BenchReference

5/16″-18 UNC Thread

Recommended tap drill F
Tap drill Ø 0.2570″ / 6.528 mm
Major Ø 0.3125″ / 7.938 mm
Threads per inch 18

The chart recommendation gives approximately 76.9% thread engagement — the conventional target for average materials. It is not the only valid choice; see the options below.

Specification

Designation5/16″-18 UNC
Thread familyUnified National Coarse (UNC)
Nominal size5/16
Basic major diameter 0.3125″ / 7.938 mm
Threads per inch / pitch18 TPI (1.411 mm pitch)
Recommended tap drill F (0.2570″ / 6.528 mm)
P = 0.0556″ (18 TPI) 60° major Ø 0.3125″ 60° thread form (UN / ISO) — schematic

Tap drill options by thread engagement

Any of these standard drills produces a usable engagement for 5/16″-18 UNC; smaller drills give stronger threads but higher tapping torque. Percentages use the conventional approximation (60° thread form).

6.4 mm 83.9% F ★ 76.9% 6.8 mm 62.1% shaded = share of the full thread height the tap forms · accent line = drilled hole wall
★ = chart recommendation. A smaller drill leaves more of the thread height for the tap to form.
Drill Decimal in mm Approx. engagement
6.4 mm 0.2520 6.4 83.9%
6.5 mm 0.2559 6.5 78.4%
F — chart recommendation 0.2570 6.528 76.9%
6.6 mm 0.2598 6.6 73%
G 0.2610 6.629 71.4%
6.7 mm 0.2638 6.7 67.5%
17/64″ 0.2656 6.747 65%
H 0.2660 6.756 64.4%
6.8 mm 0.2677 6.8 62.1%

Nearest drills to the tap drill in other systems

Drill Size vs F Resulting engagement
6.5 mm 0.2559″ / 6.5 mm −0.028 mm 78.4%
1/4″ 0.2500″ / 6.35 mm −0.178 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.

Tapped hole Clearance hole F tap drill threads cut into the material 11/32" (normal fit) the tinted gap lets the bolt pass freely
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 5/16-18 hex nut
Head — top view Bolt — side view Nut — side view width across flats = 1/2″ wrench head height 13/64″ thickness 17/64″

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

Related threads

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.