ASME B18.2.1

Covers — complete dimensional and general data for inch-series square, hex, heavy hex, and askew-head bolts and hex/heavy-hex/flange/lag screws: head dimensions, thread lengths, and tolerances.

Does NOT cover (exclusions) — the following are governed elsewhere:

• Bolt material and strength → ASTM A193 / A307 / A320.
• Nut dimensions → ASME B18.2.2.
• Metric fastener dimensions → ISO 4014 / 4017; metric properties → ISO 898.

B18.2.1 fixes the head geometry; the grade/strength is a separate material spec.

B18.2.1 sets minimum requirements for bolt/screw dimensions:

• Head Dimensions — width across flats/corners and head height by product type (hex, heavy hex, square).
• Thread Length — standard and short thread lengths by nominal size and length.
• Tolerances — dimensional tolerances so bolts fit standard wrenches and spotfaces.
• Product Types — hex bolts, heavy-hex bolts/screws, flange screws, lag screws.

ASME B18.2.1 is the dimensional standard for inch-series bolts and screws — square, hex, and heavy hex — defining head sizes, thread lengths, and tolerances so fasteners fit standard wrenches and flange spotfaces. Heavy-hex heads, with their larger across-flats, are specified on pressure flange bolting to distribute the bearing load.

It governs geometry only; the material grade and strength come from ASTM A193/A307/A320, and nuts from B18.2.2. Metric equivalents are ISO 4014/4017.

Engineering Note: Specify heavy-hex (not regular hex) for rated flange joints — regular-hex heads can be under-sized for the bearing area and wrench flats expected in pressure service.