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5/16-24 UNF — Full Fastener Spec
ASME B1.1 (Unified) · Unf series · Tap drill, clearance hole, torque, strength and tool sizes — every number on one page.
Tap drill (75% thread)
I (0.2720")
6.9 mm · Clearance hole (normal fit): 0.3440" (11/32) ·
Grade 5 torque: 19.3 lb-ft dry
Thread dimensions
Major diameter
0.3125 "
Threads per inch
24
Pitch
1.058 mm
Pitch diameter
0.2854 "
Minor diameter
0.2674 "
Tensile stress area
0.05807 in²
Tap drill sizes for 5/16-24 UNF
Percent of thread is a trade-off: 75% is the shop default because the last 25% of engagement adds only a few percent of strength while roughly doubling tapping torque. Drop to 50–65% in tough or gummy material, or for blind holes where a snapped tap means scrapping the part.
| Thread engagement | Exact | Inch drill | Nearest metric drill |
|---|---|---|---|
| 50% | 0.2854" | 9/32 (0.2812") | 7.3 mm |
| 65% | 0.2773" | J (0.2770") | 7 mm |
| 75% | 0.2719" | I (0.2720") | 6.9 mm |
| 85% | 0.2665" | H (0.2660") | 6.8 mm |
Roll (form) taps need a larger hole than cut taps — add roughly
0.4× the pitch to the 75% cut-tap drill and confirm against your tap maker's chart.
Clearance hole for 5/16-24 UNF
| Fit | Drill | Use |
|---|---|---|
| Close | 0.3280" (21/64) | Located joints, dowel-like alignment |
| Normal | 0.3440" (11/32) | General assembly — use this unless you have a reason not to |
| Loose | 0.3590" (23/64) | Weldments, sheet stacks, tolerance build-up |
5/16-24 UNF torque specs
Calculated with the short-form equation T = K · D · F, using K = 0.2 (plain, dry, as-received steel) and a target clamp load of 75% of proof load — the standard preload target for a reusable joint.
| Grade | Dry | Lubricated | Dry (N·m) | Clamp load |
|---|---|---|---|---|
| Grade 2 Low/medium carbon steel, unmarked head | 12.5 lb-ft | 9.36 lb-ft | 16.9 N·m | 2,395 lb |
| Grade 5 Medium carbon Q&T, 3 radial head marks | 19.3 lb-ft | 14.5 lb-ft | 26.1 N·m | 3,701 lb |
| Grade 8 Medium carbon alloy Q&T, 6 radial head marks | 27.2 lb-ft | 20.4 lb-ft | 36.9 N·m | 5,225 lb |
| 18-8 SS 304/18-8 stainless | 9.07 lb-ft | 6.8 lb-ft | 12.3 N·m | 1,741 lb |
The nut factor K dominates this result. Zinc-plated, waxed, anti-seize
and lubricated threads all shift K between roughly 0.10 and 0.25, which is a 2.5×
spread in torque for identical preload. If preload actually matters, torque-to-yield,
angle control or a load-indicating washer beats a torque wrench every time.
Tool sizes
Hex head wrench
1/2"
Hex key (socket head)
1/4