ISO 6621-2:2020

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

General Information

• Status: Published
• Publication Date: 02-Mar-2020
• 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: 9060 - Close of review
• Completion Date: 02-Sep-2030

Relations

• Revises: ISO 6621-2:2003 - Internal combustion engines - Piston rings - Part 2: Inspection measuring principles
• Effective Date: 21-Nov-2015

Overview - ISO 6621-2:2020 (Piston rings - Inspection measuring principles)

ISO 6621-2:2020 specifies the measuring principles used to inspect piston rings for reciprocating internal combustion engines (rings up to and including 200 mm diameter). The document defines how to measure geometric, dimensional and performance-related characteristics of rings - including ring width, radial wall thickness, free and closed gaps, forces, ovality, surface texture and coating depth. It can also be applied to piston rings for compressors operating under analogous conditions. The standard references GPS surface texture standards (ISO 4287, ISO 4288) and the ISO 6621 series vocabulary and ring-type standards.

Key technical topics and requirements

The standard defines measurement methods and units (millimetres, micrometres, Newtons, degrees) for many ring characteristics, for example:

• Ring geometry

  • Ring width (h1, h3), radial wall thickness (a1)
  • Keystone angle, land width/offset, twist, obliqueness, helix
  • Ovality, point deflection, free flatness, unevenness

• Gaps and forces

  • Total free gap (m, p) and closed gap (s)
  • Tangential force (Ft) and diametral force (Fd) - measured in Newtons

• Peripheral surface features

  • Taper, barrel (t2, t3), circumferential waviness, light tightness (percentage)
  • Surface roughness parameters (Ra, Rz) per ISO 4287/4288

• Materials and coatings

  • Plating/coating thickness, nitrided case depth (hardness ≥ 700 HV0.1)

• General measurement conditions

  • Rings normally rest on a reference plane in the free/open state; some checks require the ring closed in a gauge of nominal bore size
  • Instruments shall have sufficient resolution (measurement resolution not to exceed 10% of the applicable tolerance)
  • Specific procedures are given for single-piece vs multi-piece rings and for rings with features such as internal notches

Practical applications and who uses this standard

ISO 6621-2 is used by:

• Engine and compressor designers specifying ring geometry and performance
• Piston ring manufacturers for quality control and incoming inspection
• Metrology and test laboratories performing dimensional and surface inspections
• OEMs, aftermarket suppliers and quality assurance teams ensuring ring conformity
• R&D teams evaluating ring designs (taper, barrel, twist, coatings, nitriding)

Benefits include consistent ring measurement practice, clearer specification of functional properties (sealing, oil control, mechanical fit), and reduced ambiguity in supplier/manufacturer communications.

Related standards

• ISO 6621-1 (Vocabulary)
• Other parts of ISO 6621 (Part 1–5 cover types, materials and dimensional tables)
• ISO 6624 series (keystone rings - cast iron/steel variants)
• ISO 4287, ISO 4288 (surface texture measurement)

Keywords: ISO 6621-2:2020, piston rings, inspection measuring principles, ring measurement, tangential force, diametral force, free gap, ovality, surface roughness, plating thickness, nitrided case depth.

Standards Content (Sample)

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: [phone]
Fax: [phone]
Email: [e-mail]
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
diameter
Note 1 to entry: See Figure 22.
Note 2 to entry: Point deflection is measured in millimetres.
3.10
light tightness
ability of the peripheral surface of a ring when mounted in a gauge of nominal cylinder bore diameter to
exclude the passage of light
Note 1 to entry: See Figure 23.
Note 2 to entry: Areas of the ring showing pinpoint, burry or fuzzy light shall be considered as light tight.
Note 3 to entry: Light tightness is measured in percentage of ring circumference.
3.11
taper on peripheral surface
intentional angular deviation of the peripheral surface from a line perpendicular to the reference plane
Note 1 to entry: See Figure 24.
Note 2 to entry: In the case of the taper faced peripheral surface with partly cylindrical area both measuring
points shall be placed on the taper area.
Note 3 to entry: Taper on peripheral surface is measured in micrometres or degrees.
3.12
barrel on peripheral surface
intentional convex deviation of the peripheral surface from a line perpendicular to the reference plane
Note 1 to entry: See Figure 26 for a symmetrical barrel and Figure 28 for an asymmetrical barrel.
Note 2 to entry: t is used for lower barrel face and t for upper barrel face.
2 3
Note 3 to entry: Barrel on a peripheral surface is measured in millimetres.
3.13
land width
h
h
width of the land which theoretically should be in contact with the cylinder bore
Note 1 to entry: See Figure 29.
Note 2 to entry: Land width is measured in millimetres.
3.14
land offset
displacement of the two peripheral surfaces of a slotted or drilled oil control ring in relation to each
other in a radial direction
Note 1 to entry: See Figure 31.
Note 2 to entry: Land offset is measured in millimetres.
3.15
plating/coating thickness
distance between the outer surface of the plating/coating and the surface of the base ring material
connected with the different configurations of platings/coatings
Note 1 to entry: See Figure 33.
Note 2 to entry: Plating/coating thickness is measured in millimetres.
3.16
nitrided case depth
thickness of the surface layer with a hardness value ≥700 HV 0,1 measured perpendicular to the ring
peripheral surface or side faces.
Note 1 to entry: See Figures 34 and 35.
Note 2 to entry: Nitrided case depth is measured in millimetres.
3.17
keystone angle
angle enclosed by the two sides of the ring or alternatively, the sum of both side face angles, i.e.
included angle
Note 1 to entry: See Figure 36.
Note 2 to entry: Keystone angle is measured in degrees.
3.18
obliqueness
unintentional deviation of the bisector of the keystone included angle from parallelism with the
reference plane
Note 1 to entry: Not applicable to rings with designed twist (3.19).
Note 2 to entry: See Figure 42.
Note 3 to entry: Obliqueness is measured in degrees.
3.19
twist
intentional torsional deviation of the section of the ring from the reference plane when the ring
is restricted to nominal diameter, as in the case of asymmetrical rings such as those internally or
externally stepped or bevelled
Note 1 to entry: See Figure 43.
Note 2 to entry: Twist is measured in millimetres.
4 © ISO 2020 – All rights reserved

3.20
unevenness
Te
unintentional deviation of the sides of the ring from parallelism to the reference plane, i.e. twisted or
dished rings
Note 1 to entry: See Figures 46 and 48.
Note 2 to entry: Not applicable to rings with designed twist (3.19), as covered by 4.2.18.
3.21
helix
displacement of the gap ends perpendicular to the reference plane
Note 1 to entry: See Figure 51.
Note 2 to entry: Helix is measured in millimetres.
3.22
free flatness
relationship between the ring in the free state and a plane parallel to its reference plane
Note 1 to entry: Free flatness is measured in millimetres.
3.23
circumferential waviness
three or more lobes showing a continuous pattern of peaks and valleys spaced at a set frequency
circumferentially around the bottom side face of the ring
Note 1 to entry: Circumferential waviness is measured in micrometres.
4 Measuring principles
4.1 General measuring conditions
The following general requirements are applicable to all measuring principles unless otherwise
specified:
a) the ring shall rest on the reference plane in the free or open condition. No additional force shall be
applied to load the ring on the reference plane; except when measuring "unevenness" in accordance
with 4.2.19 or "helix" in accordance with 4.2.20;
b) certain measurements are made with the ring in the closed condition in a gauge of nominal cylinder
bore diameter. When orientated rings are measured in this way, they shall be so placed that the top
side of the ring is towards the reference plane;
c) measurements shall be made using instruments with a resolution not to exceed 10 % of the
tolerance of the dimension being measured.
For further terms and definitions see ISO 6621-1.
4.2 Characteristics and measuring principles
4.2.1 Ring width
4.2.1.1 Parallel sided rings, h (in millimetres)
Measuring principle
Measure with spherical measuring probes each of radius 1,5 mm ± 0,05 mm, exerting a measuring force of
approximately 1 N (see Figure 3).
In the case of slotted oil rings, the measurement shall be made between the slots and not across them (see
Figure 2).
Key
h  ring width
Figure 1
Key
h  ring width
Figure 2
Figure 3
4.2.1.2 Keystone rings, half keystone rings, h
Measuring principle
Method A
This method determines h for a specified value of a (see Figure 4).
3 6
Key
h  keystone ring width
a  keystone ring depth
Figure 4
6 © ISO 2020 – All rights reserved

Measuring principle
Measure with spherical measuring probes each radius 1,5 mm ± 0,05 mm exerting a measuring force of ap-
proximately 1 N (see Figure 5).
If the measuring equipment is calibrated with parallel gauges instead of keystone gauges, the use of spherical
measuring probes for the measurement according to Figure 4 will give rise to an error as follows:
— for 6° keystone angle: 0,004 mm (Ring types: T, TB, TBA, and TM)
— for 7° keystone angle: 0,012 mm (Ring types: HK, and HKB)
— for 15° keystone angle: 0,026 mm. (Ring types: K, KB, KBA, and KM)
To obtain the correct measured width of the keystone ring, the above values shall be deducted from the meas-
ured values.
Values of a are given in ISO 6624-1 to ISO 6624-4.
Key
a  keystone ring depth
Figure 5
Method B
This method determines a for a specified value of h (see Figure 4).
6 3
Measure with a flat face probe exerting a measuring force of approximately 1 N. The ring shall be placed between
two sharp-edged (radius ≤0,01 mm) circular discs which are spaced apart at the specified gauge width h (see
Figure 6).
Values of h are given in ISO 6624-1 to ISO 6624-4.
Key
h  keystone ring width
a  keystone ring depth
Figure 6
4.2.2 Radial wall thickness, a
Measuring principle
1. Measure radially between a flat measuring surface on the peripheral surface and a special measuring
surface of the radius approximately 4 mm on the bore, and using a measuring force of 3 N to 10 N (see
Figure 8).
Key
a  radial wall thickness
Figure 7
2. Measure radially between cylindrical inserts or rollers of radius approximately 4 mm and with a measuring
force of 3 N to 10 N. The peripheral surface of the rollers shall be perpendicular to the reference plane.
Figure 8
The length of the rollers shall be greater than the axial ring width (see Figure 9).
Figure 9
4.2.3 Total free gap m, p
Measuring principle
Measure with a steel rule to the nearest 0,25 mm.
Optionally, this feature can be measured with callipers.
8 © ISO 2020 – All rights reserved

Measuring principle
Key
m  free gap
a  radial wall thickness
Figure 10
Key
p  free gap
a  radial wall thickness
Figure 11
4.2.4 Closed gap, s
Measuring principle
Measure in a bore gauge of nominal diameter using a wedge gauge or feeler gauges and using a measuring force
of approximately 1 N (see Figure 12).
For closed gap tolerances lower than values in the ISO standard tables (ISO 6622, ISO 6623, ISO 6624, ISO 6625,
ISO 6626, ISO 6627) measuring methods with improved accuracy are required. One example would be to use
an optical technique. The gap area shall be thoroughly cleaned to obtain an accurate measurement.
+0,001 xd
The diameter of the bore gauge shall comply with the following: d
Correction shall be made for any deviation of the bore gauge from the nominal ring diameter.
Key
d  nominal diameter
s  closed gap
Figure 12
4.2.5 Tangential force, F (in Newton)
t
4.2.5.1 For single-piece rings
NOTE Not recommended for rings d < 50mm; for these rings, see 4.2.6.
Measuring principle
1. Tape method (see Figure 13)
The encircling steel tape of thickness 0,08 mm to 0,10 mm is carried around 10 mm diameter rollers set
20 mm apart (see Figure 13). In tightening the tape, the ring is closed to the point where the gap ends
touch and then open to the closed gap dimension previously measured. The ring force is then read off from
the precision measuring scale. The gap of the ring shall be symmetrically disposed between the rollers.
An alternative method to set up the tangential loading of the force measuring instrument is using a solid
disc of nominal bore diameter ±0,005 mm to set up the length of the tape. The gauge disc is inserted into
the tape and the tape length adjusted until the specified mean limit tangential force is indicated.
Key
1  measuring scale
2  diameter of rollers 10
...