Letter O drill size
A letter O drill is 0.3160 in (8.03 mm). It sits between letter N and letter P in the letter series; the nearest metric drill is 8.0 mm.
Letter O at a glance
Decimal, metric and the nearest sizes in the other series.
Nearest metric drills
Metric drills step by 0.05 mm below 3 mm and by 0.1 mm from 3 mm. Difference is metric minus inch size.
| Metric drill | Inch | Difference | In inches | Hole |
|---|---|---|---|---|
| 8.0 mm | 0.3150" | −0.026 mm | −0.0010" | Smaller |
| 8.1 mm | 0.3189" | +0.074 mm | +0.0029" | Larger |
Nearby inch drills
The next size down and up in each inch series, so you can pick a substitute from the drawer.
| Drill | Series | Inch | mm | Difference |
|---|---|---|---|---|
| #1 | Number | 0.2280" | 5.79 mm | −0.0880" |
| Letter N | Letter | 0.3020" | 7.67 mm | −0.0140" |
| 5/16" | Fractional | 0.3125" | 7.94 mm | −0.0035" |
| Letter O | Letter | 0.3160" | 8.03 mm | this size |
| Letter P | Letter | 0.3230" | 8.20 mm | +0.0070" |
| 21/64" | Fractional | 0.3281" | 8.33 mm | +0.0121" |
Threads a Letter O drill can tap
Threads from the tap tables on this site that this drill can tap, with the thread percentage it leaves (55–85 % shown).
| Thread | Standard drill | Thread with this drill | Suits |
|---|---|---|---|
| 3/8-16 UNC | 5/16 | 73 % | Good in steel |
Clearance for screws and bolts
Bolts and screws a Letter O hole clears, with the diametral clearance and the gap each side. Metric sizes are compared with ISO 273.
| Fastener | Diameter | Clearance | Per side | ISO 273, metric only |
|---|---|---|---|---|
| 5/16" bolt | 0.3125" | 0.0035" | 0.0018" | – |
| M8 bolt | 8 mm | 0.03 mm | 0.01 mm | Tighter than close fit (8.4 mm) |
Inch clearance is the drill size minus the basic major diameter. A drilled hole usually cuts slightly oversize.
Letter drill index
Every size in the series, holes drawn to scale.
Common questions
What size is a letter O drill?
A letter O drill is 0.3160 in, which is 8.03 mm (8.0264 mm exactly).
What is the metric equivalent of a letter O drill?
The nearest metric drill is 8.0 mm, 0.026 mm smaller. The other close size is 8.1 mm.
What thread can I tap with a letter O drill?
It leaves about 73 % thread in 3/8-16 UNC.
Which fractional drill is closest to letter O?
5/16 in (0.3125 in) is the closest, 0.0035 in smaller.
What does a letter O drill clear?
It clears a M8 bolt with 0.03 mm total clearance (0.01 mm per side); for metric bolts: tighter than close fit.