M27×2 Tap Drill Size
The number you came for, then the reasoning behind it.
M27×2 tap drill — 75% thread
25 mm
25.051 mm exact · nearest inch drill 63/64 (0.9844")
How it's calculated
Tap drill = major diameter − pitch → 27 − 2 = 25 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% | 25.701 mm | 25.7 mm | 1.0156" (65/64 drill) |
| 65% | 25.311 mm | 25.3 mm | 1.0000" (1 drill) |
| 75% | 25.051 mm | 25 mm | 0.9844" (63/64 drill) |
| 85% | 24.792 mm | 24.8 mm | 0.9688" (31/32 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.