1-8 tap drill size
7/8"
- Inches
- 0.8750
- Millimetres
- 22.22
- Thread
- 77%
Drill a 7/8" hole before cutting a 1-8 thread. That is the standard tap drill for about 75% thread.
1-8 thread and hole sizes
| Thread | 1 diameter, 8 threads per inch, coarse (UNC) |
|---|---|
| Major diameter | 1.0000 in 25.40 mm |
| Pitch | 0.1250 in 3.175 mm |
| Tap drill, about 75% thread | 7/8" 0.8750 in |
| Tap drill, about 50% thread | 59/64" 0.9219 in, for stainless and other hard materials |
| Clearance hole, close fit | 1-1/64" 1.0156 in |
| Clearance hole, free fit | 1-1/32" 1.0312 in |
| Nearest metric bit | 22.2 mm |
Why 7/8"
A tap cuts threads into the wall of a hole, so the hole has to be smaller than the screw. For a 1-8 thread the hole that leaves exactly 75% thread is 0.8782 in. The closest standard bit is 7/8" at 0.8750 in, which leaves 77%.
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
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