5/16″-24 UNF Thread

Recommended tap drill I

Tap drill Ø 0.2720″ / 6.909 mm

Major Ø 0.3125″ / 7.938 mm

Threads per inch 24

The chart recommendation gives approximately 74.8% thread engagement — the conventional target for average materials. It is not the only valid choice; see the options below.

Specification

Designation	5/16″-24 UNF
Thread family	Unified National Fine (UNF)
Nominal size	5/16
Basic major diameter	0.3125″ / 7.938 mm
Threads per inch / pitch	24 TPI (1.058 mm pitch)
Recommended tap drill	I (0.2720″ / 6.909 mm)

Tap drill options by thread engagement

Any of these standard drills produces a usable engagement for 5/16″-24 UNF; smaller drills give stronger threads but higher tapping torque. Percentages use the conventional approximation (60° thread form).

★ = chart recommendation. A smaller drill leaves more of the thread height for the tap to form.

Drill	Decimal in	mm	Approx. engagement
6.8 mm	0.2677	6.8	82.7%
6.9 mm	0.2717	6.9	75.5%
I — chart recommendation	0.2720	6.909	74.8%
7 mm	0.2756	7	68.2%
J	0.2770	7.036	65.6%
7.1 mm	0.2795	7.1	60.9%

Nearest drills to the tap drill in other systems

Drill	Size	vs I	Resulting engagement
6.9 mm	0.2717″ / 6.9 mm	−0.009 mm	75.5%
17/64″	0.2656″ / 6.747 mm	−0.162 mm	86.6%

Clearance holes

For a bolt or screw of this size to pass through freely. Recommended drills per ASME B18.2.8: close, normal, and loose fits.

Schematic — proportions exaggerated for clarity; use the table values.

Fit	Drill / Ø	Decimal in	mm
close	21/64″	0.3281	8.334
normal	11/32″	0.3438	8.731
loose	23/64″	0.3594	9.128

Hex bolt & nut dimensions

For a standard hex cap screw (ASME B18.2.1) and finished hex nut (ASME B18.2.2) in this size.

Wrench / socket size (bolt head)	1/2″
Head height	13/64″ (0.2031″)
Nut thickness	17/64″ (0.2656″)
Compatible nut	5/16-24 hex nut

Basic (nominal) dimensions. See the wrench & socket size charts for all sizes and the metric ↔ SAE socket comparison.

Related threads

5/16″-18 UNC — same nominal size, 18 TPI (Unified National Coarse (UNC)) · compare
1/4″-28 UNF — next size down in UNF
3/8″-24 UNF — next size up in UNF

Data provenance. ASME B1.1, Unified Inch Screw Threads (ASME). Machinery’s Handbook — tap drill charts (Industrial Press). ASME B18.2.8, Clearance Holes for Bolts, Screws, and Studs (ASME). ASME B18.2.1, Hex Cap Screws (head dimensions) (ASME). ASME B18.2.2, Finished Hex Nuts (ASME). Engagement percentages use the conventional approximation (engagement = (major − drill) ÷ (1.29904 × pitch)) and are guidance, not a tolerance specification. See all data sources.