Home › 1/4-20 UNC › 1/4-20 UNC tap drill size
1/4-20 UNC Tap Drill Size
The number you came for, then the reasoning behind it.
1/4-20 UNC tap drill — 75% thread
#7
0.2010" · 5.1 mm metric equivalent
How it's calculated
Tap drill = major diameter − (0.01299 × %thread) / TPI → 0.2500 − (0.01299 × 75) / 20 = 0.2013"
The nearest standard drill above that is #7 at 0.2010".
All engagement percentages
| % thread | Exact hole | Inch drill | Metric equivalent |
|---|---|---|---|
| 50% | 0.2175" | 7/32 (0.2188") | 5.5 mm |
| 65% | 0.2078" | #4 (0.2090") | 5.3 mm |
| 75% | 0.2013" | #7 (0.2010") | 5.1 mm |
| 85% | 0.1948" | #9 (0.1960") | 4.9 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.