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3/4-16 UNF Tap Drill Size
The number you came for, then the reasoning behind it.
3/4-16 UNF tap drill — 75% thread
11/16
0.6875" · 17.5 mm metric equivalent
How it's calculated
Tap drill = major diameter − (0.01299 × %thread) / TPI → 0.7500 − (0.01299 × 75) / 16 = 0.6891"
The nearest standard drill above that is 11/16 at 0.6875".
All engagement percentages
| % thread | Exact hole | Inch drill | Metric equivalent |
|---|---|---|---|
| 50% | 0.7094" | 45/64 (0.7031") | 18 mm |
| 65% | 0.6972" | 45/64 (0.7031") | 17.7 mm |
| 75% | 0.6891" | 11/16 (0.6875") | 17.5 mm |
| 85% | 0.6810" | 11/16 (0.6875") | 17.3 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.