ISO 6621-3:2021

Internal combustion engines - Piston rings - Part 3: Material specifications

This document classifies materials intended for the manufacture of piston rings, based on their mechanical properties and the stresses the materials are capable of withstanding. This document is applicable to piston rings for reciprocating internal combustion engines up to and including those of 200 mm in diameter. It is also applicable to piston rings of compressors working under similar conditions.

Moteurs à combustion interne — Segments de piston — Partie 3: Spécifications des matériaux

General Information

• Status: Published
• Publication Date: 15-Jun-2021
• ICS: 43.060.10 - Engine block and internal components
• Technical Committee: ISO/TC 22/SC 34 - Propulsion, powertrain and powertrain fluids
• Drafting Committee: ISO/TC 22/SC 34/WG 4 - Piston rings
• Current Stage: 6060 - International Standard published
• Start Date: 16-Jun-2021
• Due Date: 29-Nov-2021
• Completion Date: 16-Jun-2021

Relations

• Revises: ISO 6621-3:2000 - Internal combustion engines - Piston rings - Part 3: Material specifications
• Effective Date: 15-Dec-2017

Overview

ISO 6621-3:2021 provides a standardized classification of materials intended for the manufacture of piston rings. The document covers piston rings for reciprocating internal combustion engines up to and including 200 mm in diameter and is also applicable to compressor rings operating under similar conditions. The standard focuses on mechanical properties, hardness measurement and the grouping of materials into classes and subclasses to support consistent selection and specification.

Key aspects emphasized by the standard include:

• Classification by mechanical strength and modulus, which influence ring contact pressure
• Use of Vickers hardness testing (ISO 6507-1) for base material hardness measurement
• Consideration of final coatings, engine rating, liner surface and microstructural features (graphite, cementite, ferrite) when selecting materials

Key Topics

• Material classes and subclasses: Materials are grouped into classes and more detailed subclasses (document uses MC codes) so manufacturers and clients can match ring performance requirements without relying solely on chemical composition. This allows different manufacturing routes to be compared by mechanical outcome rather than formulation alone.

• Mechanical properties: Selection criteria are based primarily on modulus and bending strength; modulus determines the pressure a ring exerts on the cylinder wall for a given geometry. The standard highlights relationships between modulus and strength while noting exceptions based on manufacturing and heat treatment.

• Hardness measurement: ISO 6507-1 is cited for Vickers HV30 testing of piston-ring base material. Hardness values in the document are averages from multiple measurements around the ring gap and are intended for classification; alternative hardness methods and equivalents are to be agreed between manufacturer and client.

• Manufacturing and microstructure considerations: The document advises that final selection should account for coatings, heat treatment, and microstructure since these factors affect wear, friction and long-term performance.

Applications

ISO 6621-3:2021 is practically useful for:

• Engine designers specifying piston rings for automotive, industrial and small- to medium-sized engines (diameter ≤ 200 mm)
• Compressor manufacturers seeking ring material guidance under comparable operating conditions
• Ring manufacturers and suppliers aligning material selection and testing with international practice
• Quality and procurement teams using standardized material classes to specify rings, coatings and hardness requirements

Benefits include clearer communication between client and manufacturer, consistent testing procedures, and reduced risk from mismatched material properties.

Related Standards

• ISO 6621-1: Vocabulary for piston rings
• ISO 6621-2: Inspection and measuring principles
• ISO 6621-4 and ISO 6621-5: General specifications and quality requirements for piston rings
• ISO 6507-1: Vickers hardness test method (normative reference for hardness measurements)

For selection decisions, consult ISO 6621-3 alongside the related ISO 6621 series parts and discuss final subclass, coating and testing arrangements directly with ring manufacturers to ensure compliance with application-specific demands.

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 6621-3
Third edition
2021-06
Internal combustion engines — Piston
rings —
Part 3:
Material specifications
Moteurs à combustion interne — Segments de piston —
Partie 3: Spécifications des matériaux
Reference number
©
ISO 2021
© ISO 2021
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting
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ISO copyright office
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Published in Switzerland
ii © ISO 2021 – All rights reserved

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Mechanical properties . 1
Bibliography . 4
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www .iso .org/ patents).
Any trade name used in this document is information given for the convenience of users and does not
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For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see www .iso .org/
iso/ foreword .html.
This document was prepared by Technical Committee ISO/TC 22, Road vehicles, Subcommittee SC 34,
Propulsion, powertrain and powertrain fluids.
This third edition cancels and replaces the second edition (ISO 6621-3:2000), which has been technically
revised.
The main changes compared to the previous edition are as follows:
— a material's new subclass was added.
A list of all parts in the ISO 6621 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www .iso .org/ members .html.
iv © ISO 2021 – All rights reserved

Introduction
This document is one of a number of series of International Standards dealing with piston rings for
[5] [6] [7]
reciprocating internal combustion engines. Others are ISO 6622-1 and ISO 6622-2 , ISO 6623 ,
[8] [9] [10] [11] [12] [13] [14]
ISO 6624-1 , ISO 6624-2 , ISO 6624-3 and ISO 6624-4 , ISO 6625 , ISO 6626 , ISO 6626-2 ,
[15] [16]
ISO 6626-3 , and ISO 6627 .
This document provides a user guide to the types of materials available for piston rings.
Many such materials are available, made by different manufacturers using different casting and
machining techniques, with each suited to a particular application. In many instances, their chemical
compositions differ, but the method of manufacture and the heat treatment, if any, result in materials
from different manufacturers with similar mechanical properties. The performance of rings made from
two different materials might be very similar; i.e. several subclasses of materials could meet a given
requirement.
In ring manufacture it is convenient to group materials into classes according to their moduli, since
for a ring of given dimensions, the pressure it exerts on the cylinder wall is determined only by the
modulus. The material strength is also generally related to modulus, i.e. the higher the modulus, the
greater the strength, although there are exceptions depending on the method of manufacture. Material
hardness, on the other hand, is determined by both chemical composition and heat treatment; this is
made clear by the division of classes into subclasses. Because of this, the final choice of material and
subclass is agreed between the manufacturer and client.
INTERNATIONAL STANDARD ISO 6621-3:2021(E)
Internal combustion engines — Piston rings —
Part 3:
Material specifications
1 Scope
This document classifies materials intended for the manufacture of piston rings, based on their
mechanical properties and the stresses the materials are capable of withstanding.
This document is applicable to piston rings for reciprocating internal combustion engines up to and
including those of 200 mm in diameter. It is also applicable to piston rings of c
...