#31 Drill Size
number drill
#31 0.120 in 3.05 mm
A #31 drill measures 0.120 in. It taps 2 threads in this chart.
What a #31 drill taps
Every thread this drill cuts between 50% and 85% of thread, deepest first. Outside that band it is the wrong drill for the job, so nothing below that line is listed.
| Thread | % thread | Chart drill | Chart % |
|---|---|---|---|
| M3.5 x 0.6 | 58.0% | 2.9 mm | 77.0% |
| #6-40 | 55.4% | #33 | 77.0% |
Other jobs for a #31
- Hard-material tap drill
- The size to use in stainless or tool steel for #6-40, where it leaves a shallower thread than the standard drill and cuts tapping torque.
- Clearance hole
- The through hole a bolt passes in, not a tapped hole: close fit for #4-40, close fit for #4-48. See the clearance hole chart.
Drills either side
#31 drill: common questions
What size is a #31 drill?
0.120 inches, which is 3.05 mm. It is a number drill.
What can I tap with a #31 drill?
2 threads: M3.5 x 0.6 at 58.0%, #6-40 at 55.4%.
Is a #31 drill used for clearance holes?
Yes: close fit for #4-40, close fit for #4-48. A clearance hole is not tapped; the bolt passes through it.
What drill is next to a #31?
Down: 3 mm at 0.1181 in. Up: #30 at 0.1285 in.