Q drill bit size
0.332 in
8.43 mm · letter drill · tapping drill for 3/8-24
| Inches | 0.332 in |
| Millimetres | 8.43 mm |
| Nearest metric drill | 8.4 mm |
| System | Letter (A to Z) |
| Taps | 3/8-24 |
What Q means
A bit marked Q is a letter drill. Letters run A to Z and get larger as they advance, covering 0.234 to 0.413 inches — the gap between the largest number drill and the point where fractional sizes become fine enough to be useful.
The same hole in the other systems
Three drill systems overlap across this range and a workshop rarely has all three. The nearest bit of each other kind to Q:
| System | Nearest size | Difference |
|---|---|---|
| number | #1 — 0.228 in | 0.104 in smaller |
| fractional | 21/64" — 0.3281 in | 0.0039 in smaller |
| metric | 8.4 mm | 0.033 mm smaller |
The closest substitute is 0.0039 inches away. For a clearance hole that is nothing; for a tapping hole it moves the thread engagement by several per cent, which is the difference between an easy tap and a broken one.
What Q is for
Q is the standard tapping drill for 3/8-24 at 75% thread engagement. That is not a coincidence of the tables: the theoretical figure for 3/8-24 works out at 0.3344 inches, and Q at 0.332 is the nearest drill that exists.
What gets drilled at 0.332 inches
Clearance holes for bolts up to about half an inch, and tapping drills for the larger unified threads. Above a quarter inch a hand drill starts to want two hands and a slower speed, and the bit wants a pilot hole.
Either side of it
| Drill | Inches | Millimetres |
|---|---|---|
| O | 0.316 in | 8.03 mm |
| P | 0.323 in | 8.2 mm |
| 21/64" | 0.3281 in | 8.33 mm |
| Q | 0.332 in | 8.43 mm |
| R | 0.339 in | 8.61 mm |
| 11/32" | 0.3438 in | 8.73 mm |
| S | 0.348 in | 8.84 mm |
Frequently asked questions
What size is a Q drill bit?
0.332 inches, which is 8.43 mm. Letter drills run A to Z and get larger as the letter advances, filling the gap between the largest number drill and the fractional sizes.
What is a Q drill bit in mm?
8.43 mm. A metric index will offer 8.4 mm as the nearest stocked size, which is 0.033 mm smaller — enough to matter when tapping and not enough to matter for a clearance hole.
What tap does a Q drill bit go with?
3/8-24 at the standard 75% thread. That thread comes out at a theoretical diameter this drill is the nearest stocked size to.
What is the next size up from Q?
R at 0.339 inches — 0.007 inches larger. Below it is 21/64" at 0.3281.
Other sizes
- R drill bit — 0.339 in, 8.61 mm
- 11/32" drill bit — 0.3438 in, 8.73 mm
- S drill bit — 0.348 in, 8.84 mm
- T drill bit — 0.358 in, 9.09 mm
- 23/64" drill bit — 0.3594 in, 9.13 mm
- U drill bit — 0.368 in, 9.35 mm
- 3/8" drill bit — 0.375 in, 9.52 mm
- V drill bit — 0.377 in, 9.58 mm