1/4-28 tap drill size
#3
- Inches
- 0.2130
- Millimetres
- 5.41
- Thread
- 80%
Drill a #3 hole before cutting a 1/4-28 thread. That is the standard tap drill for about 75% thread.
No #3 bit? A 7/32" bit (0.2188 in) gives about 67% thread, which holds fine in most materials.
1/4-28 thread and hole sizes
| Thread | 1/4 diameter, 28 threads per inch, fine (UNF) |
|---|---|
| Major diameter | 0.2500 in 6.35 mm |
| Pitch | 0.0357 in 0.907 mm |
| Tap drill, about 75% thread | #3 0.2130 in |
| Tap drill, about 50% thread | #1 0.2280 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 | 5.4 mm |
Why #3
A tap cuts threads into the wall of a hole, so the hole has to be smaller than the screw. For a 1/4-28 thread the hole that leaves exactly 75% thread is 0.2152 in. The closest standard bit is #3 at 0.2130 in, which leaves 80%.
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
- 1/4-28 tap
- #3 drill bit
- Tap and drill set
- Drill bit size gauge
- Thread pitch gauge
- Cutting oil for tapping
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