ISO 13921:2008

Overview

ISO 13921:2008 is an international aerospace standard that specifies the dimensions for 100° reduced countersunk head screws with an internal offset cruciform (ribbed or unribbed) drive. Published as the second edition in 2008, ISO 13921:2008 replaces the 1996 edition and is intended for use in the compilation of aerospace product standards. The standard covers screws with normal shank (close or large tolerance), short or medium length MJ threads, metallic material (coated or uncoated), and strength classes ≤ 1 100 MPa. Dimensions and tolerances are expressed in millimetres.

Key topics and technical requirements

• Head and drive geometry: 100° reduced countersunk head with internal offset cruciform drive - available ribbed or unribbed for anti-slip features.
• Threads and shank: MJ series threads (short or medium length) and normal shank with close or large tolerance options.
• Materials and finishes: Metallic screws, coated or uncoated; dimensions apply after coating but before lubrication (with specific pre-coating shank tolerances noted for some coatings).
• Strength classes: Applicable to screws with strength classes less than or equal to 1 100 MPa.
• Dimensions and tolerances: Provided in millimetres via figure(s) and Table 1 in the standard; tolerances of form and position default to ISO 7913 unless otherwise stated.
• Normative references: Interacts with other standards such as ISO 286-2 (limits and fits), ISO 3353-1 (lead/runout threads), ISO 5855-2 (MJ threads limits), ISO 7913 (form/position tolerances), ISO 14275 and ISO 14276 (internal offset cruciform drives).

Applications and users

ISO 13921:2008 is used wherever consistent, interchangeable aerospace fasteners are required:

• Aerospace OEMs and tier suppliers specifying screw dimensions on component drawings and product standards.
• Fastener manufacturers producing MJ-threaded, reduced countersunk screws for aircraft and spacecraft hardware.
• Design and mechanical engineers selecting screws for flush mounting, structural and secondary assemblies.
• Procurement and quality assurance teams verifying dimensional conformity and acceptance criteria.
• Maintenance and repair organizations ensuring replacement parts meet original specification for safety and airworthiness.

Related standards

• ISO 286-2 - Limits and fits (tolerance grades)
• ISO 3353-1 - Lead and runout threads (rolled external threads)
• ISO 5855-2 - MJ threads - limit dimensions for bolts and nuts
• ISO 7913 - Tolerances of form and position for bolts and screws
• ISO 14275 / ISO 14276 - Internal offset cruciform drives (ribbed/unribbed)

Keywords: ISO 13921:2008, aerospace screws, 100° reduced countersunk head, internal offset cruciform drive, MJ threads, screw dimensions, aerospace fastener standards.