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9/16-18 UNF Tap Drill Size
The number you came for, then the reasoning behind it.
9/16-18 UNF tap drill — 75% thread
33/64
0.5156" · 12.9 mm metric equivalent
How it's calculated
Tap drill = major diameter − (0.01299 × %thread) / TPI → 0.5625 − (0.01299 × 75) / 18 = 0.5084"
The nearest standard drill above that is 33/64 at 0.5156".
All engagement percentages
| % thread | Exact hole | Inch drill | Metric equivalent |
|---|---|---|---|
| 50% | 0.5264" | 17/32 (0.5312") | 13.4 mm |
| 65% | 0.5156" | 33/64 (0.5156") | 13.1 mm |
| 75% | 0.5084" | 33/64 (0.5156") | 12.9 mm |
| 85% | 0.5012" | 1/2 (0.5000") | 12.7 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.