#12-28 tap drill size
#14
- Inches
- 0.1820
- Millimetres
- 4.62
- Thread
- 73%
Drill a #14 hole before cutting a #12-28 thread. That is the standard tap drill for about 75% thread.
No #14 bit? A 3/16" bit (0.1875 in) gives about 61% thread, which holds fine in most materials.
#12-28 thread and hole sizes
| Thread | #12 diameter, 28 threads per inch, fine (UNF) |
|---|---|
| Major diameter | 0.2160 in 5.49 mm |
| Pitch | 0.0357 in 0.907 mm |
| Tap drill, about 75% thread | #14 0.1820 in |
| Tap drill, about 50% thread | #10 0.1935 in, for stainless and other hard materials |
| Clearance hole, close fit | #2 0.2210 in |
| Clearance hole, free fit | #1 0.2280 in |
| Nearest metric bit | 4.6 mm |
Why #14
A tap cuts threads into the wall of a hole, so the hole has to be smaller than the screw. For a #12-28 thread the hole that leaves exactly 75% thread is 0.1812 in. The closest standard bit is #14 at 0.1820 in, which leaves 73%.
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
- #12-28 tap
- #14 drill bit
- Tap and drill set
- Drill bit size gauge
- Thread pitch gauge
- Cutting oil for tapping
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