The tap drill for a 1/4-20 UNC thread is #7 drill (0.2010 inch, 5.11 mm), which gives about 75% thread, the standard every tap drill chart uses.
Taps and dies for 1/4-20
Which drill gives which percentage of thread?
| Target thread | Ideal drill (in) | Nearest drill | Drill (in) | Actual thread |
|---|---|---|---|---|
| 60% | 0.2110 | 5.4 mm | 0.2126 | 58% |
| 65% | 0.2078 | 5.3 mm | 0.2087 | 64% |
| 70% | 0.2045 | 5.2 mm | 0.2047 | 70% |
| 75% | 0.2013 | #7 | 0.2010 | 75% |
| 80% | 0.1980 | #8 | 0.1990 | 79% |
What are the 1/4-20 thread dimensions?
| Series | Unified UNC |
| Pitch | 20 TPI (1.270 mm pitch) |
| Major diameter | 0.2500" / 6.35 mm |
| Basic pitch diameter | 0.2175" / 5.53 mm |
| Basic minor diameter (external) | 0.1850" / 4.70 mm |
| Standard tap drill (about 75%) | #7 = 0.2010" |
| Clearance, close fit (rule of thumb) | 6.6 mm = 0.2598" |
| Clearance, free fit (rule of thumb) | 7.0 mm = 0.2756" |
How do you tap 1/4-20 cleanly?
This is the easy middle of the size range: a hand tap in a T-handle or bar wrench, started square and fed with cutting oil, will cut it cleanly in aluminum, brass and mild steel. In stainless, drop to about 65% thread (the larger drill below) and never let the tap rub without cutting.
Other threads at this diameter: 1/4-28.
Drills and cutting fluid for tapping
Frequently asked questions
What size drill for a 1/4-20 tap?
The tap drill for a 1/4-20 UNC thread is #7 drill (0.2010 inch, 5.11 mm), which gives about 75% thread, the standard every tap drill chart uses. For hard or tough material a slightly larger drill giving about 65% thread taps much more easily; the table on this page lists it.
What size clearance hole for 1/4-20?
For a bolt to pass through, a close fit is about 6.6 mm (0.2598") and a looser free fit about 7.0 mm (0.2756"). These are rule-of-thumb sizes of about 3% and 8% over the 0.2500" major diameter; use the close fit where location matters and the free fit where parts must line up easily.
What is the minor diameter of 1/4-20?
The basic minor diameter of a 1/4-20 external thread is about 0.1850" (4.70 mm) and the basic pitch diameter about 0.2175" (5.53 mm). An internal thread's minor diameter is set by the tap drill, which is why the drill decides how much thread is cut.
Nearby thread sizes
Keep going
Tap drills per the standard 75% thread convention used by published charts. Clearance sizes are rules of thumb, not ASME B18.2.8 values. Not professional advice.