1-1/4-7 tap drill size
1-7/64"
- Inches
- 1.1094
- Millimetres
- 28.18
- Thread
- 76%
Drill a 1-7/64" hole before cutting a 1-1/4-7 thread. That is the standard tap drill for about 75% thread.
1-1/4-7 thread and hole sizes
| Thread | 1-1/4 diameter, 7 threads per inch, coarse (UNC) |
|---|---|
| Major diameter | 1.2500 in 31.75 mm |
| Pitch | 0.1429 in 3.629 mm |
| Tap drill, about 75% thread | 1-7/64" 1.1094 in |
| Nearest metric bit | 28.2 mm |
Why 1-7/64"
A tap cuts threads into the wall of a hole, so the hole has to be smaller than the screw. For a 1-1/4-7 thread the hole that leaves exactly 75% thread is 1.1108 in. The closest standard bit is 1-7/64" at 1.1094 in, which leaves 76%.
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
- 1-1/4-7 tap
- 1-7/64" drill bit
- Tap and drill set
- Drill bit size gauge
- Thread pitch gauge
- Cutting oil for tapping
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