9/16-18 tap drill size
33/64"
- Inches
- 0.5156
- Millimetres
- 13.10
- Thread
- 65%
Drill a 33/64" hole before cutting a 9/16-18 thread. That is the standard tap drill for about 75% thread.
9/16-18 thread and hole sizes
| Thread | 9/16 diameter, 18 threads per inch, fine (UNF) |
|---|---|
| Major diameter | 0.5625 in 14.29 mm |
| Pitch | 0.0556 in 1.411 mm |
| Tap drill, about 75% thread | 33/64" 0.5156 in |
| Tap drill, about 50% thread | 17/32" 0.5312 in, for stainless and other hard materials |
| Clearance hole, close fit | 37/64" 0.5781 in |
| Clearance hole, free fit | 19/32" 0.5938 in |
| Nearest metric bit | 13.1 mm |
Why 33/64"
A tap cuts threads into the wall of a hole, so the hole has to be smaller than the screw. For a 9/16-18 thread the hole that leaves exactly 75% thread is 0.5084 in. The closest standard bit is 33/64" at 0.5156 in, which leaves 65%.
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
- 9/16-18 tap
- 33/64" drill bit
- Tap and drill set
- Drill bit size gauge
- Thread pitch gauge
- Cutting oil for tapping
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