BenchReference

1/4″-28 UNF Thread

Recommended tap drill #3
Tap drill Ø 0.2130″ / 5.41 mm
Major Ø 0.2500″ / 6.35 mm
Threads per inch 28

The chart recommendation gives approximately 79.8% thread engagement — the conventional target for average materials. It is not the only valid choice; see the options below.

Specification

Designation1/4″-28 UNF
Thread familyUnified National Fine (UNF)
Nominal size1/4
Basic major diameter 0.2500″ / 6.35 mm
Threads per inch / pitch28 TPI (0.907 mm pitch)
Recommended tap drill #3 (0.2130″ / 5.41 mm)
P = 0.0357″ (28 TPI) 60° major Ø 0.2500″ 60° thread form (UN / ISO) — schematic

Tap drill options by thread engagement

Any of these standard drills produces a usable engagement for 1/4″-28 UNF; smaller drills give stronger threads but higher tapping torque. Percentages use the conventional approximation (60° thread form).

5.4 mm 80.6% #3 ★ 79.8% #2 62.5% 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
5.4 mm 0.2126 5.4 80.6%
#3 — chart recommendation 0.2130 5.41 79.8%
5.5 mm 0.2165 5.5 72.1%
7/32″ 0.2188 5.556 67.4%
5.6 mm 0.2205 5.6 63.6%
#2 0.2210 5.613 62.5%

Nearest drills to the tap drill in other systems

Drill Size vs #3 Resulting engagement
5.4 mm 0.2126″ / 5.4 mm −0.01 mm 80.6%
7/32″ 0.2188″ / 5.556 mm +0.146 mm 67.4%

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 #3 tap drill threads cut into the material 9/32" (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 17/64″ 0.2656 6.747
normal 9/32″ 0.2813 7.144
loose 19/64″ 0.2969 7.541

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) 7/16″
Head height 5/32″ (0.1563″)
Nut thickness 7/32″ (0.2188″)
Compatible nut 1/4-28 hex nut
Head — top view Bolt — side view Nut — side view width across flats = 7/16″ wrench head height 5/32″ thickness 7/32″

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.