5/16-24 UNF tap drill size
Use a letter I drill (0.2720 in, 6.91 mm) for 5/16-24 UNF. It is the standard chart size and leaves about 75% thread, enough for full practical strength without excessive tapping torque.
| Drill | inch | mm | Thread % | Typical use |
|---|---|---|---|---|
| 17/64" | 0.2656 | 6.75 | 86.6% | Tight: high tapping torque, tap breakage risk |
| H | 0.2660 | 6.76 | 85.9% | Tight: high tapping torque, tap breakage risk |
| 6.9 mm (metric drill) | 0.2717 | 6.9 | 75.5% | General purpose |
| I (chart drill) | 0.2720 | 6.91 | 74.8% | General purpose |
| J | 0.2770 | 7.04 | 65.6% | Hard or tough material, deep or blind holes |
| K | 0.2810 | 7.14 | 58.2% | Low engagement: check strength |
| 9/32" | 0.2813 | 7.14 | 57.7% | Low engagement: check strength |
| Thread | Tap drill | Tap drill (mm) | Clearance close / free |
|---|---|---|---|
| #12-28 UNF | #14 (0.1820 in) | 4.62 mm | #2 / #1 |
| 1/4-28 UNF | #3 (0.2130 in) | 5.41 mm | F / H |
| 5/16-24 UNF (this page) | I (0.2720 in) | 6.91 mm | P / Q |
| 3/8-24 UNF | 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" |
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 0.2917 in (7.41 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 5/16 screw: close fit letter P (0.3230 in, 8.2 mm), free fit letter Q (0.3320 in, 8.43 mm).
FAQ
What size drill for a 5/16-24 UNF tap?
The standard tap drill for 5/16-24 UNF is a letter I drill, 0.2720 in (6.91 mm), which gives about 75% thread.
What clearance hole for a 5/16 UNF screw?
Use letter P (0.3230 in, 8.2 mm) for a close fit or letter Q (0.3320 in, 8.43 mm) for a free fit. Clearance depends on diameter only, so it is the same for UNC and UNF 5/16 screws.
What metric drill can I use for 5/16-24 UNF?
The closest metric drill is 6.9 mm (0.2717 in), giving about 75% thread on 5/16-24 UNF.
What is the difference between 5/16-24 UNF and 5/16-18 UNC?
Both are 5/16 diameter. 5/16-24 UNF has 24 threads per inch and uses a I tap drill (0.2720 in); 5/16-18 UNC has 18 TPI and uses a F drill (0.2570 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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