Letter Q drill size
A letter Q drill is 0.3320 in (8.43 mm). It is the standard tap drill for 3/8-24 UNF; the nearest metric drill is 8.4 mm.
Letter Q at a glance
Decimal, metric and the nearest sizes in the other series.
- Decimal inch
- 0.3320"
- Millimetres
- 8.43 mm 8.4328
- Nearest metric drill
- 8.4 mm −0.033 mm
- Nearest fractional
- 21/64" 0.3281", −0.0039"
- Hole area
- 0.0866 in² 55.85 mm²
- Tap drill for
- 3/8-24 UNF
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 |
|---|---|---|---|---|
| 8.4 mm | 0.3307" | −0.033 mm | −0.0013" | Smaller |
| 8.5 mm | 0.3346" | +0.067 mm | +0.0026" | 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 | −0.1040" |
| Letter P | Letter | 0.3230" | 8.20 mm | −0.0090" |
| 21/64" | Fractional | 0.3281" | 8.33 mm | −0.0039" |
| Letter Q | Letter | 0.3320" | 8.43 mm | this size |
| Letter R | Letter | 0.3390" | 8.61 mm | +0.0070" |
| 11/32" | Fractional | 0.3438" | 8.73 mm | +0.0117" |
Threads a Letter Q 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 |
|---|---|---|---|
| 3/8-24 UNF | Q | 79 % | Standard drill |
| M10 × 1.5 | 8.5 mm | 80 % | Soft metals, plastics |
Clearance for screws and bolts
Bolts and screws a Letter Q 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" | 0.0195" | 0.0098" | – |
| M8 bolt | 8 mm | 0.43 mm | 0.22 mm | Close fit (close 8.4, normal 9 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 Q drill?
A letter Q drill is 0.3320 in, which is 8.43 mm (8.4328 mm exactly).
What is the metric equivalent of a letter Q drill?
The nearest metric drill is 8.4 mm, 0.033 mm smaller. The other close size is 8.5 mm.
What tap uses a letter Q drill?
A letter Q drill is the standard tap drill for 3/8-24 UNF (about 79 % thread).
Which fractional drill is closest to letter Q?
21/64 in (0.3281 in) is the closest, 0.0039 in smaller.
What does a letter Q drill clear?
It clears a M8 bolt with 0.43 mm total clearance (0.22 mm per side); for metric bolts: close fit.