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1-1/8-12 UNF Tap Drill Size
The number you came for, then the reasoning behind it.
1-1/8-12 UNF tap drill — 75% thread
67/64
1.0469" · 26.5 mm metric equivalent
How it's calculated
Tap drill = major diameter − (0.01299 × %thread) / TPI → 1.1250 − (0.01299 × 75) / 12 = 1.0438"
The nearest standard drill above that is 67/64 at 1.0469".
All engagement percentages
| % thread | Exact hole | Inch drill | Metric equivalent |
|---|---|---|---|
| 50% | 1.0709" | 69/64 (1.0781") | 27 mm |
| 65% | 1.0546" | 67/64 (1.0469") | 27 mm |
| 75% | 1.0438" | 67/64 (1.0469") | 26.5 mm |
| 85% | 1.0330" | 33/32 (1.0312") | 26 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.