ASME B18.3 is the governing dimensional standard for fine-thread socket head cap screws. This page sets out the scope of the standard, reproduces the full size table it specifies, and lists the DIN/ISO/ASME cross-references commonly cited in datasheets and drawings.
ASME B18.3 — Scope
ASME B18.3 is "Socket cap, shoulder, set screws, and hex keys (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 B18.3 does not divide fine-thread socket head cap screws 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 B18.3
| Thread series | Drive | Material options | Typical application |
|---|
| metric fine (DIN 912 / ISO 4762) | internal hex | carbon steel / alloy steel / 304 / 316 stainless | precision machinery and moulds |
| UNF (ASME B18.3) | internal hex | carbon steel / alloy steel / 304 / 316 stainless | automotive components |
| UNF (ASME B18.3) | internal hex | alloy steel | general equipment assembly |
Equivalent Standards
Where direct equivalents exist (ISO ↔ DIN, ISO ↔ ASME), they are listed on the parent Fine-Thread Socket Head Cap Screws page.
Always confirm against the dimensions on this page, since some "equivalent" standards differ in tolerances and head geometry.
When to Specify ASME B18.3
ASME B18.3 is the inch-based reference; for metric programs choose the ISO equivalent instead.
Custom manufacturing · Made to drawing
Need a screw to this standard in a non-standard size?
Parts built to ASME B18 3 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 warm forging.
Minimum order from 2,000 pieces; samples within 7 days of drawing approval.
Custom screw manufacturingSend a drawing
Applications & Industries
The sectors that buy Fine-Thread Socket Head Cap Screws to DIN 912 / ISO 4762 and ASME B18.3 (UNF), and the duty each one buys them for. Stocked from metric fine (DIN 912 / ISO 4762) to UNF (ASME B18.3) (nominal diameter).
Industries that specify Fine-Thread Socket Head Cap Screws
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Precision machinery
Closer preload control than a coarse thread.
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Automotive components
UNF variants for inch-threaded assemblies.
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Equipment assembly
Stainless options where the joint is exposed.
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Mould and die work
Fine pitch holds setting in a joint that is adjusted often.
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Energy & infrastructure
Tower, pipework, switchgear and plant fixings specified for long service life and inspectable joints.
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Maintenance & MRO
Stocked replacements matched to an existing property class, thread and finish.
Typical applications
- matching metric fine or UNF to the tapped hole
- choosing 304 or 316 for the exposure
- using a correctly sized hex key to protect the socket
- selecting fine pitch where preload must be set precisely
- bolting brackets, motors and gearboxes to a frame
- assemblies that are dismantled for service and re-torqued
Service conditions to specify against
- Marine and coastal
- Available in A4 (316) stainless for chloride exposure — coastal structures, pool halls, water treatment and deck hardware, where A2 will eventually pit.
- High load
- Supplied in ISO 898-1 property classes, so the joint can be designed to a stated proof load and tightened to a calculated torque rather than by feel.
- Torque and preload
- The joint is designed around bolt tension, so lubrication, coating and the friction coefficient all change the torque figure — state them together.
Where to use something else
- Do not force a fine-thread screw into a coarse tapped hole; a few turns of apparent engagement destroys both threads.
- Do not use a machine thread into unthreaded sheet or timber; use a tapping screw, rivet nut or insert made for that parent material.
Full application guide: where Fine-Thread Socket Head Cap Screws are used, how to specify them, and the cases they are wrong for
