M8 x 1.25 tap drill size
6.8 mm
- Inches
- 0.2677
- Inch bit
- Letter H
- Thread
- 74%
Drill a 6.8 mm hole before cutting an M8 thread with 1.25 mm pitch. The rule for metric taps is diameter minus pitch.
No metric bits? A Letter H bit (0.2660 in) gives about 77% thread.
M8 x 1.25 thread and hole sizes
| Thread | M8, 1.25 mm pitch, coarse |
|---|---|
| Major diameter | 8 mm 0.3150 in |
| Tap drill | 6.8 mm 0.2677 in |
| Closest inch bit | Letter H 0.2660 in |
| Clearance hole, close fit | 8.4 mm |
| Clearance hole, medium fit | 9 mm |
Why 6.8 mm
Subtract the pitch from the diameter: 8 minus 1.25 is 6.75 mm, and charts list the nearest stocked bit, 6.8 mm. That leaves 74% 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
- M8 x 1.25 tap
- 6.8 mm drill bit
- Tap and drill set
- Drill bit size gauge
- Thread pitch gauge
- Cutting oil for tapping
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