M10×1.25 Tap Drill Size
The number you came for, then the reasoning behind it.
M10×1.25 tap drill — 75% thread
8.8 mm
8.782 mm exact · nearest inch drill S (0.3480")
How it's calculated
Tap drill = major diameter − pitch → 10 − 1.25 = 8.75 mm
The exact 75% figure is major − (pitch × 75 / 76.98). The simple subtract-the-pitch rule lands within a hundredth of a millimetre of it, which is why every machinist uses it.
All engagement percentages
| % thread | Exact hole | Metric drill | Inch equivalent |
|---|---|---|---|
| 50% | 9.188 mm | 9.2 mm | 0.3594" (23/64 drill) |
| 65% | 8.945 mm | 8.9 mm | 0.3480" (S drill) |
| 75% | 8.782 mm | 8.8 mm | 0.3480" (S drill) |
| 85% | 8.620 mm | 8.6 mm | 0.3390" (R drill) |
Which percentage should you actually use?
75% for through-holes in mild steel and aluminium. 65% for blind holes, stainless, titanium and anything where the tap is at risk. 50% for hand-tapping deep holes. The strength you give up going from 75% to 60% is around 5%; the tapping torque you give up is around 30%. In practice the threads almost never fail — the tap does.
75% for through-holes in mild steel and aluminium. 65% for blind holes, stainless, titanium and anything where the tap is at risk. 50% for hand-tapping deep holes. The strength you give up going from 75% to 60% is around 5%; the tapping torque you give up is around 30%. In practice the threads almost never fail — the tap does.