BS A 104-2025 PDF

Full title and description

STB BS A 104-2025 — Aerospace Series. Bolts, hexagonal heads with optional safety/locking wire holes, unified threads, corrosion‑resisting or heat‑resisting steels, plain finish. Specification covering dimensions, materials, mechanical properties, classification and tests for aerospace hexagonal‑head bolts.

Abstract

This standard specifies requirements for aerospace hexagonal‑head bolts with unified threads, including optional safety/locking wire holes, permitted materials (corrosion‑resisting and heat‑resisting steels), mechanical property classifications and test methods. It includes minimum tensile strength classifications for CR and HR steels, dimensional and marking requirements, and guidance on inspection and acceptance for aerospace applications.

General information

• Status: Active / Definitive (current BSI aerospace series specification).
• Publication date: 19 August 2025 (standard edition BS A 104:2025).
• Publisher: British Standards Institution (BSI).
• ICS / categories: 49.030.20 — Bolts, screws, studs (Aerospace fasteners).
• Edition / version: 2025 edition (STB prefix indicates localized/translated copy of BS A 104:2025 where applicable; tracked‑changes and TC variants also exist).
• Number of pages: 23 pages (typical published PDF edition).

Scope

The standard applies to hexagonal‑head bolts used in aerospace applications that have unified threads and a plain finish. It sets out material classes (corrosion‑resisting and heat‑resisting steels), mechanical property requirements including minimum tensile strengths over the stated temperature ranges, dimensional and thread tolerances, optional provisions for safety/locking wire holes, marking, testing and inspection procedures, and conformity/acceptance criteria for procurement and manufacture.

Key topics and requirements

• Bolt geometry and dimensional tables for hexagonal‑head bolts with unified threads.
• Material specification: corrosion‑resisting (CR) and heat‑resisting (HR) steels suitable for aerospace use.
• Mechanical classifications and minimum tensile strength values — e.g. CR steel Rm ≥ 127 600 lbf/in² (≈880 MPa) and HR steel Rm ≥ 130 500 lbf/in² (≈900 MPa) over the specified temperature ranges.
• Optional safety/locking wire hole provisions and related dimensional/strength requirements.
• Surface finish: plain finish requirements and any acceptance criteria for coatings or corrosion protection where referenced.
• Testing and inspection: tensile testing, dimensional verification, and acceptance/rejection criteria for production lots.
• Marking, traceability and documentation needed for aerospace procurement and certification.

Typical use and users

Used by aerospace manufacturers, fastener suppliers, procurement engineers, quality and inspection departments, design offices and maintenance organisations. Applicable where high‑strength, temperature‑resistant or corrosion‑resistant hexagonal‑head bolts are required (airframe assemblies, engine components and other critical aerospace structures).

Related standards

Replaces and updates earlier related British fastener standards (for example BS 4A 104:2002+A2:2012); related documents include other BS aerospace fastener series standards, applicable ISO/ASME/NAS/AMS specifications for aerospace fasteners and material standards referenced for corrosion‑resisting and heat‑resisting steels. Users should consult the normative references listed in the standard for full cross‑reference details.

Keywords

Aerospace bolts; hexagonal‑head bolts; locking wire holes; unified thread; corrosion‑resisting steel; heat‑resisting steel; tensile strength; fastener specification; BSI; BS A 104:2025.

FAQ

Q: What is this standard?

A: STB BS A 104-2025 is the 2025 BSI aerospace‑series specification for hexagonal‑head bolts (unified threads) intended for aerospace use, covering materials, mechanical properties, dimensions and inspection.

Q: What does it cover?

A: It covers bolt geometry, material classes (CR and HR steels), minimum tensile strength classifications, optional locking/safety wire holes, plain finish requirements, testing, marking and acceptance criteria for aerospace bolts.

Q: Who typically uses it?

A: Aerospace manufacturers, fastener suppliers, procurement and quality engineers, designers and maintenance organisations that require standardized high‑performance bolts for aircraft and associated equipment.

Q: Is it current or superseded?

A: It is current (2025 edition) and replaces the earlier BS 4A 104:2002+A2:2012 series for these bolt types; tracked‑changes (TC) and draft/comment variants have also been published for transition and amendment control.

Q: Is it part of a series?

A: Yes — it is part of the BSI Aerospace Series of fastener specifications. Other related BS A and aerospace series documents cover complementary fasteners, materials and test methods.

Q: What are the key keywords?

A: Hexagonal‑head bolts, aerospace fasteners, unified threads, locking wire holes, corrosion‑resisting steel, heat‑resisting steel, tensile strength, BSI, specification.