Letter X drill size
A letter X drill is 0.3970 in (10.08 mm). It sits between letter W and letter Y in the letter series; the nearest metric drill is 10.1 mm.
Letter X 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 |
|---|---|---|---|---|
| 10.0 mm | 0.3937" | −0.084 mm | −0.0033" | Smaller |
| 10.1 mm | 0.3976" | +0.016 mm | +0.0006" | 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 | −0.1690" |
| Letter W | Letter | 0.3860" | 9.80 mm | −0.0110" |
| 25/64" | Fractional | 0.3906" | 9.92 mm | −0.0064" |
| Letter X | Letter | 0.3970" | 10.08 mm | this size |
| Letter Y | Letter | 0.4040" | 10.26 mm | +0.0070" |
| 13/32" | Fractional | 0.4062" | 10.32 mm | +0.0092" |
Threads a Letter X 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 |
|---|---|---|---|
| M12 × 1.75 | 10.2 mm | 84 % | Soft metals, plastics |
| 7/16-20 UNF | 25/64 | 62 % | Hard, tough metals |
Clearance for screws and bolts
Bolts and screws a Letter X 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 |
|---|---|---|---|---|
| 3/8" bolt | 0.3750" | 0.0220" | 0.0110" | – |
| M10 bolt | 10 mm | 0.08 mm | 0.04 mm | Tighter than close fit (10.5 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 X drill?
A letter X drill is 0.3970 in, which is 10.08 mm (10.0838 mm exactly).
What is the metric equivalent of a letter X drill?
The nearest metric drill is 10.1 mm, 0.016 mm larger. The other close size is 10.0 mm.
What thread can I tap with a letter X drill?
It leaves about 84 % thread in M12 × 1.75 and 62 % in 7/16-20 UNF.
Which fractional drill is closest to letter X?
25/64 in (0.3906 in) is the closest, 0.0064 in smaller.
What does a letter X drill clear?
It clears a M10 bolt with 0.08 mm total clearance (0.04 mm per side); for metric bolts: tighter than close fit.