3/8-24 UNF tap drill size
Use a letter Q drill (0.3320 in, 8.43 mm) for 3/8-24 UNF. It is the standard chart size and leaves about 79% thread, enough for full practical strength without excessive tapping torque.
| Drill | inch | mm | Thread % | Typical use |
|---|---|---|---|---|
| 21/64" | 0.3281 | 8.33 | 86.6% | Tight: high tapping torque, tap breakage risk |
| 8.4 mm (metric drill) | 0.3307 | 8.4 | 81.8% | General purpose |
| Q (chart drill) | 0.3320 | 8.43 | 79.4% | General purpose |
| R | 0.3390 | 8.61 | 66.5% | Hard or tough material, deep or blind holes |
| 11/32" | 0.3438 | 8.73 | 57.7% | Low engagement: check strength |
| Thread | Tap drill | Tap drill (mm) | Clearance close / free |
|---|---|---|---|
| 1/4-28 UNF | #3 (0.2130 in) | 5.41 mm | F / H |
| 5/16-24 UNF | I (0.2720 in) | 6.91 mm | P / Q |
| 3/8-24 UNF (this page) | Q (0.3320 in) | 8.43 mm | W / X |
| 7/16-20 UNF | 25/64" (0.3906 in) | 9.92 mm | 29/64" / 15/32" |
| 1/2-20 UNF | 29/64" (0.4531 in) | 11.51 mm | 33/64" / 17/32" |
Practical notes
Spiral-point taps suit through holes; spiral-flute taps lift chips out of blind holes. In a blind hole, drill a few threads deeper than the full-thread depth you need.
Typical engagement lengths: about 1 × diameter in steel, 1.5 × in cast iron or brass and 2 × in aluminium. In tough stainless or hardened steel a slightly larger drill (60–70% thread) cuts torque and tap breakage with little loss of strength.
Thread-forming (roll) taps need a larger hole, roughly 0.3542 in (9.0 mm, D − 0.5 × pitch). Check the tap maker's chart; forming suits ductile metals such as aluminium, mild steel and brass, not cast iron.
UNF (fine) threads have more threads per inch than UNC, a larger tensile stress area and better resistance to loosening, but they strip more easily in soft material and are more sensitive to drill size.
Clearance drills for a 3/8 screw: close fit letter W (0.3860 in, 9.8 mm), free fit letter X (0.3970 in, 10.08 mm).
FAQ
What size drill for a 3/8-24 UNF tap?
The standard tap drill for 3/8-24 UNF is a letter Q drill, 0.3320 in (8.43 mm), which gives about 79% thread.
What clearance hole for a 3/8 UNF screw?
Use letter W (0.3860 in, 9.8 mm) for a close fit or letter X (0.3970 in, 10.08 mm) for a free fit. Clearance depends on diameter only, so it is the same for UNC and UNF 3/8 screws.
What metric drill can I use for 3/8-24 UNF?
The closest metric drill is 8.4 mm (0.3307 in), giving about 82% thread on 3/8-24 UNF.
What is the difference between 3/8-24 UNF and 3/8-16 UNC?
Both are 3/8 diameter. 3/8-24 UNF has 24 threads per inch and uses a Q tap drill (0.3320 in); 3/8-16 UNC has 16 TPI and uses a 5/16" drill (0.3125 in). The threads are not interchangeable.
Sources: ASME B1.1 — Unified inch screw threads (UN and UNR form) · Machinery's Handbook — tap drill sizes and percentage-of-thread formula · ASME B94.11M — twist drills (number and letter drill sizes) · Common machinist clearance-drill chart (close / free fit)
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