#2-56 UNC Tap Drill Size
The number you came for, then the reasoning behind it.
#2-56 UNC tap drill — 75% thread
#50
0.0700" · 1.7 mm metric equivalent
How it's calculated
Tap drill = major diameter − (0.01299 × %thread) / TPI → 0.0860 − (0.01299 × 75) / 56 = 0.0686"
The nearest standard drill above that is #50 at 0.0700".
All engagement percentages
| % thread | Exact hole | Inch drill | Metric equivalent |
|---|---|---|---|
| 50% | 0.0744" | #49 (0.0730") | 1.9 mm |
| 65% | 0.0709" | #50 (0.0700") | 1.8 mm |
| 75% | 0.0686" | #50 (0.0700") | 1.7 mm |
| 85% | 0.0663" | #51 (0.0670") | 1.7 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.