Overview
ISO 965-6:2025 specifies the limits of sizes for major, pitch and minor diameters of ISO general‑purpose metric screw threads (M). It applies to metric fastening screw threads that conform to ISO 261 and use basic and design profiles per ISO 68‑1, for the ten recommended tolerance classes (4H, 5H, 6H, 7H, 6G, 4h, 6h, 6g, 6f and 6e). The standard provides formulae (rounded to the third decimal place) and tabulated limits across nominal diameter ranges for internal and external threads, focused on fine and medium tolerance qualities (first and second choices).
Keywords: ISO 965-6:2025, metric screw threads, limits of sizes, tolerance classes, major diameter, pitch diameter, minor diameter.
Key technical topics and requirements
- Scope and applicability
- Limits of sizes for internal and external M screw threads conforming to ISO 261 and ISO 68‑1.
- Covers the ten tolerance classes recommended in ISO 965‑1.
- Dimensions defined
- Major diameter, pitch diameter, minor diameter for internal and external threads.
- Calculation and tables
- Limits derived from fundamental deviations and tolerances in ISO 965‑1.
- Formulae provided for calculation of limits; results rounded to three decimal places.
- Extensive tabulated values (multiple tables by tolerance class and nominal diameter) for practical selection.
- Gauging and measurement notes
- Maximum major diameter of internal thread is not specified.
- Minimum major diameter of internal thread and maximum minor diameter of external thread are usually gauged by the crest diameter of GO thread gauges (ISO 1502).
- Measurement of maximum and minimum minor diameters for external threads is decided by product documentation; measurement may not always be required.
- Normative references
Practical applications and users
- Who uses it
- Mechanical designers, fastener manufacturers, quality engineers, metrology and inspection teams, and standards/ compliance specialists.
- Use cases
- Specifying thread tolerances for components and assemblies to ensure interchangeability.
- Setting inspection and gauging criteria for bolts, nuts and threaded components.
- Product specification in automotive, aerospace, machinery, and general manufacturing where metric M threads are used.
- Benefits
- Ensures consistent functional fit between internal and external threads.
- Facilitates international interchangeability and reduces rework and assembly failure.
Related standards
For implementation or procurement, consult the full ISO 965‑6:2025 document and relevant national standards bodies.
Open preview (PDF)ISO general purpose metric screw threads — Tolerances — Part 6: Limits of sizes for internal and external threads (fine and medium tolerance qualities, first and second choices)