1/4-20 tap drill size
#7
- Inches
- 0.2010
- Millimetres
- 5.11
- Thread
- 75%
Drill a #7 hole before cutting a 1/4-20 thread. That is the standard tap drill for about 75% thread.
No #7 bit? A 13/64" bit (0.2031 in) gives about 72% thread, which holds fine in most materials.
1/4-20 thread and hole sizes
| Thread | 1/4 diameter, 20 threads per inch, coarse (UNC) |
|---|---|
| Major diameter | 0.2500 in 6.35 mm |
| Pitch | 0.0500 in 1.270 mm |
| Tap drill, about 75% thread | #7 0.2010 in |
| Tap drill, about 50% thread | 7/32" 0.2188 in, for stainless and other hard materials |
| Clearance hole, close fit | Letter F 0.2570 in |
| Clearance hole, free fit | Letter H 0.2660 in |
| Nearest metric bit | 5.1 mm |
Why #7
A tap cuts threads into the wall of a hole, so the hole has to be smaller than the screw. For a 1/4-20 thread the hole that leaves exactly 75% thread is 0.2013 in. The closest standard bit is #7 at 0.2010 in, which leaves 75%.
More thread than that adds almost no strength and makes the tap much easier to break. In hard or tough material, go one step larger.
Tools for this job
- 1/4-20 tap
- #7 drill bit
- Tap and drill set
- Drill bit size gauge
- Thread pitch gauge
- Cutting oil for tapping
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