Overview
ISO 261:1998 provides the general plan for ISO general-purpose metric screw threads (M). The standard establishes the framework for selecting nominal diameters and thread pitches and ties the metric thread form to the basic profile in ISO 68-1, with basic dimensions in ISO 724 and tolerances referenced to ISO 965-1. ISO 261 is intended as a practical guide for designers, manufacturers and engineers working with standard metric threads.
Key Topics
- Scope: Specifies ISO general-purpose metric screw threads (M) with the basic profile defined by ISO 68-1. Basic dimensions are given in ISO 724; tolerances are specified in ISO 965-1.
- Designation: Thread designation shall follow the rules in ISO 965-1.
- Diameter and pitch selection:
- Preference is given to diameters listed in the standard in a prioritized set of columns (choose column 1 first, then column 2, then column 3).
- For each selected nominal diameter (or diameter range), select one of the pitches shown on the corresponding line.
- The standard distinguishes “coarse” and “fine” pitches to reflect common practice; these terms do not imply quality differences.
- Some pitches shown in parentheses are to be avoided where possible.
- A specific list of finer pitches that may be used when finer than table values are required is provided: 3 mm, 2 mm, 1.5 mm, 1 mm, 0.75 mm, 0.5 mm, 0.35 mm, 0.25 mm, 0.2 mm.
- The standard notes increasing difficulty in meeting tolerances for larger diameters with a given pitch.
- Special cases: Certain diameters and pitches are reserved for specific applications (for example, some sizes are noted only for spark plugs or for locking nuts for bearings).
Applications
ISO 261 is useful for:
- Mechanical designers specifying fasteners for assemblies where ISO metric threads are required.
- Manufacturers and procurement teams looking to standardize screw, bolt and nut sizes across product ranges.
- Quality and inspection personnel referencing the appropriate system for designation, pitch selection and conformity checks.
By following ISO 261, users ensure compatibility with ISO profile and dimension standards, simplify interchangeability, and reduce the need for bespoke thread forms.
Related Standards
- ISO 68-1 - Basic profile for metric screw threads (referenced for thread form)
- ISO 724 - Basic dimensions for ISO metric screw threads
- ISO 965-1 - Principles and basic data for tolerances and designation
- ISO 262 and ISO 5408 are additional normative references related to sizes and vocabulary
Following ISO 261:1998 ensures a harmonized approach to metric thread selection and designation in line with international practice.
