1/2-13 tap drill size
27/64"
- Inches
- 0.4219
- Millimetres
- 10.72
- Thread
- 78%
Drill a 27/64" hole before cutting a 1/2-13 thread. That is the standard tap drill for about 75% thread.
1/2-13 thread and hole sizes
| Thread | 1/2 diameter, 13 threads per inch, coarse (UNC) |
|---|---|
| Major diameter | 0.5000 in 12.70 mm |
| Pitch | 0.0769 in 1.954 mm |
| Tap drill, about 75% thread | 27/64" 0.4219 in |
| Tap drill, about 50% thread | 29/64" 0.4531 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 | 10.7 mm |
Why 27/64"
A tap cuts threads into the wall of a hole, so the hole has to be smaller than the screw. For a 1/2-13 thread the hole that leaves exactly 75% thread is 0.4251 in. The closest standard bit is 27/64" at 0.4219 in, which leaves 78%.
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
- 1/2-13 tap
- 27/64" drill bit
- Tap and drill set
- Drill bit size gauge
- Thread pitch gauge
- Cutting oil for tapping
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