Overview
ISO 8538:1999 defines the dimensions for aerospace hexagonal, self-locking nuts with a counterbore and captive washer using MJ threads. It covers nuts of classification 1 100 MPa (ambient)/235 °C, 1 100 MPa (ambient)/315 °C, and 1 100 MPa (ambient)/425 °C. This International Standard is intended for the compilation of aerospace product standards and provides dimensional requirements to support procurement and manufacturing compliance.
Key topics and requirements
- Scope: Dimensional specification for self-locking hexagonal nuts with counterbore and captive washer (MJ threads) for specified strength/temperature classifications.
- Dimensions and tolerances: All dimensions and tolerances are given in millimetres. Values apply after any surface coating(s) but before lubricant application.
- Threading: MJ thread limits and basic fit are referenced to ISO 5855‑2. Thread tolerances in the self-locking zone apply before forming out-of-round.
- Self-locking feature: Achieved by localized out-of-round form in the nut (tooling marks permissible in that area). The captive washer must be free to rotate on the nut.
- Form and position tolerances: Conform to ISO 8788 (tolerances of form and position for nuts).
- Compatibility: Dimensions were determined to satisfy the procurement specification ISO 5858 (self-locking nuts with maximum operating temperature ≤ 425 °C).
- Product configuration: Figure and table layouts define key dimensions (bearing diameter, wrench engagement, chamfer, etc.) and include notes on surface texture and manufacturing discretion where details are not specified.
Applications and users
- Who uses ISO 8538:
- Aerospace design and structural engineers specifying fasteners for airframes, engines, and high-temperature assemblies.
- Fastener manufacturers producing MJ-thread hex nuts with captive washers.
- Quality and inspection teams verifying dimensional conformity.
- Procurement specialists and standards writers compiling aerospace product standards.
- Practical applications:
- High-temperature aerospace joints where self-locking capability and captive washer retention are required.
- Components exposed to elevated temperatures (up to classification limits) in propulsion, bleed air systems, and hot-structure assemblies.
- Standardizing parts lists and procurement documents to ensure interchangeability and traceability.
Related standards
- ISO 5855‑2 - MJ threads: limit dimensions for bolts and nuts.
- ISO 5858 - Procurement specification for self-locking nuts (≤ 425 °C).
- ISO 8788 - Fastener tolerances of form and position.
Keywords: ISO 8538:1999, aerospace nuts, hexagonal self-locking nuts, MJ threads, captive washer, counterbore, dimensions, high-temperature fasteners, aerospace standards.