#8-36 tap drill size
#29
- Inches
- 0.1360
- Millimetres
- 3.45
- Thread
- 78%
Drill a #29 hole before cutting a #8-36 thread. That is the standard tap drill for about 75% thread.
No #29 bit? A 9/64" bit (0.1406 in) gives about 65% thread, which holds fine in most materials.
#8-36 thread and hole sizes
| Thread | #8 diameter, 36 threads per inch, fine (UNF) |
|---|---|
| Major diameter | 0.1640 in 4.17 mm |
| Pitch | 0.0278 in 0.706 mm |
| Tap drill, about 75% thread | #29 0.1360 in |
| Tap drill, about 50% thread | #26 0.1470 in, for stainless and other hard materials |
| Clearance hole, close fit | #18 0.1695 in |
| Clearance hole, free fit | #16 0.1770 in |
| Nearest metric bit | 3.5 mm |
Why #29
A tap cuts threads into the wall of a hole, so the hole has to be smaller than the screw. For a #8-36 thread the hole that leaves exactly 75% thread is 0.1369 in. The closest standard bit is #29 at 0.1360 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
- #8-36 tap
- #29 drill bit
- Tap and drill set
- Drill bit size gauge
- Thread pitch gauge
- Cutting oil for tapping
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