Overview
ISO 3221:1998 specifies the dimensions for aerospace anchor nuts that are self-locking, fixed at a 90° corner, and provided with a counterbore and MJ threads. The standard covers nuts of classifications: 1 100 MPa (at ambient temperature) / 235 °C, 1 100 MPa / 315 °C, and 1 100 MPa / 425 °C. It is intended exclusively for the compilation of aerospace product standards and was prepared by ISO/TC 20/SC 4 (Aerospace fastener systems).
Key points from the standard:
- Dimensions and tolerances are expressed in millimetres.
- Values apply after any surface coating(s) but before the application of lubricants.
- The metallic cap (optional) shall remain joined to the nut at the maximum operating temperature; test conditions are referenced in ISO 5858.
- Tolerances of form and position conform to ISO 8788.
Note: The classification notes in ISO 3221 clarify that "1 100 MPa" corresponds to the minimum tensile stress the nut can withstand at ambient temperature, and the listed temperatures are maximum operating temperatures conditioned by surface treatment.
Key Topics
- Product scope: Self-locking, fixed, 90° corner anchor nuts with counterbore and MJ threads for aerospace applications.
- Classifications: Three maximum operating temperature classes paired with the 1 100 MPa ambient tensile designation (235 °C, 315 °C, 425 °C).
- Dimensional data: Configuration is illustrated in Figure 1 and detailed in Table 1 of the standard; Table 1 lists diameter/thread codes (MJ3 to MJ10) and their associated dimensional parameters.
- Manufacturing & inspection: Dimensions apply after coating; tolerances of form and position reference ISO 8788; where details are not specified, manufacturer discretion applies.
- Compatibility: Dimensions were determined to permit production of parts that satisfy procurement requirements specified in ISO 5858.
Applications
These nuts are specified for use in aerospace product standards where:
- Interchangeability of anchor nuts and verified fit with corresponding bolts (MJ threads) is required.
- High-temperature resistance classes are needed due to operating environment or surface treatment limitations.
- Procurement and manufacturing specifications demand consistent dimensional and form tolerances for assembly and safety-critical joints.
Practical benefits include standardized dimensions to support repeatable manufacturing, simplified procurement, and reliable mating with MJ-threaded bolts in aerospace assemblies.
Related Standards
- ISO 5855-2:1988 - MJ threads: limit dimensions for bolts and nuts.
- ISO 5858:1991 - Procurement specification for self-locking nuts with maximum operating temperature ≤ 425 °C.
- ISO 8788:1987 - Tolerances of form and position for nuts.
For detailed drawings, dimensional tables and mandatory notes, consult the full ISO 3221:1998 publication, Figure 1 and Table 1, and the cited normative references to ensure compliance with procurement and test conditions.