ISO 965-3:2021

Overview

ISO 965-3:2021 - "ISO general purpose metric screw threads - Tolerances - Part 3: Limit deviations for screw threads" specifies the limit deviations for pitch and crest diameters of ISO general-purpose metric screw threads (M) that conform to ISO 261 and use the basic thread profile in ISO 68‑1. The values in this part are derived from the fundamental deviations and tolerances defined in ISO 965‑1 and are presented in detailed tables for recommended tolerance classes.

Key topics

• Scope and applicability: Limit deviations apply to pitch and crest diameters for internal and external ISO metric threads (M) with recommended tolerance classes.
• Numeric limits: Table 1 (in the standard) gives the upper (ES, es) and lower (EI, ei) deviation values by basic major diameter, pitch and tolerance class.
• Profile and contour requirements: The actual root contour of internal and external threads must not transgress the basic profile and the specified fundamental deviation at any point.
• Coated threads: Tolerances are specified for parts before coating; after coating the thread profile must not transgress maximum material limits for positions H (internal) or h (external). Note: provisions are intended for thin coatings (e.g., electroplating).
• Relationship to other parts: Values are derived from ISO 965‑1; terminology references ISO 5408. The document is one part of the ISO 965 series on thread tolerances.

Applications and who uses it

ISO 965‑3:2021 is a practical reference for:

• Design engineers specifying thread tolerance classes for mechanical assemblies and fasteners.
• Manufacturers and machinists setting machine tool processes, thread-cutting and rolling parameters.
• Quality engineers and inspectors verifying pitch and crest diameters against limit deviations for acceptance and interchangeability.
• Fastener suppliers and procurement teams ensuring supplied threads meet international tolerance limits.
• Standards bodies and technical committees coordinating compatibility of metric screw threads between components and suppliers.

Typical uses include tolerance selection for mating parts, acceptance criteria in inspection plans, drawing notes for threaded features, and ensuring global interchangeability of metric fasteners.

Related standards

• ISO 965‑1 (Fundamental deviations and tolerances)
• ISO 261 (ISO general purpose metric screw threads - Designation and dimensions)
• ISO 68‑1 (Basic profile for metric threads)
• ISO 5408 (Screw threads - Vocabulary)
• Other parts of the ISO 965 series (for comprehensive tolerance systems)

Keywords: ISO 965-3:2021, metric screw threads, limit deviations, thread tolerances, pitch diameter, crest diameter, ISO 261, ISO 68-1, ISO 965-1, tolerance classes, coated threads.