#4-48 UNF Tap Drill Size
The number you came for, then the reasoning behind it.
#4-48 UNF tap drill — 75% thread
#42
0.0935" · 2.3 mm metric equivalent
How it's calculated
Tap drill = major diameter − (0.01299 × %thread) / TPI → 0.1120 − (0.01299 × 75) / 48 = 0.0917"
The nearest standard drill above that is #42 at 0.0935".
All engagement percentages
| % thread | Exact hole | Inch drill | Metric equivalent |
|---|---|---|---|
| 50% | 0.0985" | #40 (0.0980") | 2.5 mm |
| 65% | 0.0944" | 3/32 (0.0938") | 2.4 mm |
| 75% | 0.0917" | #42 (0.0935") | 2.3 mm |
| 85% | 0.0890" | #43 (0.0890") | 2.3 mm |
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.