ISO 8788:2020

Overview - ISO 8788:2020 (Aerospace nuts, metric - tolerances of form and position)

ISO 8788:2020 specifies the tolerances of form and position for metric nuts intended for aerospace construction. This third edition (2020) is a minor revision prepared by ISO/TC 20/SC 4 and aligns geometrical tolerancing with ISO 1101, ISO 2692 and ISO 5459. The standard defines which geometric characteristics are controlled for different nut types and provides illustrated requirements and numeric tolerance tables for implementation.

Key technical topics and requirements

The standard covers tolerancing for a wide range of aerospace nut types, including:

• Plain nuts and slotted nuts
• Self‑locking nuts (plastic locking ring; hexagonal/bihexagonal formed out‑of‑round)
• Clinch/shank nuts
• Anchor nuts (two-lug, single-lug, corner)

Primary geometric characteristics and positional controls specified:

• Flatness of bearing surface
• Squareness (perpendicularity) of bearing surface
• Across‑flats symmetry
• Symmetry of slots
• Locking wire hole position
• Flange run‑out and flange symmetry
• Counterbore coaxiality and clinching/shank coaxiality
• Rivet hole position

Practical implementation guidance:

• Illustrated examples and explanatory drawings are provided in Annexes A–I.
• Numeric tolerances and dependency rules are given in Tables 12 and 13, where values depend on the nominal thread diameter and specific reference dimensions (e.g., dimension K).
• Tolerancing follows GPS principles as defined in ISO 1101 (form, orientation, location, run‑out), ISO 2692 (MMR/LMR/RPR), and ISO 5459 (datums and datum systems).

Practical applications and users

ISO 8788 is essential where precision, interchangeability and airworthiness are required:

• Aerospace design and stress engineers specifying fasteners for primary and secondary structures
• Fastener manufacturers producing metric nuts for aircraft and spacecraft
• Quality and metrology teams performing inspection and acceptance of nuts against geometric tolerances
• Purchasing/procurement teams requiring conformity evidence from suppliers
• Certification bodies and maintenance organizations verifying part conformity

Applying ISO 8788 reduces assembly misfit, improves load distribution, and supports traceability and regulatory compliance in safety‑critical aerospace applications.

Related standards (for reference)

• ISO 1101 - Geometrical product specifications (GPS) - Geometrical tolerancing
• ISO 2692 - GPS - Maximum/least material requirement reciprocity
• ISO 5459 - GPS - Datums and datum systems

Keywords: ISO 8788, aerospace nuts, metric nuts, tolerances of form and position, geometrical tolerancing, flatness, squareness, across‑flats symmetry, anchor nuts, clinch/shank nuts.