Letter U drill size
A letter U drill is 0.3680 in (9.35 mm). It is the standard tap drill for 7/16-14 UNC; the nearest metric drill is 9.3 mm.
Letter U at a glance
Decimal, metric and the nearest sizes in the other series.
- Decimal inch
- 0.3680"
- Millimetres
- 9.35 mm 9.3472
- Nearest metric drill
- 9.3 mm −0.047 mm
- Nearest fractional
- 3/8" 0.3750", +0.0070"
- Hole area
- 0.106 in² 68.6 mm²
- Tap drill for
- 7/16-14 UNC
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 |
|---|---|---|---|---|
| 9.3 mm | 0.3661" | −0.047 mm | −0.0019" | Smaller |
| 9.4 mm | 0.3701" | +0.053 mm | +0.0021" | 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 | −0.1400" |
| Letter T | Letter | 0.3580" | 9.09 mm | −0.0100" |
| 23/64" | Fractional | 0.3594" | 9.13 mm | −0.0086" |
| Letter U | Letter | 0.3680" | 9.35 mm | this size |
| 3/8" | Fractional | 0.3750" | 9.52 mm | +0.0070" |
| Letter V | Letter | 0.3770" | 9.58 mm | +0.0090" |
Threads a Letter U 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 |
|---|---|---|---|
| 7/16-14 UNC | U | 75 % | Standard drill |
Clearance for screws and bolts
Bolts and screws a Letter U 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" | 0.0555" | 0.0277" | – |
| M8 bolt | 8 mm | 1.35 mm | 0.67 mm | Normal fit (normal 9, loose 10 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 U drill?
A letter U drill is 0.3680 in, which is 9.35 mm (9.3472 mm exactly).
What is the metric equivalent of a letter U drill?
The nearest metric drill is 9.3 mm, 0.047 mm smaller. The other close size is 9.4 mm.
What tap uses a letter U drill?
A letter U drill is the standard tap drill for 7/16-14 UNC (about 75 % thread).
Which fractional drill is closest to letter U?
3/8 in (0.3750 in) is the closest, 0.0070 in larger.
What does a letter U drill clear?
It clears a M8 bolt with 1.35 mm total clearance (0.67 mm per side); for metric bolts: normal fit.