M33×2 Tap Drill Size
The number you came for, then the reasoning behind it.
M33×2 tap drill — 75% thread
31 mm
31.051 mm exact · nearest inch drill 39/32 (1.2188")
How it's calculated
Tap drill = major diameter − pitch → 33 − 2 = 31 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% | 31.701 mm | 31.5 mm | 1.2344" (79/64 drill) |
| 65% | 31.311 mm | 31.5 mm | 1.2344" (79/64 drill) |
| 75% | 31.051 mm | 31 mm | 1.2188" (39/32 drill) |
| 85% | 30.792 mm | 31 mm | 1.2188" (39/32 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.