Inch series · technical reference

Hex Head Bolt Dimensions

Reference head dimensions for finished hex cap screws. UNC and UNF share the nominal head geometry shown here; select the exact thread and length from the product listing.

Source: ASME B18.2.1 record in U.S. DoD ASSIST.

StandardASME B18.2.1.6 · Inch Series
RevisionSource review · 2026-08
Unitsinches
CAD ORTHOGRAPHIC REFERENCEASME B18.2.1.6 · INCH SERIES REFERENCE
ASME B18.2.1 hex head bolt dimension drawing, side view with dimension callouts, 1/4-20 UNC reference size
SIDE VIEW · LENGTH REFERENCE · 1/4-20 UNC
ASME B18.2.1 hex head bolt dimension drawing, end view showing width across flats and head height, 1/4-20 UNC reference size
END VIEW · HEAD / DRIVE · 1/4-20 UNC

Reference size shown: 1/4-20 UNC. The drawing identifies dimension locations only. Use the table for nominal data and confirm the exact product configuration before ordering.

Reference head dimensions · inch series
Nominal threadUNC / UNF (TPI)Across flats (F) max / minHead height (H) max / minAcross corners max (G)Thread length (LT) ≤6 in / >6 in
1/4 in20 / 280.438 / 0.4280.163 / 0.1500.5050.750 / 1.000
5/16 in18 / 240.500 / 0.4890.211 / 0.1950.5770.875 / 1.125
3/8 in16 / 240.562 / 0.5510.243 / 0.2260.6501.000 / 1.250
1/2 in13 / 200.750 / 0.7360.323 / 0.3020.8661.250 / 1.500
5/8 in11 / 180.938 / 0.9220.403 / 0.3781.0831.500 / 1.750
3/4 in10 / 161.125 / 1.1000.483 / 0.4551.2991.750 / 2.000
1 in8 / 121.500 / 1.4690.627 / 0.5911.7322.250 / 2.500
1-1/4 in7 / 121.875 / 1.8120.813 / 0.7492.1652.750 / 3.000
1-3/8 in6 / 122.062 / 1.9940.878 / 0.8102.3823.000 / 3.250
1-1/2 in6 / 122.250 / 2.1750.974 / 0.9022.5983.250 / 3.500

Reference dimensions for product selection. Verify the current governing standard, thread class, material, finish, length and thread engagement before engineering release or installation.

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