Overview
ISO 3353-1:2020 - "Aerospace - Lead and runout threads - Part 1: Rolled external threads" defines the form, limits and inspection method for incompletely formed rolled external threads used in aerospace construction. The document specifies what constitutes lead threads, runout threads and completely formed threads, sets root-radius and profile requirements, and prescribes an optical inspection procedure to be used in case of dispute. It is applicable whenever a definition document references it.
Key topics and technical requirements
- Scope and definitions
- Clear definitions of lead thread, runout thread and completely formed thread to standardize interpretation across manufacturers and inspectors.
- Geometric requirements
- Incomplete-thread flanks at the root shall be joined by a radius (or two radii and a flat) that is smooth and free of abrupt tool marks.
- The root radius of incompletely formed threads must be greater than or equal to the minimum root radius specified in the applicable definition document.
- Lead thread limits
- The lead-thread region must lie within the limits defined in the applicable thread definition document; profile-projection comparator charts (Figure 10) are provided for inspection.
- Runout thread configurations
- Runout requirements cover several shank types: normal shank, pitch-diameter shank, stepped shank, screws threaded to the head / bolts to a shoulder, and oversized bolts. Each configuration has corresponding inspection charts (Figures 11–14).
- Inspection method
- Manufacturers may use any method that demonstrates conformity, but optical projection is mandated in disputes.
- Charts are intended for use with a profile projection comparator having magnification ≥ ×20.
- Practical inspection steps include locating the first/last completely formed thread, then aligning the specified flank to the chart reference lines.
- Practical note (Annex A)
- Assembling the bolt with a GO screw ring gauge (without entering the chamfer) is the preferred, faster method for checking lead and runout threads; profile projection is more accurate but operator-dependent and slower.
Applications and users
- Intended for aerospace fastener manufacturers, designers of threaded components, quality/inspection engineers, maintenance and repair organizations, and procurement/specification writers.
- Used to ensure consistent rolled external thread starts and finishes on bolts and screws in aircraft and space hardware, to support interchangeability, fatigue performance and assembly reliability.
- Also referenced in definition documents and repair/overhaul procedures where threaded terminations are critical.
Related standards
- Part of the ISO 3353 series addressing incomplete threads for aerospace; users should consult the relevant thread definition documents and other parts of the ISO 3353 series as referenced.
- Prepared by ISO/TC 20 (Aircraft and space vehicles), SC 4 (Aerospace fastener systems).
Keywords: ISO 3353-1, lead threads, runout threads, rolled external threads, aerospace fasteners, thread inspection, profile projection comparator, GO screw ring gauge.
