1/2-20 tap drill size
29/64"
- Inches
- 0.4531
- Millimetres
- 11.51
- Thread
- 72%
Drill a 29/64" hole before cutting a 1/2-20 thread. That is the standard tap drill for about 75% thread.
1/2-20 thread and hole sizes
| Thread | 1/2 diameter, 20 threads per inch, fine (UNF) |
|---|---|
| Major diameter | 0.5000 in 12.70 mm |
| Pitch | 0.0500 in 1.270 mm |
| Tap drill, about 75% thread | 29/64" 0.4531 in |
| Tap drill, about 50% thread | 15/32" 0.4688 in, for stainless and other hard materials |
| Clearance hole, close fit | 33/64" 0.5156 in |
| Clearance hole, free fit | 17/32" 0.5312 in |
| Nearest metric bit | 11.5 mm |
Why 29/64"
A tap cuts threads into the wall of a hole, so the hole has to be smaller than the screw. For a 1/2-20 thread the hole that leaves exactly 75% thread is 0.4513 in. The closest standard bit is 29/64" at 0.4531 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
- 1/2-20 tap
- 29/64" drill bit
- Tap and drill set
- Drill bit size gauge
- Thread pitch gauge
- Cutting oil for tapping
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