M56×4 Tap Drill Size
The number you came for, then the reasoning behind it.
M56×4 tap drill — 75% thread
52 mm
52.103 mm exact · no standard inch drill this large
How it's calculated
Tap drill = major diameter − pitch → 56 − 4 = 52 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% | 53.402 mm | 53.5 mm | no standard inch drill |
| 65% | 52.622 mm | 52.5 mm | no standard inch drill |
| 75% | 52.103 mm | 52 mm | no standard inch drill |
| 85% | 51.583 mm | 51.5 mm | no standard inch 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.