M42×4.5 Tap Drill Size
The number you came for, then the reasoning behind it.
M42×4.5 tap drill — 75% thread
37.5 mm
37.616 mm exact · nearest inch drill 47/32 (1.4688")
How it's calculated
Tap drill = major diameter − pitch → 42 − 4.5 = 37.5 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% | 39.077 mm | 39 mm | 1.5312" (49/32 drill) |
| 65% | 38.200 mm | 38 mm | 1.5000" (3/2 drill) |
| 75% | 37.616 mm | 37.5 mm | 1.4688" (47/32 drill) |
| 85% | 37.031 mm | 37 mm | 1.4531" (93/64 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.