7/8-9 tap drill size
49/64"
- Inches
- 0.7656
- Millimetres
- 19.45
- Thread
- 76%
Drill a 49/64" hole before cutting a 7/8-9 thread. That is the standard tap drill for about 75% thread.
7/8-9 thread and hole sizes
| Thread | 7/8 diameter, 9 threads per inch, coarse (UNC) |
|---|---|
| Major diameter | 0.8750 in 22.22 mm |
| Pitch | 0.1111 in 2.822 mm |
| Tap drill, about 75% thread | 49/64" 0.7656 in |
| Tap drill, about 50% thread | 51/64" 0.7969 in, for stainless and other hard materials |
| Clearance hole, close fit | 57/64" 0.8906 in |
| Clearance hole, free fit | 29/32" 0.9062 in |
| Nearest metric bit | 19.4 mm |
Why 49/64"
A tap cuts threads into the wall of a hole, so the hole has to be smaller than the screw. For a 7/8-9 thread the hole that leaves exactly 75% thread is 0.7668 in. The closest standard bit is 49/64" at 0.7656 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
- 7/8-9 tap
- 49/64" drill bit
- Tap and drill set
- Drill bit size gauge
- Thread pitch gauge
- Cutting oil for tapping
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