ASME B18.2.1 covers the dimensional requirements for square-head bolts. Reproduced below: the complete size table, and the companion and equivalent standards referenced when ordering to this document.
ASME B18.2.1 — Scope
ASME B18.2.1 is "Square, hex, heavy hex, and askew head bolts and hex, heavy hex, hex flange, lobed head, and lag screws (inch series)". An inch-series document.
Material, property class, surface finish and packaging are governed by separate documents — ISO 898-1 and ISO 898-2 for mechanical property classes, ISO 4042 for electroplated finishes.
Tolerance & Product Grades
ASME B18.2.1 does not divide square-head bolts into product grades. Tolerances are given as limits on the individual dimensions in the table.
Where a general-tolerance class is cited on the drawing instead, it applies to the dimensions not otherwise toleranced.
Dimensions Specified by ASME B18.2.1
| Size | Thread form | Head width (in) | Head height (in) | Tensile strength (psi) | Material / grade | Reference standard |
|---|---|---|---|---|---|---|
| 1/4-20 | fully or partially threaded | 3/8 | 11/64 | 120000 | yellow-zinc-chromate steel, Grade 5 | ASME B18.2.1 / SAE J429 |
| M6 / M8 / M10 / M12 / M14 / M16 / M20 / M24 / M30 | fully or partially threaded | yellow-zinc-chromate steel, Grade 5 | ASME B18.2.1 / SAE J429 | |||
| 5/16-18 through 1-1/4-7 | fully or partially threaded | yellow-zinc-chromate steel, Grade 5 | ASME B18.2.1 / SAE J429 |
Equivalent Standards
Where direct equivalents exist (ISO ↔ DIN, ISO ↔ ASME), they are listed on the parent Square-Head Bolts page.
Always confirm against the dimensions on this page, since some "equivalent" standards differ in tolerances and head geometry.
Companion Standards
ASME B18.2.1 controls only the geometry of square-head bolts. The mechanical strength is set by ISO 898-1 (bolts, screws and studs) or ISO 898-2 (nuts); the surface finish is governed by separate finish standards such as ISO 4042 for electroplating. For corrosion testing, the relevant document is ISO 9227 (neutral salt spray).
Custom manufacturing · Made to drawing
Need a bolt to this standard in a non-standard size?
Parts built to ASME B18 2 1 are already in the catalog, so a special is usually a variation on existing tooling rather than a new design — which is what keeps the cost down. Made by cold heading or hot forging.
Minimum order from 2,000 pieces; samples within 7 days of drawing approval.
