3/8-24 Tap Drill Size
Unified fine (UNF) · 24 threads per inch · ASME B1.1, class 2B
For a 3/8 tap: 3/8-16 coarse takes a 5/16" drill, 3/8-24 fine takes a Q drill.
What size drill bit for a 3/8-24 tap?
To scale section of the tapped hole
Drill options % of thread
| Drill | Inch | mm | Thread | 2B |
|---|---|---|---|---|
| 8.4 mm | 0.3307 | 8.40 | ✓ | |
| QUse | 0.3320 | 8.43 | ✓ | |
| 8.5 mm | 0.3347 | 8.50 | ✓ | |
| 8.6 mm | 0.3386 | 8.60 | ✓ | |
| R | 0.3390 | 8.61 | ✓ | |
| 8.7 mm | 0.3425 | 8.70 | ✗ | |
| 11/32" | 0.3438 | 8.73 | ✗ |
By material Aluminum, brass, mild steel: Q (79%) · Stainless, tool steel: R (67%)
75% (orange tick) is the usual target. Hard or gummy metals like stainless are often tapped at 60 to 65% to spare the tap; aluminum, brass and plastic take 75% or a little more. The last column checks the hole against the 2B minor diameter limits (0.330 in to 0.340 in).
3/8-24 UNF ASME B1.1, class 2B
| Major diameter | 0.3750 in / 9.53 mm |
|---|---|
| Threads per inch | 24 TPI (pitch 0.0417 in) |
| Pitch diameter (basic) | 0.3479 in |
| Minor diameter 2B | 0.3299–0.3396 in |
| Thread-forming tap drill | T 0.3580 in |
Clearance hole ASME B18.2.8
For a screw that passes through the part. Normal fit is the default; close fit locates the part more accurately but leaves no room for hole position error.
Hole depth rule of thumb
- 0.375 inthread length for full strength in steel (1 × D)
- 0.563 inin cast iron or brass (1.5 × D), up to 0.750 in in aluminum
- +0.208 inextra drill depth below the thread for a plug tap (5 pitches), +0.083 in with a bottoming tap
Shop notes
Q is the chart drill at about 79% thread, a little on the full side for a fine pitch. Most shops will get a better hole from the 8.5 mm drill, which gives about 75% and is already in any kit with an M10 x 1.5 tap, since that is its tap drill too. For stainless or tough steel, an R (about 67%) still keeps the hole inside the 2B minor diameter limits and takes real load off the tap.
The drill to leave in the index is 11/32. It is the fraction that looks right, and at 0.3438 in it is too big: about 58% thread and outside the limits. On a 24-pitch thread, R at 0.3390 in is acceptable and 11/32 at 0.3438 in is out of spec.
The coarse sibling, 3/8-16, is drilled 5/16, and that drill is far too small for 3/8-24: at 0.3125 in it is well under the 0.3299 in minimum minor diameter. If a 3/8 fine tap is suddenly very hard to turn, check which drill made the hole.
You will find 3/8-24 as the mounting thread on many drill chucks of the 3/8 in size, so a chuck that came off a drill usually wants this tap, not the coarse one.
Questions
What size is a letter Q drill bit?
A letter Q drill is 0.3320 in (8.43 mm), between 21/64 (0.3281 in) and 11/32 (0.3438 in). It is the standard tap drill for 3/8-24.