# ISO metric threads

Metric fastening-thread designations, pitch and tolerance classes, coating state and dimensional versus joint-performance requirements.

Reviewed: 2026-11-01

Tags: parts-and-design, specifications, threads, fasteners

## Identity and applicable standards

ISO general-purpose metric fastening threads use an M designation and a 60° form. In M10 × 1.5, 10 mm is nominal major diameter and 1.5 mm is axial pitch. [Accu: metric thread identification](https://www.accu.co.uk/p/544-how-to-cut-tap-threads-a-complete-guide)

As a source-scoped example, Bossard's **01-2025** coarse-thread table gives M10 external **6g**, for listed engagement **5–15 mm**, a major-diameter range of **9.732–9.968 mm** and pitch-diameter range of **8.862–8.994 mm**. These are published table values, not dimensions recalculated from the 2026 standard or a strength-qualified engagement range. [Bossard, page 97](https://assets.eu.ctfassets.net/0vp0u5uh75zd/2tYqENAuufvdsudjM8Qbrc/b3461eaa59c2203d3a8b9509ac24dacf/096_098_Metric_ISOthreads_Fastening_EN_01_2025.pdf)

The relevant standards perform different jobs:

| Standard | Role and verified edition |
| --- | --- |
| ISO 68-1:2023 | Basic and design profiles; edition 2, October 2023 |
| ISO 261:1998 | General thread plan; edition 2, December 1998, confirmed 2024 |
| ISO 965-1:2026 | Tolerance-system principles and basic data; edition 5, April 2026 |
| ISO 1502:1996 | Gauges and gauging; edition 2, February 1996, confirmed 2021; published and under review |

These dates follow the ISO publication fields. ISO 965-1:2026 replaces the 2013 edition and its 2021 amendment. Keep the edition required by the drawing or purchase specification explicit instead of silently updating an existing contract. [ISO 68-1](https://www.iso.org/standard/85107.html), [ISO 261](https://www.iso.org/standard/4165.html), [ISO 965-1](https://www.iso.org/standard/87889.html), [ISO 1502](https://www.iso.org/standard/6092.html)

## Read the complete designation

A practical thread requirement identifies nominal diameter, pitch, internal or external tolerance class, handedness where relevant, and usable thread extent. For a proposed internal `M10 × 1.5-6H` and external `M10 × 1.5-6g` pair, the pitch matches, but the material, finish and remaining drawing requirements still need agreement. These example labels do not prescribe the appropriate fit for every joint.

Tolerance classes combine a number and letter. Uppercase letters identify internal-thread tolerance positions and lowercase letters external-thread positions; the number identifies tolerance grade. Accu illustrates `5g6g` as pitch-diameter class 5g followed by major-diameter class 6g, and explains that identical external classes can be abbreviated to `6g`. Do not interpret the number 6 as a fixed micrometer clearance. [Accu: thread tolerance notation](https://www.accu.co.uk/p/134-iso-metric-thread-tolerances)

Keep a thread tolerance class separate from a fastener's mechanical property class. A designation such as 6g addresses dimensional tolerancing; it does not specify a bolt's material strength. Likewise, do not confuse `M10 × 1.5` thread diameter/pitch notation with a product description giving nominal diameter and fastener length. Resolve ambiguous catalog or drawing shorthand before ordering tooling or mating parts.

## Coarse and fine pitches

A nominal diameter can have multiple standardized pitches. Bossard's January 2025 guide lists **M10 coarse pitch as 1.5 mm** and includes fine-thread examples such as **M10 × 1.25** and **M10 × 1**. These are distinct mating geometries, not interchangeable alternatives. State pitch explicitly when ambiguity would affect procurement or manufacture. [Bossard: metric ISO threads, pages 97–98](https://assets.eu.ctfassets.net/0vp0u5uh75zd/2tYqENAuufvdsudjM8Qbrc/b3461eaa59c2203d3a8b9509ac24dacf/096_098_Metric_ISOthreads_Fastening_EN_01_2025.pdf)

Choose the pitch against the actual joint and manufacturing process rather than assuming “fine” means universally stronger or more precise. Consider available wall and engagement, required adjustment, materials, service loading and production capability. The dimensional thread plan alone does not resolve those design choices.

**Original bounded pitch calculation:** assume a single-start screw with 1.5 mm pitch, without backlash or elastic movement. One revolution advances it 1.5 mm relative to its mating thread; eight revolutions advance it **12 mm**. A single-start 1.25 mm-pitch screw advances **10 mm** in eight revolutions. The resulting **2 mm** difference illustrates why apparently similar M10 parts cannot be substituted on diameter alone. It is not a lead-accuracy specification or an engagement-length recommendation; multi-start threads require their actual lead to be considered.

## Diameter limits and a bounded table example

Major, pitch and minor diameters describe different parts of the profile. A caliper reading across external crests is not a complete check of pitch diameter or functional thread conformity. Obtain the limits for the specified diameter, pitch, class, engagement range and delivery condition.

For an original arithmetic check, that listed major-diameter tolerance width is `9.968 − 9.732 = 0.236 mm`; the pitch-diameter width is `8.994 − 8.862 = 0.132 mm`. Their difference reinforces why a single generic “thread tolerance” value is inadequate. Use the governing specification for acceptance of an actual part.

## Coatings and the delivered condition

Coating consumes dimensional clearance. Bossard illustrates bright 6g external threads with 6H internal threads and identifies 6h external limits after coating; its footnote warns that 6H nuts need unused allowance within their manufactured field for added protection. This is a bounded guide example, not permission for arbitrary plating thickness. [Bossard, page 96](https://assets.eu.ctfassets.net/0vp0u5uh75zd/2tYqENAuufvdsudjM8Qbrc/b3461eaa59c2203d3a8b9509ac24dacf/096_098_Metric_ISOthreads_Fastening_EN_01_2025.pdf)

Specify whether limits and gauging apply before or after coating, together with the coating system and any special allowance. Check the actual process specification for thick coatings or other special systems. Do not repair a tight coated assembly by uncontrolled chasing or forcing without resolving the dimensional and protective-finish requirements.

## Gauging, strength and tightening

ISO 1502 specifies manufacture and use of gauges for checking internal and external metric threads against limits. Select calibrated, appropriate gauges and an acceptance method for the specified class and condition. A convenient mating bolt or nut is not automatically an adequate substitute for that inspection. [ISO gauging scope](https://www.iso.org/standard/6092.html)

Verify usable full-thread length, runout and blind-hole clearance separately from drill depth. Include the actual mating part and coating state in a production trial when assembly behavior matters. Investigate burrs, damage or misalignment before interpreting resistance as a tolerance-class problem.

Thread dimensional compatibility does not establish joint load capacity or installation torque. Bossard's torque guidance distinguishes fastener type, material and head shape, and explicitly says its approximate values do not replace a bolted-joint calculation. Its friction guidance identifies thread/bearing surfaces and lubrication as inputs to tightening-torque selection. [Bossard: preload and tightening torque](https://www.bossard.com/ch-en/knowledge-hub/resources/technical-information/preload-and-tightening-torque/), [friction guidance](https://www.bossard.com/in-en/knowledge-hub/resources/technical-information/friction-and-friction-coefficients/)

For an actionable handoff, record the full thread designation, governing editions, usable extent, mating requirement, material/property class, finish state and dimensional inspection. Record joint-strength and tightening requirements separately. The cited Bossard guide prints **01-2025**; live Accu and Bossard web tutorials have no verified technical revision date here.