1-3/8-6 tap drill size
1-7/32"
- Inches
- 1.2188
- Millimetres
- 30.96
- Thread
- 72%
Drill a 1-7/32" hole before cutting a 1-3/8-6 thread. That is the standard tap drill for about 75% thread.
1-3/8-6 thread and hole sizes
| Thread | 1-3/8 diameter, 6 threads per inch, coarse (UNC) |
|---|---|
| Major diameter | 1.3750 in 34.92 mm |
| Pitch | 0.1667 in 4.233 mm |
| Tap drill, about 75% thread | 1-7/32" 1.2188 in |
| Nearest metric bit | 31 mm |
Why 1-7/32"
A tap cuts threads into the wall of a hole, so the hole has to be smaller than the screw. For a 1-3/8-6 thread the hole that leaves exactly 75% thread is 1.2126 in. The closest standard bit is 1-7/32" at 1.2188 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
- 1-3/8-6 tap
- 1-7/32" drill bit
- Tap and drill set
- Drill bit size gauge
- Thread pitch gauge
- Cutting oil for tapping
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