5/8-18 tap drill size
37/64"
- Inches
- 0.5781
- Millimetres
- 14.68
- Thread
- 65%
Drill a 37/64" hole before cutting a 5/8-18 thread. That is the standard tap drill for about 75% thread.
5/8-18 thread and hole sizes
| Thread | 5/8 diameter, 18 threads per inch, fine (UNF) |
|---|---|
| Major diameter | 0.6250 in 15.88 mm |
| Pitch | 0.0556 in 1.411 mm |
| Tap drill, about 75% thread | 37/64" 0.5781 in |
| Tap drill, about 50% thread | 19/32" 0.5938 in, for stainless and other hard materials |
| Clearance hole, close fit | 41/64" 0.6406 in |
| Clearance hole, free fit | 21/32" 0.6562 in |
| Nearest metric bit | 14.7 mm |
Why 37/64"
A tap cuts threads into the wall of a hole, so the hole has to be smaller than the screw. For a 5/8-18 thread the hole that leaves exactly 75% thread is 0.5709 in. The closest standard bit is 37/64" at 0.5781 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
- 5/8-18 tap
- 37/64" drill bit
- Tap and drill set
- Drill bit size gauge
- Thread pitch gauge
- Cutting oil for tapping
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