Overview
ISO 5855-1:1999 - Aerospace - MJ threads - Part 1: General requirements defines the fundamental geometry, designation and tolerance principles for MJ threads used in aerospace construction. The standard establishes the basic triangular thread profile, a system for designating diameter × pitch combinations, tables of basic dimensions and tolerances for nominal diameters from 1.6 mm to 300 mm, and calculation methods for any diameter/pitch combination (including sizes > 300 mm).
Keywords: ISO 5855-1, MJ threads, aerospace threads, metric MJ threads, thread profile, pitch diameter, thread tolerances.
Key topics
- Basic profile and symbols - defines major, pitch and minor diameters and the height of the fundamental triangle used to describe MJ thread geometry (see clauses 3–4).
- Limit profiles - specifies allowable form and position limits for external and internal threads; permits continuous non-reversing curves within limits and minimum root radii (clause 5).
- Dimension tables - provides tabulated basic dimensions and tolerances for a broad selection of diameter × pitch combinations between 1.6 mm and 300 mm (clause 1 and tables).
- Tolerances and grades - requires use of tolerance grades and deviations per ISO 965‑1 for pitch and diameter tolerances; includes rules for pitch and flank-angle deviations and coated-thread provisions (clause 6).
- Designation rules - standardizes naming (e.g., MJ6 × 1-4h6h for external; MJ6 × 1-4H5H for internal), with special-case and left‑hand thread notation (clause 9).
- Calculation formulae - provides mathematical formulas to compute limit dimensions and tolerances for non-tabulated combinations and sizes exceeding 300 mm (clause 10).
Applications
ISO 5855-1 is used wherever high-reliability metric MJ threads are required in aerospace structures and components. Typical applications include:
- Aerospace fastener design and specification
- Manufacturing and machining of threaded parts for aircraft and spacecraft
- Quality control, inspection and acceptance testing of threaded connections
- Component procurement and supplier standards compliance
- Design of large-diameter threaded fittings where tabulated sizes are not available
Who uses this standard
- Aerospace engineers and designers specifying threaded interfaces
- Fastener manufacturers and machine shops producing MJ threads
- Quality / metrology engineers performing inspection and tolerance verification
- Procurement teams and certification authorities ensuring standards compliance
Related standards
- ISO 965-1 - general-purpose metric screw threads: tolerances (normative reference for tolerance grades)
- ISO 5855-2 - limit dimensions for bolts and nuts (complements Part 1)
- ISO 5855-3 - limit dimensions for fittings for fluid systems
Using ISO 5855-1 helps ensure interoperable, repeatable and inspectable MJ thread interfaces in aerospace applications, supporting safety and supply-chain consistency.
