mm
- Inches
- 0.6102
- Inch bit
- 39/64"
- Thread
- 77%
Drill a 15.5 mm hole before cutting an M18 thread with 2.5 mm pitch. The rule for metric taps is diameter minus pitch.
No metric bits? A 39/64" bit (0.6094 in) gives about 78% thread.
M18 x 2.5 thread and hole sizes
| Thread | M18, 2.5 mm pitch, coarse |
|---|---|
| Major diameter | 18 mm 0.7087 in |
| Tap drill | 15.5 mm 0.6102 in |
| Closest inch bit | 39/64" 0.6094 in |
| Clearance hole, close fit | 19 mm |
| Clearance hole, medium fit | 20 mm |
Why 15.5 mm
Subtract the pitch from the diameter: 18 minus 2.5 is 15.5 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
- M18 x 2.5 tap
- 15.5 mm drill bit
- Tap and drill set
- Drill bit size gauge
- Thread pitch gauge
- Cutting oil for tapping
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