7/16-28 tap drill size
Letter Y
- Inches
- 0.4040
- Millimetres
- 10.26
- Thread
- 72%
Drill a Letter Y hole before cutting a 7/16-28 thread. That is the standard tap drill for about 75% thread.
No Letter Y bit? A 13/32" bit (0.4062 in) gives about 67% thread, which holds fine in most materials.
7/16-28 thread and hole sizes
| Thread | 7/16 diameter, 28 threads per inch, extra fine (UNEF) |
|---|---|
| Major diameter | 0.4375 in 11.11 mm |
| Pitch | 0.0357 in 0.907 mm |
| Tap drill, about 75% thread | Letter Y 0.4040 in |
| Tap drill, about 50% thread | Letter Z 0.4130 in, for stainless and other hard materials |
| Clearance hole, close fit | 29/64" 0.4531 in |
| Clearance hole, free fit | 15/32" 0.4688 in |
| Nearest metric bit | 10.3 mm |
Why Letter Y
A tap cuts threads into the wall of a hole, so the hole has to be smaller than the screw. For a 7/16-28 thread the hole that leaves exactly 75% thread is 0.4027 in. The closest standard bit is Letter Y at 0.4040 in, which leaves 72%.
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
- 7/16-28 tap
- Letter Y drill bit
- Tap and drill set
- Drill bit size gauge
- Thread pitch gauge
- Cutting oil for tapping
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