Overview
ISO 3353-2:2020 - "Aerospace - Lead and runout threads - Part 2: Internal threads" specifies lead, runout and undercut requirements for internal threads (blind tapped holes) used in aerospace construction. Prepared by ISO/TC 20/SC 4 (Aerospace fastener systems), this second edition updates definitions and editorial content from the 2002 edition. The standard is intended for use whenever it is referenced in a product definition document and complements product-specific thread standards.
Key Topics
- Scope and applicability: Requirements apply to internal (blind) threads in aerospace components when ISO 3353-2 is called out in a definition or product standard.
- Terms and definitions: Clear definitions for lead thread, runout thread, undercut, and completely formed thread are provided for consistent interpretation.
- Symbols: Standard thread symbols such as major, pitch and minor diameters (D3, D2, D1) and pitch (P) are used throughout.
- Surface and root geometry: Flank and root radii for incompletely formed threads must be smooth, free of abrupt tool marks and at least the calculated radius associated with the maximum profile specified in ISO 5855-1.
- Lead thread requirements: The standard defines the geometry and axial length of the lead threads (the fully formed portion at the start of the thread) and references dimensional relationships to pitch and Dmax.
- Runout requirements: Defines the axial runout zone where incompletely formed threads transition to the blind hole bottom and requires the thread form to lie within the limits specified by the applicable product standard.
- Undercut dimensions: Provides dimensional allowances for undercuts at the hole bottom to limit the extent of completely formed threads. The standard includes numerical allowances tied to pitch (for example, additions to Dmax for certain pitch ranges are specified in the document).
Applications
ISO 3353-2:2020 is used by:
- Aerospace designers and engineers specifying threaded blind holes on drawings and product standards
- Fastener and insert manufacturers establishing tap and tool geometry
- Machinists programming blind tapped hole processes
- Quality and inspection engineers verifying lead/runout and undercut conformity
- Standards writers referencing consistent internal thread terminations in aerospace assemblies
Practical benefits include consistent thread entry/exit geometry, controlled thread depth for blind holes, and improved interchangeability and inspectionability of aerospace fasteners and threaded components.
Related Standards
- ISO 5855-1 - Aerospace MJ threads - Part 1: General requirements (normative reference cited in ISO 3353-2)
- Other parts of the ISO 3353 series (see ISO website for full listing)
Keywords: ISO 3353-2:2020, aerospace internal threads, blind tapped holes, lead threads, runout threads, undercuts, thread requirements, ISO 5855-1, aerospace fastener systems.
