Overview - ISO 8399-2:1998 (Aerospace accessory drives & mounting flanges - Dimensions)
ISO 8399-2:1998 defines the metric-series dimensions for couplings, spigots and mounting flanges used with quick attach/detach provisions in aircraft gearboxes and engine accessories. As Part 2 of ISO 8399, it complements Part 1 (design criteria) by providing the detailed dimensional, fit and surface requirements that support interchangeability, manufacture and inspection of accessory drives and mounting flanges.
Key topics and technical requirements
- Scope and configuration: Dimensioned layouts, figures and tables specify the geometry for accessory flanges and mating engine/gearbox flanges (spigots, collars, sockets).
- Spigot and sealing: Detailed spigot dimensions and sealing groove guidance. Radial compression and assembly stretch assumptions are based on nitrile, fluorocarbon and silicone elastomers with ~80% stretch capability and 70–80 IRHD hardness. Groove modifications are recommended if other elastomers are used.
- Locating features: Locating pin and socket dimensions are provided for accurate flange alignment, including transfer tube and transfer-tube seal arrangements for larger spigot sizes.
- O-ring specification: Transfer-tube O-rings specified with nominal inside diameter 6.9 −0.14 mm and cross-section 1.8 −0.08 mm (aligned with ISO 3601 series).
- Involute splines: Internal and external spline characteristics follow ISO 4156 - number of teeth and module listed in tables; pressure angle 30°; fillet root; tolerance class 5; fit class H/d; side fit. Separate dimensions are given for lubricated and unlubricated splines.
- Surface finish & tolerances: Surface roughness per ISO 4287 and ISO 1302 (values in figures), with permission to double roughness for aluminium sealing surfaces after treatment. Limit deviations and fits reference ISO 286-2; general tolerances default to ISO 2768-m where not otherwise specified.
- Documentation: Figures and tables supply the specific numeric dimensions, control diameters and inspection notes required for manufacturing and quality control.
Applications and who uses it
ISO 8399-2 is applied across the aerospace supply chain where standardized accessory drive interfaces are critical:
- Aircraft and engine OEMs specifying accessory drive/mounting flanges
- Accessory manufacturers (pumps, generators, starters) for mechanical interface design
- Gearbox and accessory mounting designers ensuring interchangeability
- Manufacturing, inspection and quality teams using the dimensional tables and surface/tolerance requirements
- MRO and certification engineers verifying compliant replacement parts
Related standards
Keywords: ISO 8399-2, accessory drives, mounting flanges, aerospace couplings, spigot dimensions, quick attach/detach, involute splines, O-ring, surface roughness, metric series.