ISO 6621-1:2018

INTERNATIONAL ISO
STANDARD 6621-1
Third edition
2018-02
Internal combustion engines —
Piston rings —
Part 1:
Vocabulary
Moteurs à combustion interne — Segments de piston —
Partie 1: Vocabulaire
Reference number
©
ISO 2018
© ISO 2018
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ii © ISO 2018 – All rights reserved

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Piston ring classification . 5
5 Piston ring types . 6
5.1 Cross-section configuration . 6
5.2 Peripheral surface configuration . 7
5.3 Edge configuration . 9
5.4 Coated, plated and nitrided surfaces configuration .10
5.5 Joint configuration .11
6 Piston ring nomenclature .11
6.1 Free (unstressed) ring .11
6.2 Closed ring .11
6.3 Assembled ring in closed condition .12
6.4 Edges, surfaces and faces .13
6.5 Chamfered edges .14
6.6 Scraper ring .14
6.7 Oil control ring .15
Annex A (informative) List of equivalent terms in English, French, Russian, German,
Spanish, Portuguese, Italian, Japanese and Chinese .17
Bibliography .33
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
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electrotechnical standardization.
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URL: 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 6626-1:2007), which has been technically
revised. The main changes compared to the previous edition are as follows:
— Annex A was updated with improved translations in Portuguese, Japanese and Russian and a new
term “witness line” was added; and
— Figure quality was improved.
iv © ISO 2018 – All rights reserved

Introduction
ISO 6621 is one of the series of International Standards dealing with piston rings for reciprocating
[6][7] [8] [9][10][11][12]
internal combustion engines. Others are ISO 6622 , ISO 6623:2013 , ISO 6624 ,
[13] [14][15][16][17] [18]
ISO 6625:1986 , ISO 6626 and ISO 6627:2011 .
INTERNATIONAL STANDARD ISO 6621-1:2018(E)
Internal combustion engines — Piston rings —
Part 1:
Vocabulary
1 Scope
This document defines the most commonly used terms for piston rings. These terms designate either
types of piston rings or certain characteristics and phenomena of piston rings.
The terms and definitions in this document apply to piston rings for reciprocating internal combustion
engines. They may also be used for piston rings of compressors working under analogous conditions.
NOTE 1 Further terms and definitions covering measuring principles are given in ISO 6621-2:2003.
NOTE 2 This document gives the equivalent terms in the Chinese, English, French, German, Italian, Japanese,
Portuguese, Russian and Spanish language.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— IEC Electropedia: available at http:// www .electropedia .org/
— ISO Online browsing platform: available at http:// www .iso .org/ obp
3.1 Types of piston ring
3.1.1
piston ring
outward expanding annular ferrous spring, fitting into a piston groove, sealing against pressure
differential of gases or liquids between the peripheral and side faces of the ring and the bore and piston
groove respectively
3.1.2
single-piece ring
piston ring (3.1.1) formed from only one part that is intended for installation in a single ring groove
3.1.3
multi-piece ring
piston ring (3.1.1) comprising two or more component parts that are intended for installation in a single
ring groove
3.1.4
compression ring
piston ring (3.1.1) whose primary purpose is to prevent leakage of gas past the piston
3.1.5
oil control ring
piston ring (3.1.1) with oil return slots or an equivalent whose primary purpose is to scrape oil from the
cylinder wall
3.1.6
rectangular ring
compression ring (3.1.4) with a rectangular cross-section whose geometrically simple form provides an
adequate seal under normal engine operating conditions
3.1.7
keystone ring
compression ring (3.1.4) with both sides tapered
Note 1 to entry: The keystone ring is used in those cases when the ring sticking can be expected. Due to its
wedge shape, any radial movement of the ring will alter its axial clearance and thus minimize the build-up of
combustion residues.
3.1.8
half keystone ring
compression ring (3.1.4) with one side face tapered
Note 1 to entry: Usually, the tapered side face is the one that faces the combustion chamber.
3.1.9
scraper ring
ring with a rectangular shaped step on the lower peripheral edge to scrape oil from the cylinder wall
Note 1 to entry: It can also act as a lower compression ring (3.1.4).
3.1.10
Napier ring
scraper ring (3.1.9) with a radiused undercut step
3.1.11
slotted oil control ring
oil control ring (3.1.5) with parallel side faces and two contact lands
Note 1 to entry: Due to the narrow lands of this type of ring, a high unit pressure is achieved.
3.1.12
bevelled-edge oil control ring
slotted oil control ring (3.1.11) with lands that are chamfered on their outer edges
Note 1 to entry: The peripheral edges of both lands are chamfered in order to achieve a further increase in unit
pressure and thereby a better oil scraping effect.
3.1.13
double-bevelled oil control ring
slotted oil control ring (3.1.11) with lands that are chamfered on their upward facing edges
Note 1 to entry: By chamfering the edges of both lands in the same direction, the oil scraping effect is even further
improved.
3.1.14
coil-spring-loaded slotted oil control ring
slotted oil control ring (3.1.11) whose radial pressure is increased by means of a cylindrical coil spring
Note 1 to entry: This spring acts equally in all directions against the inside of the ring.
3.1.15
coil-spring-loaded bevelled-edge oil control ring
coil-spring-loaded slotted oil control ring (3.1.14) with lands that are chamfered on their outer edges
2 © ISO 2018 – All rights reserved

3.1.16
coil-spring-loaded double-bevelled oil control ring
coil-spring-loaded slotted oil control ring (3.1.14) with lands that are chamfered on their upward
facing edges
3.1.17
coil-spring-loaded bevelled-edge chromium-plated oil control ring
coil-spring-loaded slotted oil control ring (3.1.14) with lands that are chromium-plated and chamfered on
their inner and outer edges
Note 1 to entry: May or may not be profile ground.
3.1.18
expander/segment oil control ring
multi-piece (3.1.3) oil control ring comprised of an expander-spacer and two segments
Note 1 to entry: Expander-spacer design will vary with manufacturer.
3.2 Physical characteristics of rings
3.2.1
nominal ring diameter
nominal diameter, d , identical to the nominal cylinder bore, in accordance with ISO 286-1
3.2.2
witness line
narrow continuous line of contact lapped on the peripheral surface of the ring which can be seen around
the circumference with normal vision
3.2.3
joint
location on a piston ring (3.1.1) where the butt ends come together
3.2.4
butting
touching of the two ring gap faces
3.2.5
effective free gap
free gap, m, minus the measured closed gap, s
Note 1 to entry: See Figure 2 for m; see Figure 3 for s .
Note 2 to entry: Free gap used in the formulae for calculation of E value, tangential force, F , diametral force, F ,
t d
and stresses.
3.2.6
pressure pattern
contact pressure distribution around the circumference of the ring when closed in its nominal
cylinder bore
3.2.7
contact pressure
pressure that a ring exerts radially against the cylinder wall
Note 1 to entry: Pressure is expressed in N/mm .
3.2.8
pin point
burry light
intermittent pinpoints of bright light or hazy light, but not bright direct light, observed in the test for
light-tightness
3.3 Piston part
3.3.1
ring groove
groove in the piston in which the piston ring (3.1.1) is fitted
3.4 Measuring devices
3.4.1
ring gauge
solid annular gauge having an inside diameter of the nominal cylinder bore
3.4.2
reference plane (datum surface)
plane on which the piston ring (3.1.1) is placed for measurements, except where otherwise specified
4 © ISO 2018 – All rights reserved

4 Piston ring classification
See Figure 1 for piston ring classification.
Figure 1 — Piston ring classification
Key
1 construction
2 function
3 cross-section configuration
4 peripheral surface configuration
5 edge configuration
6 surface layer configuration
7 spring type
5 Piston ring types
5.1 Cross-section configuration
The more common cross-section configurations in general use are shown in Table 1. Combinations of
configurations listed in Tables 2 to 5 along with those in Table 1 are shown as “common features” in the
relevant International Standards referenced in each table.
Table 1 — Cross-section configuration
Type Cross-section Relevant
International Standards
ISO 6622-1:2003
Rectangular ring
ISO 6622-2:2013
ISO 6624-1:2017,
Keystone ring
ISO 6624-3:2017
ISO 6624-2:2016
Half keystone ring
ISO 6624-4:2016
Scraper ring (stepped) ISO 6623:2013
Napier ring (undercut stepped) ISO 6623:2013
Slotted oil control ring ISO 6625:1986
Double-bevelled-edge oil control ring ISO 6625:1986
Bevelled-edge oil control ring ISO 6625:1986
ISO 6626-1,
Coil-spring-loaded slotted oil control ring
ISO 6626-2
ISO 6626-1,
Coil-spring-loaded double-bevelled-edge
oil control ring
ISO 6626-2
6 © ISO 2018 – All rights reserved

Table 1 (continued)
Type Cross-section Relevant
International Standards
ISO 6626-1,
Coil-spring-loaded bevelled-edge oil
control ring
ISO 6626-2
Steel oil control ring with V-groove ISO 6626-3:2008
Expander/segment oil control ring ISO 6627:2011
5.2 Peripheral surface configuration
The more common peripheral surface configurations in general use are shown in Table 2.
Table 2 — Peripheral surface configuration
Configuration Cross-section Relevant Internation-
al Standards
ISO 6622-1:2003/
ISO 6622-2:2013,
ISO 6623:2013,
Peripheral surface straight-faced
ISO 6624-1:2017/
ISO 6624-2:2016,
ISO 6624-3:2017/
ISO 6624-4:2016
ISO 6622-1:2003/
ISO 6622-2:2013,
Peripheral surface symmetric bar- ISO 6624-1:2017/
rel-faced ISO 6624-2:2016,
ISO 6624-3:2017/
ISO 6624-4:2016
ISO 6622-1:2003/
ISO 6622-2:2013,
Peripheral surface asymmetric bar-
rel-faced
ISO 6624-1:2017/
ISO 6624-3:2017,
ISO 6622-1:2003
ISO 6622-2:2013,
Peripheral surface-taper-faced ISO 6623:2013/
ISO 6624-1:2017,
ISO 6624-3:2017
ISO 6622-1:2003/
ISO 6622-2:2013,
Peripheral surface-taper-faced partially
ISO 6623/
cylindrical-machined or -lapped
ISO 6624-1:2017,
ISO 6624-3:2017
8 © ISO 2018 – All rights reserved

5.3 Edge configuration
The more common edge configurations in general use are shown in Table 3.
Table 3 — Edge configuration
Configuration Cross-section Relevant
International Standard
ISO 6622-1:2003/
ISO 6622-2:2013,
Internal bevel top side (positive twist
type)
ISO 6624-1:2017/
ISO 6624-3:2017,
ISO 6622-1:2003,
Internal step top side (positive twist type)
ISO 6624-1:2017
ISO 6622-1:2003,
Internal bevel bottom side (negative
twist type)
ISO 6622-2:2013
Internal step bottom side (negative twist
ISO 6622-1:2003
type)
ISO 6622-1:2003/
ISO 6623,
Inside edges chamfered ISO 6624-1:2017/
ISO 6624-2:2016,
ISO 6625:1986
ISO 6622-2:2013,
Inside edges rounded
ISO 6624-3:2017/
ISO 6624-4:2016
ISO 6622-1:2003,
Outside edges chamfered
ISO 6624-2:2016
ISO 6622-2:2013,
Outside edges rounded
ISO 6624-3:2017/
ISO 6624-4:2016
5.4 Coated, plated and nitrided surfaces configuration
The more common coated, plated and nitrided surface configurations in general use are shown in
Table 4.
Table 4 — Surface layer configuration
Configuration Cross-section Relevant
International Standards
ISO 6622-1:2003/
ISO 6623,
ISO 6624-1:2017/
Peripheral surface plain, i.e. uncoated,
ISO 6624-2:2016,
unplated and not nitrided
ISO 6625:1986,
ISO 6626-1/ISO 6626-2
ISO 6622-1:20031/
ISO 6622-2:2013,
ISO 6623/
ISO 6624-1:2017,
Peripheral surface-coated or –plated ISO 6624-2:2016/
“fully-faced configuration” ISO 6624-3:2017,
ISO 6624-4:2016/
ISO 6626-1,
ISO 6626-2/
ISO 6627:2011
ISO 6622-1:2003,
Peripheral surface-coated or –plated
ISO 6624-1:2017/
“semi-inlaid configuration”
ISO 6624-2:2016
ISO 6622-1:2003/
ISO 6622-2:2013,
ISO 6623/
Peripheral surface-coated or –plated
ISO 6624-1:2017,
“inlaid configuration”
ISO 6624-2:2016/
ISO 6624-3:2017,
ISO 6624-4:2016
ISO 6622-2:2013/
ISO 6624-3:2017,
Piston-ring-nitrided on all surfaces ISO 6624-4:2016/
ISO 6627:2011/
ISO 6626-3:2008
Piston-ring-nitrided on specific surfaces ISO 6624-3:2017/
only (e.g. “peripheral surface only”) ISO 6624-4:2016
10 © ISO 2018 – All rights reserved

5.5 Joint configuration
The more common joint configurations in general use are shown in Table 5.
Table 5 — Joint configuration
Relevant
Configuration Joint configuration
International Standards
Joint with side notch ISO 6621-4:2015
Joint with internal notch ISO 6621-4:2015
6 Piston ring nomenclature
6.1 Free (unstressed) ring
Terms commonly used to describe free (unstressed) rings are shown in Figure 2.
8 8
Key
1 peripheral surface 6 inside surface
2 back of the ring 7 ring width, h
3 free gap, m 8 side faces
4 butt ends 9 peripheral edges
5 radial wall thickness, a 10 coil spring excursion, f
1 1
Figure 2 — (Unstressed) ring
6.2 Closed ring
Additional terms commonly used to describe closed rings are shown in Figure 3.
Key
1 closed gap, s
2 cylinder bore/nominal ring diameter, d
Figure 3 — Closed ring
6.3 Assembled ring in closed condition
Terms commonly used to describe piston rings assembled in the piston groove are shown in Figure 4.
a₆
a) Rectangular ring b) Keystone ring
Key
1 groove root radius 6 side clearance
2 groove root diameter 7 groove width
3 radial clearance 8 ring width, h
4 nominal diameter, d 9 nominal ring width, h
1 1
5 radial wall thickness, a
Method A: a ref…h measured
6 3
Method B: h ref…a measured
3 6
Figure 4 — Ring clearances
12 © ISO 2018 – All rights reserved
h₃
6.4 Edges, surfaces and faces
Terms commonly used to describe edges, surfaces and faces on the rings are shown in Figure 5.
Key
1 inside edges 8 gap face
2 inside surface 9 outside gap corners
3 inside edge of gap 10 slot face
4 peripheral surface 11 inside gap corners
5 peripheral edges 12 side face
6 opposite gap corners 13 outside groove face
7 inside edge of slot
Figure 5 — Edges, surfaces and faces
6.5 Chamfered edges
Terms commonly used to describe edge chamfers on rings are shown in Figure 6.
Key
1 outside-chamfered edge
2 peripheral edge
3 side face edge
4 inside-chamfered edge
Figure 6 — Chamfered edges
6.6 Scraper ring
Terms commonly used to describe scraper and Napier rings are shown in Figure 7.
Key
1 Napier angle
2 step width, h
3 undercut radius, r
4 Napier undercut and step
5 step depth, a
Figure 7 — Section of Napier ring
14 © ISO 2018 – All rights reserved

6.7 Oil control ring
Terms commonly used to describe coil-spring-loaded slotted oil control rings are shown in Figure 8.
3 7
15 16
Key
1 angular slot spacing 10 coil spring diameter, d
2 angular pitch at gap 11 coil spring groove diameter, d
3 bridge length, w 12 coil spring
4 slot spacing, w 13 land width, h
3 5
5 slot length, w 14 land spacing, B
1 3
6 slot 15 external land depth, a
7 bridge 16 groove depth, a
8 radial thickness with coil spring, a 17 groove depth and bridge, a
12 13
9 slot width, c 18 radial wall thickness, a
1 1
Figure 8 — Coil-spring-loaded slotted oil control ring
Terms commonly used to describe expander/segment oil control rings are shown in Figure 9.
Key
1 seating tab 10 spacer radial thickness, a
2 nominal ring assembly diameter, d 11 expander radial thickness, a
1 9
3 segment closed gap, s 12 seating tab thickness, a
1 14
4 expander-spacer ends 13 spacer width, h
5 peripheral surface 14 seating tab angle,
6 nominal assembly width, h 15 expander width, h
11 9
7 assembly radial thickness, a 16 segment width, h
11 12
8 expander-spacer 17 segment radial wall thickness, a
9 segment
a
Stagger segment gaps and expander ends (all three components).
Figure 9 — Expander/segment oil control ring
16 © ISO 2018 – All rights reserved

Annex A
(informative)
List of equivalent terms in English, French, Russian, German,
Spanish, Portuguese, Italian, Japanese and Chinese
18 © ISO 2018 – All rights reserved
Table A.1
Chinese
English French Russian German Spanish Portuguese Italian Japanese
barrel-faced face bombée с бочкообразной Balligkeit bombeado Face abaulada Bombatura
バレルフェース
рабочей
桶面
поверхностью
barrel on bombé sur la бочкообразность ballige Lauffläche periferia Abaulado na face Bombatura sul 外周面バレル
peripheral périphérie по рабочей bombeada de contato diametro esterno
外圆面桶面度
surface поверхности
bevelled-edge oil segment racleur маслосъёмное Dachfasen-Ölab- segmento de en- Anel de óleo de Anello raccogl- ベベルオイルコント
control ring régulateur d’huile кольцо со streifring grase de patines chanfros simétri- iolio con smussi ロールリング
异向倒角油环
chanfreiné встречной фаской achaflanados cos convergenti
symétrique simétricos
butting arc-boutement смыкание замка Berührung der contacto de las Toque das pontas Contatto delle 合い口突き当たり
Stoßflächen puntas estremità
抵头
dell’anello
cam turned tourné en forme обточенный по formgedreht torneado de Torneamento de Tornito di forma カム旋削加工
suivant une came копиру forma forma (con camma)
凸轮仿形车削
closed gap jeu à la coupe тепловой зазор Stoßspiel ajuste de puntas Folga entre pontas Gioco al taglio 合い口すきま
闭口间隙
coating layer épaisseur du толщина Schichtdicke espesor del recu- Espessura da cob- Spessore rivesti-
コーティング厚さ
镀层厚度
thickness revêtement покрытия brimiento ertura mento
coil-spring- segment racleur маслосъёмное Ölabstreifring mit segmento de Anel de óleo com Anello raccoglio- コイルエキスパンダ
loaded oil control d’huile à ressort кольцо с витым Schlauchfeder engrase con ex- mola helicoidal lio caricato con 付きオイルコントロ
螺旋撑簧油环
ring hélicoïdale расширителем pansor helicoidal molla elicoidale
ールリング
compression ring segment de com- компрессионное Verdichtungsring segmento de Anel de com- Anello di com- コンプレッションリ
气环
pression кольцо compresión pressão pressione ング
Chromium plating revêtu de chrome Хромирование Chromschicht Capa de Cromo/ Camada de cromo rivestimento di クロムめっき
镀铬层
Cromación cromo
(datum surface) voir plan de базовая Messebene (superficie de (Superfície de Piano di riferi- データム面
référence поверхность (для referencia) referência) mento
see reference
измерений) 基准面
plane ver: plano de Veja plano de
referencia referência
diametral force force diamétrale диаметральная Diametralkraft carga diametral Força diametral Forza diametrale 直径張力
径向弹力
сила
Table A.1 (continued)
English French Russian German Spanish Portuguese Italian Japanese Chinese
double-bevelled segment racleur маслосъёмное Gleichfasen-Ölab- segmento de en- Anel de óleo de Anello raccogl-
ダ
...