Letter W drill size
A letter W drill is 0.3860 in (9.80 mm). It sits between letter V and letter X in the letter series; the nearest metric drill is 9.8 mm.
Letter W 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 |
|---|---|---|---|---|
| 9.8 mm | 0.3858" | −0.004 mm | −0.0002" | Smaller |
| 9.9 mm | 0.3898" | +0.096 mm | +0.0038" | 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 | −0.1580" |
| 3/8" | Fractional | 0.3750" | 9.52 mm | −0.0110" |
| Letter V | Letter | 0.3770" | 9.58 mm | −0.0090" |
| Letter W | Letter | 0.3860" | 9.80 mm | this size |
| 25/64" | Fractional | 0.3906" | 9.92 mm | +0.0046" |
| Letter X | Letter | 0.3970" | 10.08 mm | +0.0110" |
Threads a Letter W 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 |
|---|---|---|---|
| 7/16-20 UNF | 25/64 | 79 % | Soft metals, plastics |
| 7/16-14 UNC | U | 56 % | Hard, tough metals |
Clearance for screws and bolts
Bolts and screws a Letter W 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 |
|---|---|---|---|---|
| M8 bolt | 8 mm | 1.80 mm | 0.90 mm | Normal fit (normal 9, loose 10 mm) |
| 3/8" bolt | 0.3750" | 0.0110" | 0.0055" | – |
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 W drill?
A letter W drill is 0.3860 in, which is 9.80 mm (9.8044 mm exactly).
What is the metric equivalent of a letter W drill?
The nearest metric drill is 9.8 mm, 0.004 mm smaller. The other close size is 9.9 mm.
What thread can I tap with a letter W drill?
It leaves about 79 % thread in 7/16-20 UNF and 56 % in 7/16-14 UNC.
Which fractional drill is closest to letter W?
25/64 in (0.3906 in) is the closest, 0.0046 in larger.
What does a letter W drill clear?
It clears a 3/8" bolt with 0.0110" total clearance (0.0055" per side).