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3/8-24 UNF Tap Drill Size
The number you came for, then the reasoning behind it.
3/8-24 UNF tap drill — 75% thread
Q
0.3320" · 8.5 mm metric equivalent
How it's calculated
Tap drill = major diameter − (0.01299 × %thread) / TPI → 0.3750 − (0.01299 × 75) / 24 = 0.3344"
The nearest standard drill above that is Q at 0.3320".
All engagement percentages
| % thread | Exact hole | Inch drill | Metric equivalent |
|---|---|---|---|
| 50% | 0.3479" | S (0.3480") | 8.8 mm |
| 65% | 0.3398" | R (0.3390") | 8.6 mm |
| 75% | 0.3344" | Q (0.3320") | 8.5 mm |
| 85% | 0.3290" | 21/64 (0.3281") | 8.4 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.