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3/8-16 UNC Tap Drill Size
The number you came for, then the reasoning behind it.
3/8-16 UNC tap drill — 75% thread
5/16
0.3125" · 8 mm metric equivalent
How it's calculated
Tap drill = major diameter − (0.01299 × %thread) / TPI → 0.3750 − (0.01299 × 75) / 16 = 0.3141"
The nearest standard drill above that is 5/16 at 0.3125".
All engagement percentages
| % thread | Exact hole | Inch drill | Metric equivalent |
|---|---|---|---|
| 50% | 0.3344" | Q (0.3320") | 8.5 mm |
| 65% | 0.3222" | P (0.3230") | 8.2 mm |
| 75% | 0.3141" | 5/16 (0.3125") | 8 mm |
| 85% | 0.3060" | N (0.3020") | 7.8 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.