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1-1/2-6 UNC Tap Drill Size
The number you came for, then the reasoning behind it.
1-1/2-6 UNC tap drill — 75% thread
43/32
1.3438" · 34 mm metric equivalent
How it's calculated
Tap drill = major diameter − (0.01299 × %thread) / TPI → 1.5000 − (0.01299 × 75) / 6 = 1.3376"
The nearest standard drill above that is 43/32 at 1.3438".
All engagement percentages
| % thread | Exact hole | Inch drill | Metric equivalent |
|---|---|---|---|
| 50% | 1.3918" | 89/64 (1.3906") | 35.5 mm |
| 65% | 1.3593" | 87/64 (1.3594") | 34.5 mm |
| 75% | 1.3376" | 43/32 (1.3438") | 34 mm |
| 85% | 1.3160" | 21/16 (1.3125") | 33.5 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.