#6-40 UNF Thread
Recommended tap drill #33
Tap drill Ø 0.1130″ / 2.87 mm
Major Ø 0.1380″ / 3.505 mm
Threads per inch 40
The chart recommendation gives approximately 77% thread engagement — the conventional target for average materials. It is not the only valid choice; see the options below.
Specification
| Designation | #6-40 UNF |
|---|---|
| Thread family | Unified National Fine (UNF) |
| Nominal size | #6 |
| Basic major diameter | 0.1380″ / 3.505 mm |
| Threads per inch / pitch | 40 TPI (0.635 mm pitch) |
| Recommended tap drill | #33 (0.1130″ / 2.87 mm) |
Tap drill options by thread engagement
Any of these standard drills produces a usable engagement for #6-40 UNF; smaller drills give stronger threads but higher tapping torque. Percentages use the conventional approximation (60° thread form).
| Drill | Decimal in | mm | Approx. engagement |
|---|---|---|---|
| #34 | 0.1110 | 2.819 | 83.1% |
| #33 — chart recommendation | 0.1130 | 2.87 | 77% |
| 2.9 mm | 0.1142 | 2.9 | 73.4% |
| #32 | 0.1160 | 2.946 | 67.7% |
| 3 mm | 0.1181 | 3 | 61.2% |
Nearest drills to the tap drill in other systems
| Drill | Size | vs #33 | Resulting engagement |
|---|---|---|---|
| 2.9 mm | 0.1142″ / 2.9 mm | +0.03 mm | 73.4% |
| 7/64″ | 0.1094″ / 2.778 mm | −0.092 mm | 88.1% |
Clearance holes
For a bolt or screw of this size to pass through freely. Recommended drills per ASME B18.2.8: close, normal, and loose fits.
Related threads
- #6-32 UNC — same nominal size, 32 TPI (Unified National Coarse (UNC)) · compare
- #5-44 UNF — next size down in UNF
- #8-36 UNF — next size up in UNF