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5/8-11 UNC Tap Drill Size
The number you came for, then the reasoning behind it.
5/8-11 UNC tap drill — 75% thread
17/32
0.5312" · 13.6 mm metric equivalent
How it's calculated
Tap drill = major diameter − (0.01299 × %thread) / TPI → 0.6250 − (0.01299 × 75) / 11 = 0.5364"
The nearest standard drill above that is 17/32 at 0.5312".
All engagement percentages
| % thread | Exact hole | Inch drill | Metric equivalent |
|---|---|---|---|
| 50% | 0.5660" | 9/16 (0.5625") | 14.4 mm |
| 65% | 0.5482" | 35/64 (0.5469") | 13.9 mm |
| 75% | 0.5364" | 17/32 (0.5312") | 13.6 mm |
| 85% | 0.5246" | 17/32 (0.5312") | 13.3 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.