ISO 6621-2:2020
(Main)Internal combustion engines — Piston rings — Part 2: Inspection measuring principles
Internal combustion engines — Piston rings — Part 2: Inspection measuring principles
This document defines the measuring principles to be used for measuring piston rings; it applies to piston rings up to and including 200 mm diameter for reciprocating internal combustion engines. This document can be used for piston rings for compressors working under analogous conditions.
Moteurs à combustion interne — Segments de piston — Partie 2: Principes de mesure pour inspection
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INTERNATIONAL ISO
STANDARD 6621-2
Third edition
2020-03
Internal combustion engines —
Piston rings —
Part 2:
Inspection measuring principles
Moteurs à combustion interne — Segments de piston —
Partie 2: Principes de mesure pour inspection
Reference number
©
ISO 2020
© ISO 2020
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
on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address
below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Fax: +41 22 749 09 47
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO 2020 – All rights reserved
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Measuring principles . 5
4.1 General measuring conditions . 5
4.2 Characteristics and measuring principles . 6
4.2.1 Ring width . 6
4.2.2 Radial wall thickness, a . 8
4.2.3 Total free gap m, p . 8
4.2.4 Closed gap, s . 9
4.2.5 Tangential force, F (in Newton) .10
t
4.2.6 Diametral force, F (in Newton) .15
d
4.2.7 Ovality, U (in millimetres) .16
4.2.8 Point deflection, W (in millimetres) .17
4.2.9 Light tightness (percentage of ring circumference) .17
4.2.10 Taper on peripheral surface (in micrometres or degrees) .18
4.2.11 Barrel on peripheral surface, t , t (in millimetres) .18
2 3
4.2.12 Land width, h , h (in millimetres) .20
4 5
4.2.13 Land offset (in millimetres) .21
4.2.14 Plating/coating thickness (in millimetres) .21
4.2.15 Nitrided case depth (in millimetres) .22
4.2.16 Keystone angle (in degrees) .22
4.2.17 Obliqueness (in degrees) .24
4.2.18 Twist (in millimetres) .25
4.2.19 Unevenness Te , Te .26
r u
4.2.20 Helix (axial displacement of gap ends) (in millimetres) .27
4.2.21 Free flatness (in millimetres) .27
4.2.22 Surface roughness Ra, Rz (in micrometers) .28
4.2.23 Circumferential waviness — Bottom side face (in micrometres) .28
Bibliography .30
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
constitute an endorsement.
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 6621-2:2003), which has been technically
revised.
The main changes compared to the previous edition are as follows:
— Oil ring diameter range for ring widths 3,0 mm and 3,5 mm increased up to 160 mm.
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 2020 – All rights reserved
Introduction
This document is one of a number of International Standards dealing with piston rings for reciprocating
internal combustion engines. Others are ISO 6621-1, ISO 6621-3, ISO 6621-4, ISO 6621-5, ISO 6622,
ISO 6623, ISO 6624, ISO 6625, ISO 6626 and ISO 6627 (see Bibliography for details).
The common features and dimensional tables presented in this document constitute a broad range of
variables, and the designer, in selecting a particular ring type, should bear in mind the conditions under
which it will be required to operate.
It is also essential that the designer refer to the specifications and requirements of ISO 6621-3 and
ISO 6621-4 before completing their selection.
INTERNATIONAL STANDARD ISO 6621-2:2020(E)
Internal combustion engines — Piston rings —
Part 2:
Inspection measuring principles
1 Scope
This document defines the measuring principles to be used for measuring piston rings; it applies to
piston rings up to and including 200 mm diameter for reciprocating internal combustion engines.
This document can be used for piston rings for compressors working under analogous conditions.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 4287, Geometrical Product Specifications (GPS) — Surface texture: Profile method — Terms, definitions
and surface texture parameters
ISO 4288, Geometrical Product Specifications (GPS) — Surface texture: Profile method — Rules and
procedures for the assessment of surface texture
ISO 6621-1, Internal combustion engines — Piston rings — Part 1: Vocabulary
ISO 6624-1, Internal combustion engines — Piston rings — Part 1: Keystone rings made of cast iron
ISO 6624-2, Internal combustion engines — Piston rings — Part 2: Half keystone rings made of cast iron
ISO 6624-3, Internal combustion engines — Piston rings — Part 3: Keystone rings made of steel
ISO 6624-4, Internal combustion engines — Piston rings — Part 4: Half keystone rings made of steel
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 6621-1 and the following apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
3.1
h
distance between the sides, at any particular point perpendicular to the reference plane measured in
millimetres
Note 1 to entry: See Figures 1 and 2.
3.2
h
distance between the sides at a specified distance a from the peripheral surface.
Note 1 to entry: See Figure 4.
Note 2 to entry: Alternatively, the ring width is controlled by a at a specified width h (see Figure 6).
6 3
3.3
radial wall thickness
a
radial distance between the peripheral surface and the inside surface of the ring measured in
millimetres
Note 1 to entry: See Figure 7.
3.4
total free gap
m
p
chordal distance between the gap ends of the ring in a free unstressed state, measured at the centre
line of the radial wall thickness (3.3)
Note 1 to entry: See Figure 10.
Note 2 to entry: For rings with an internal notch for a peg, the total free gap is defined by the chordal distance
marked as p in Figure 11.
Note 3 to entry: The total free gap is measured in millimetres.
3.5
closed gap
s
distance between the gap ends of the ring measured at the narrowest point, which the ring would have
when fitted in a gauge of nominal cylinder bore size
Note 1 to entry: See Figure 12.
Note 2 to entry: The closed gap s is related to the nominal diameter d .
1 1
3.6
tangential force
F
t
force necessary to maintain the ring at the closed gap (3.5) condition by means of a tangential pull on
the ends of a circumferential metal tape or hoop
Note 1 to entry: See Figures 13 to15.
Note 2 to entry: Tangential force is measured in Newtons.
Note 3 to entry: For single-piece rings, it is not recommended for rings d < 50mm; for these rings, see 4.2.6.
Note 4 to entry: For multi-piece rings, vibration is used to reduce friction during or prior to measurement.
3.7
diametral force
F
d
force, acting diametrically at 90° to the gap, necessary to maintain the ring at the nominal diameter
condition measured in the direction of the force
Note 1 to entry: See Figure 20.
Note 2 to entry: This method is only applicable to single-piece rings.
2 © ISO 2020 – All rights reserved
Note 3 to entry: Diametral force is measured in Newtons.
3.8
ovality
U
difference between the mutually perpendicular diameters d and d when the ring is drawn to a closed
3 4
gap (3.5) within a flexible tape
Note 1 to entry: It can be either positive (d > d ) or negative (d < d ) (see Figure 21).
3 4 3 4
Note 2 to entry: This method is only applicable to single-piece rings.
Note 3 to entry: Ovality is measured in millimetres.
3.9
point deflection
W
deviation of the butt ends from the true circle when restrained in a gauge of nominal cylinder bore
...
INTERNATIONAL ISO
STANDARD 6621-2
Third edition
2020-03
Internal combustion engines —
Piston rings —
Part 2:
Inspection measuring principles
Moteurs à combustion interne — Segments de piston —
Partie 2: Principes de mesure pour inspection
Reference number
©
ISO 2020
© ISO 2020
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
on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address
below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Fax: +41 22 749 09 47
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO 2020 – All rights reserved
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Measuring principles . 5
4.1 General measuring conditions . 5
4.2 Characteristics and measuring principles . 6
4.2.1 Ring width . 6
4.2.2 Radial wall thickness, a . 8
4.2.3 Total free gap m, p . 8
4.2.4 Closed gap, s . 9
4.2.5 Tangential force, F (in Newton) .10
t
4.2.6 Diametral force, F (in Newton) .15
d
4.2.7 Ovality, U (in millimetres) .16
4.2.8 Point deflection, W (in millimetres) .17
4.2.9 Light tightness (percentage of ring circumference) .17
4.2.10 Taper on peripheral surface (in micrometres or degrees) .18
4.2.11 Barrel on peripheral surface, t , t (in millimetres) .18
2 3
4.2.12 Land width, h , h (in millimetres) .20
4 5
4.2.13 Land offset (in millimetres) .21
4.2.14 Plating/coating thickness (in millimetres) .21
4.2.15 Nitrided case depth (in millimetres) .22
4.2.16 Keystone angle (in degrees) .22
4.2.17 Obliqueness (in degrees) .24
4.2.18 Twist (in millimetres) .25
4.2.19 Unevenness Te , Te .26
r u
4.2.20 Helix (axial displacement of gap ends) (in millimetres) .27
4.2.21 Free flatness (in millimetres) .27
4.2.22 Surface roughness Ra, Rz (in micrometers) .28
4.2.23 Circumferential waviness — Bottom side face (in micrometres) .28
Bibliography .30
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
constitute an endorsement.
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 6621-2:2003), which has been technically
revised.
The main changes compared to the previous edition are as follows:
— Oil ring diameter range for ring widths 3,0 mm and 3,5 mm increased up to 160 mm.
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 2020 – All rights reserved
Introduction
This document is one of a number of International Standards dealing with piston rings for reciprocating
internal combustion engines. Others are ISO 6621-1, ISO 6621-3, ISO 6621-4, ISO 6621-5, ISO 6622,
ISO 6623, ISO 6624, ISO 6625, ISO 6626 and ISO 6627 (see Bibliography for details).
The common features and dimensional tables presented in this document constitute a broad range of
variables, and the designer, in selecting a particular ring type, should bear in mind the conditions under
which it will be required to operate.
It is also essential that the designer refer to the specifications and requirements of ISO 6621-3 and
ISO 6621-4 before completing their selection.
INTERNATIONAL STANDARD ISO 6621-2:2020(E)
Internal combustion engines — Piston rings —
Part 2:
Inspection measuring principles
1 Scope
This document defines the measuring principles to be used for measuring piston rings; it applies to
piston rings up to and including 200 mm diameter for reciprocating internal combustion engines.
This document can be used for piston rings for compressors working under analogous conditions.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 4287, Geometrical Product Specifications (GPS) — Surface texture: Profile method — Terms, definitions
and surface texture parameters
ISO 4288, Geometrical Product Specifications (GPS) — Surface texture: Profile method — Rules and
procedures for the assessment of surface texture
ISO 6621-1, Internal combustion engines — Piston rings — Part 1: Vocabulary
ISO 6624-1, Internal combustion engines — Piston rings — Part 1: Keystone rings made of cast iron
ISO 6624-2, Internal combustion engines — Piston rings — Part 2: Half keystone rings made of cast iron
ISO 6624-3, Internal combustion engines — Piston rings — Part 3: Keystone rings made of steel
ISO 6624-4, Internal combustion engines — Piston rings — Part 4: Half keystone rings made of steel
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 6621-1 and the following apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
3.1
h
distance between the sides, at any particular point perpendicular to the reference plane measured in
millimetres
Note 1 to entry: See Figures 1 and 2.
3.2
h
distance between the sides at a specified distance a from the peripheral surface.
Note 1 to entry: See Figure 4.
Note 2 to entry: Alternatively, the ring width is controlled by a at a specified width h (see Figure 6).
6 3
3.3
radial wall thickness
a
radial distance between the peripheral surface and the inside surface of the ring measured in
millimetres
Note 1 to entry: See Figure 7.
3.4
total free gap
m
p
chordal distance between the gap ends of the ring in a free unstressed state, measured at the centre
line of the radial wall thickness (3.3)
Note 1 to entry: See Figure 10.
Note 2 to entry: For rings with an internal notch for a peg, the total free gap is defined by the chordal distance
marked as p in Figure 11.
Note 3 to entry: The total free gap is measured in millimetres.
3.5
closed gap
s
distance between the gap ends of the ring measured at the narrowest point, which the ring would have
when fitted in a gauge of nominal cylinder bore size
Note 1 to entry: See Figure 12.
Note 2 to entry: The closed gap s is related to the nominal diameter d .
1 1
3.6
tangential force
F
t
force necessary to maintain the ring at the closed gap (3.5) condition by means of a tangential pull on
the ends of a circumferential metal tape or hoop
Note 1 to entry: See Figures 13 to15.
Note 2 to entry: Tangential force is measured in Newtons.
Note 3 to entry: For single-piece rings, it is not recommended for rings d < 50mm; for these rings, see 4.2.6.
Note 4 to entry: For multi-piece rings, vibration is used to reduce friction during or prior to measurement.
3.7
diametral force
F
d
force, acting diametrically at 90° to the gap, necessary to maintain the ring at the nominal diameter
condition measured in the direction of the force
Note 1 to entry: See Figure 20.
Note 2 to entry: This method is only applicable to single-piece rings.
2 © ISO 2020 – All rights reserved
Note 3 to entry: Diametral force is measured in Newtons.
3.8
ovality
U
difference between the mutually perpendicular diameters d and d when the ring is drawn to a closed
3 4
gap (3.5) within a flexible tape
Note 1 to entry: It can be either positive (d > d ) or negative (d < d ) (see Figure 21).
3 4 3 4
Note 2 to entry: This method is only applicable to single-piece rings.
Note 3 to entry: Ovality is measured in millimetres.
3.9
point deflection
W
deviation of the butt ends from the true circle when restrained in a gauge of nominal cylinder bore
...
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