Home › 5/16-18 UNC › 5/16-18 UNC tap drill size
5/16-18 UNC Tap Drill Size
The number you came for, then the reasoning behind it.
5/16-18 UNC tap drill — 75% thread
F
0.2570" · 6.6 mm metric equivalent
How it's calculated
Tap drill = major diameter − (0.01299 × %thread) / TPI → 0.3125 − (0.01299 × 75) / 18 = 0.2584"
The nearest standard drill above that is F at 0.2570".
All engagement percentages
| % thread | Exact hole | Inch drill | Metric equivalent |
|---|---|---|---|
| 50% | 0.2764" | J (0.2770") | 7 mm |
| 65% | 0.2656" | 17/64 (0.2656") | 6.7 mm |
| 75% | 0.2584" | F (0.2570") | 6.6 mm |
| 85% | 0.2512" | E (0.2500") | 6.4 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.