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3/4-10 UNC Tap Drill Size
The number you came for, then the reasoning behind it.
3/4-10 UNC tap drill — 75% thread
21/32
0.6562" · 16.6 mm metric equivalent
How it's calculated
Tap drill = major diameter − (0.01299 × %thread) / TPI → 0.7500 − (0.01299 × 75) / 10 = 0.6526"
The nearest standard drill above that is 21/32 at 0.6562".
All engagement percentages
| % thread | Exact hole | Inch drill | Metric equivalent |
|---|---|---|---|
| 50% | 0.6851" | 11/16 (0.6875") | 17.4 mm |
| 65% | 0.6656" | 43/64 (0.6719") | 16.9 mm |
| 75% | 0.6526" | 21/32 (0.6562") | 16.6 mm |
| 85% | 0.6396" | 41/64 (0.6406") | 16.2 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.