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ASME Code
fasteners

ASME B18.2.1 Hex Head Bolts: Complete Specification Reference Reference Guide

Free ASME B18.2.1 reference guide with dimensions, thread data, torque specs, and selection guide. ISO-compatible cross-reference included.

Standard Scope

Specifies the complete dimensional, technical, and material requirements for hex and square head bolts and cap screws with Unified inch threads (UNC and UNF thread series) per ASME B18.2.1. Covers nominal diameters from #10 (0.190 in) up to 3.0 inches for structural, industrial, and automotive bolting assemblies.

Typical Applications

  • •Structural steel building framing and AISC bolted connection assemblies.
  • •Heavy machinery, gearbox, and electric motor baseplate fastening.
  • •Automotive powertrain mounting, suspension components, and chassis bolting.
  • •Piping flange bolting and pressure vessel nozzle connections with the [Bolt Torque Calculator](/calculators/bolt-torque).

Key Engineering Concepts

Width Across Flats

Head width determining imperial wrench or socket size.

Thread Classes

Unified Coarse (UNC) and Unified Fine (UNF) Class 2A external fit tolerances.

SAE Material Grades

Grade 2 (low-carbon steel), Grade 5 (medium-carbon heat-treated, 120 ksi tensile), and Grade 8 (alloy steel quenched & tempered, 150 ksi tensile).

Thread Tensile Area (A_t)

Critical cross-sectional area used for clamp force and stress calculations.

Engineering Notes & Best Practices

  • •Nut Factor Tightening: Calculate tightening torque using $T = K \cdot F_i \cdot d$ per VDI 2230 and ASME criteria with our [Bolt Torque Calculator](/calculators/bolt-torque).
  • •Thread Engagement Length: Minimum engagement length must be at least $1.0 d$ for steel nuts, $1.5 d$ for cast iron, and $2.0 d$ for aluminum; verify pullout safety with [Thread Strip Strength Calculator](/calculators/thread-strip-strength).
  • •Preload Sizing: For detailed clamped joint mechanics, see our technical guide on [Bolt Preload Explained](/knowledge/articles/bolt-preload-explained).

Official Standards Reference

ASME B18.2.1Official Store

Square, Hex, Heavy Hex, and Askew Head Bolts and Hex, Heavy Hex, Hex Flange, Lobed Head, and Lag Screws

Standard specification for Unified inch hex fasteners

Legal & Compliance Notice:MechToolsHub is an independent engineering utility. This article is an internally reviewed explanatory overview for educational reference only. To ensure compliance with copyright regulations, no standard dimensions database, lookup tables, tolerance calculators, or specifications are reproduced. Please purchase the official document directly from the standards organisation store to get the full specifications.
SAE J429Official Store

Mechanical and Material Requirements for Externally Threaded Fasteners

Strength grades for inch bolts

Legal & Compliance Notice:MechToolsHub is an independent engineering utility. This article is an internally reviewed explanatory overview for educational reference only. To ensure compliance with copyright regulations, no standard dimensions database, lookup tables, tolerance calculators, or specifications are reproduced. Please purchase the official document directly from the standards organisation store to get the full specifications.
ASTM A307 / A325 / A490Official Store

Standard Specification for Structural Bolts

ASTM standards for structural fastening

Legal & Compliance Notice:MechToolsHub is an independent engineering utility. This article is an internally reviewed explanatory overview for educational reference only. To ensure compliance with copyright regulations, no standard dimensions database, lookup tables, tolerance calculators, or specifications are reproduced. Please purchase the official document directly from the standards organisation store to get the full specifications.