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5/16-24 UNF Tap Drill Size
The number you came for, then the reasoning behind it.
5/16-24 UNF tap drill — 75% thread
I
0.2720" · 6.9 mm metric equivalent
How it's calculated
Tap drill = major diameter − (0.01299 × %thread) / TPI → 0.3125 − (0.01299 × 75) / 24 = 0.2719"
The nearest standard drill above that is I at 0.2720".
All engagement percentages
| % thread | Exact hole | Inch drill | Metric equivalent |
|---|---|---|---|
| 50% | 0.2854" | 9/32 (0.2812") | 7.3 mm |
| 65% | 0.2773" | J (0.2770") | 7 mm |
| 75% | 0.2719" | I (0.2720") | 6.9 mm |
| 85% | 0.2665" | H (0.2660") | 6.8 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.