ISO 6621-5:2013

INTERNATIONAL ISO
STANDARD 6621-5
Third edition
2013-10-01
Internal combustion engines —
Piston rings —
Part 5:
Quality requirements
Moteurs à combustion interne — Segments de piston —
Partie 5: Exigences de qualité
Reference number
ISO 6621-5:2013(E)
©
ISO 2013

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ISO 6621-5:2013(E)

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ii © ISO 2013 – All rights reserved

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ISO 6621-5:2013(E)

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 2
4 Visible defects. 2
4.1 General . 2
4.2 Pores, cavities, and sand inclusions . 2
4.3 Scratches, indentations, depressions, and cracks . 3
4.4 Edges . 3
4.5 Other characteristics subject to visual inspection only .10
5 Material .11
5.1 Specifications.11
5.2 Loss of tangential force under temperature effects .11
6 Raised material caused by marking of rings .11
7 Machining of periphery and sides — Unintentional deviation from ideal profile
and flatness .12
7.1 General .12
7.2 Permissible deviations for peripheral surface profile .12
7.3 Permissible helix .13
Bibliography .14
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ISO 6621-5:2013(E)

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. 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. 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.
The committee responsible for this document is ISO/TC 22, Road vehicles.
This third edition cancels and replaces the second edition (ISO 6621-5:2005), of which it constitutes a
minor revision.
ISO 6621 consists of the following parts, under the general title Internal combustion engines — Piston rings:
— Part 1: Vocabulary
— Part 2: Inspection measuring principles
— Part 3: Material specifications
— Part 4: General specifications
— Part 5: Quality requirements
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ISO 6621-5:2013(E)

Introduction
This International Standard is one of a number of series of International Standards dealing with piston
rings for reciprocating internal combustion engines. Others are ISO 6622, ISO 6623, ISO 6624, ISO 6625,
ISO 6626, and ISO 6627.
The common features and dimensional tables presented in this part of ISO 6621 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. The designer also refers to the specifications and requirements of
ISO 6621-3 and ISO 6621-4 before completing his selection.
The difficulty of trying to define in absolute terms the quality attainable in normal commercial
manufacture of piston rings is well known. In this part of ISO 6621, the commonly encountered aspects
of quality in terms of casting defects and other departures from ideal are quantified. Many minor defects
are clearly quite acceptable; other defects because of size or number are inadmissible.
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INTERNATIONAL STANDARD ISO 6621-5:2013(E)
Internal combustion engines — Piston rings —
Part 5:
Quality requirements
1 Scope
This part of ISO 6621 specifies those quality aspects that are capable of definition but not normally
found on a drawing specification.
It is applicable to the following:
— single-piece piston rings of grey cast iron or steel;
— multi-piece piston rings (oil control rings) consisting of cast iron parts and spring components;
— single-piece and multi-piece oil control rings of steel, i.e. oil control rings in the form of strip steel
components or steel segments (rails) with spring expander components.
In addition to specifying certain of the limits of acceptance relating to inspection measuring principles
(covered by ISO 6621-2), this part of ISO 6621 also covers those features for which no recognized
quantitative measurement procedures exist and which are only checked visually with normal eyesight
(spectacles if worn normally) and without magnification. Such features (superficial defects) are
additional to the standard tolerances of ring width, radial wall thickness, and closed gap.
This part of ISO 6621 does not establish acceptable quality levels (AQL), it being left to manufacturer and
client to decide the appropriate levels jointly. In this case, the recommendations of ISO 2859 are followed.
This part of ISO 6621 specifies the quality requirements of piston rings for reciprocating internal
combustion engines for road vehicles and other applications. It is applicable to all such rings of a nominal
diameter from 30 mm up to and including 200 mm.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are
indispensable for its application. For dated references, only the edition cited applies. For undated
references, the latest edition of the referenced document (including any amendments) applies.
ISO 6621-1, Internal combustion engines — Piston rings — Part 1: Vocabulary
ISO 6621-3, Internal combustion engines — Piston rings — Part 3: Material specifications
ISO 6622 (all parts), Internal combustion engines — Piston rings
ISO 6623, Internal combustion engines — Piston rings — Scraper rings made of cast iron
ISO 6624 (all parts), Internal combustion engines — Piston rings
ISO 6625, Internal combustion engines — Piston rings — Oil control rings
ISO 6626, Internal combustion engines — Piston rings — Coil-spring-loaded oil control rings
ISO 6626-2, Internal combustion engines — Piston rings — Part 2: Coil-spring-loaded oil control rings of
narrow width made of cast iron
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ISO 6621-5:2013(E)

ISO 6626-3, Internal combustion engines — Piston rings — Part 3: Coil-spring-loaded oil control rings
made of steel
ISO 6627, Internal combustion engines — Piston rings — Expander/segment oil-control rings
3 Terms and definitions
For the purposes of this document, the terms and definitions in ISO 6621-1 apply.
4 Visible defects
4.1 General
Visible defects are divided into two principal classes as described in 4.2 to 4.5.
The first class covers those defects frequently found in castings and includes such defects as porosity,
sand inclusions, cavities, etc.
The second class of defects covers mechanical abrasions which may occur during forming, machining, or
handling of the rings and includes scratches, dents, chipping, burrs, and cracks.
Inspection of piston rings for such defects is generally carried out visually, without magnification, by
inspectors having normal eyesight, corrected if necessary.
It is not intended that every ring be rigorously inspected for size and distribution of defects, but rather
that the values given in the tables and text be used as a general guide. However, in case of doubt, the
values given should be used as the means of judging the quality of the rings.
4.2 Pores, cavities, and sand inclusions
Such defects are permissible on uncoated surfaces and edges provided that the values given in Table 1
for size, number, and spacing are not exceeded.
Table 1 — Permissible values of size, number, and spacing of pores, cavities, and sand inclusions
Dimensions in millimetres
Defect size
max.
Nominal Number of Spacing of
c
diameter defects per ring defects
On On
On other On other
d max. min.
1
peripheral peripheral
a b
surfaces edges
a
surface edges
30 ≤ d < 60 0,1 0,3 0,1 0,1 2 4
1
60 ≤ d < 100 0,15 0,5 0,1 0,2 4 4
1
100 ≤ d < 150 0,2 0,5 0,1 0,3 6 8
1
150 ≤ d ≤ 200 0,2 0,8 0,1 0,4 8 8
1
a
The defects should not be closer to an edge than one-half of the maximum permissible size of the defect, with a minimum
of 0,2.
b
Does not apply to inside gap edges of piston rings with internal notch.
c
Spacing includes defects on adjacent or opposite surfaces.
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ISO 6621-5:2013(E)

4.3 Scratches, indentations, depressions, and cracks
4.3.1 Scratches
Isolated scratches are permissible provided that
— no burrs are produced exceeding the permissible values given in 4.4.1.1,
— on turned peripheral surfaces, they are not deeper than the tool marks,
— on non-turned peripheral surfaces, they are not deeper than 0,004 mm,
— on the side faces, they are not deeper than 0,01 mm, and
— on other surfaces, they are not deeper than 0,06 mm.
4.3.2 Indentations and depressions
Indentations and depressions are permissible provided that
— the values given in Table 1 for number and spacing of defects are met,
— no burrs are produced exceeding the permissible values given in 4.4.1.1, and
— they do not exceed the values for size and depth given in Table 2.
Rings of a plated/coated/nitrided type shall not have indentations or depressions on the periphery.
NOTE Indentations arising from hardness measurements on the side faces are acceptable provided that they
do not exceed the limits given in Tables 2 and 11.
4.3.3 Cracks
No cracks are permissible.
See also 4.5.4 for chromium-plated peripheral surfaces and 4.5.6 for nitrided surfaces.
Table 2 — Permissible size of indentations and depressions
Dimensions in millimetres
Defect size
Nominal diameter Depth
max.
d max.
1
On peripheral surface On side face
30 ≤ d < 100 0,3 0,6
1
10 % of corresponding max.
defect size
100 ≤ d ≤ 200 0,5 1
1
4.4 Edges
4.4.1 Edge configuration
All edges of the piston ring shall be sharp; ideally, they should be free from burrs and from ragged edges,
whether arising from crumbling of material or from deburring. Such conditions are almost impossible to
achieve regularly in volume production and hence both burrs and removal of edge material is permitted
up to the maximum sizes given in 4.4.1.1 and 4.4.1.2.
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ISO 6621-5:2013(E)

4.4.1.1 Burrs
Burrs are permitted up to the maximum values given in Table 3. The orientation and direction of burrs
shall relate to the functional surfaces of the piston ring; any burr present should point in the direction
of sliding motion of the ring and not normal to the direction of sliding.
Any burrs remaining on the edges of rings should be firmly attached, forming an integral part of the edge.
Table 3 — Permissible size of burrs for all sizes of ring
Dimensions in millimetres
Maximum size of burr
Cast iron and Expander/segment oil control rings
Burrs on edges adjacent to:
steel single and
Nitrided seg-
two-piece rings Expander Plated segment
ment
Peripheral surface 0,006 0,01 0,004 0,004
Side faces 0,006 0,01 0,02 0,01
Butt ends (gap surface) 0,04 0,1 0,1 0,04
The outside groove face (oil
0,2 — — —
rings)
The inside surface and the ends
0,5 0,5 0,1 0,1
of the slots (oil rings)
All other surfaces 0,1 0,1 0,1 0,1
4.4.1.2 Edge material removal
To eliminate protruding burrs in any direction, it is permissible to remove material from the edges to
the values given in Table 4.
Table 4 — Edge material removal in deburr operations
Dimensions in millimetres
Removal of material
Location of edge
max.
On peripheral edges 0,08
a
On peripheral edges of the gap 0,15
0,5 in circumferential direction
On inside edges of the gap
0,25 in radial direction
On other edges 0,25
a
Does not apply to rings which have specified gap edge chamfers.
4.4.2 Chipping and similar defects
...