ISO/DIS 6621-1

ISO/DIS 6621-1: Internal combustion engines — Piston rings — Part 1: Vocabulary

ISO/DIS 6621-1:2025(en)

ISO/TC 22/SC 34

Secretariat: JISC

Date: 2025-11-05

Internal combustion engines — Piston rings — Part 1: Vocabulary

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

Email: copyright@iso.org

Website: www.iso.org

Published in Switzerland

Contents

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 8

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 12

6.3 Assembled ring in closed condition 12

6.4 Edges, surfaces and faces 13

6.5 Chamfered edges 14

6.6 Scraper and Napier 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

Annex B (informative) Alphabetical Index 25

Bibliography 40

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).

ISO draws attention to the possibility that the implementation of this document may involve the use of (a) patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a) patent(s) which may be required to implement this document. However, implementers are cautioned that this may not represent the latest information, which may be obtained from the patent database available at www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.

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 fourth edition cancels and replaces the third edition (ISO 6621-1:2018), which has been technically revised.

The main changes are as follows:

— Annex A was updated with improved translations in Portuguese, Japanese and Russian and a new term “witness line” was added;

— the alphabetical index was introduced as Annex B;

— figure quality was improved.

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.

Introduction

This document is one of a series of International Standards dealing with piston rings for reciprocating internal combustion engines. Others are the rest in the ISO 6621 series, the ISO 6622 series, ISO 6623, the ISO 6624 series, ISO 6625, the ISO 6626 series and ISO 6627 (see the Bibliography).

Internal combustion engines — Piston rings — Vocabulary

1.0 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.

2.0 Normative references

There are no normative references in this document.

3.0 Terms and definitions

For the purposes of this document, the following terms and definitions apply.

ISO and IEC maintain terminology databases for use in standardization at the following addresses:

— ISO Online browsing platform: available at https://www.iso.org/obp

— IEC Electropedia: available at https://www.electropedia.org/

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 undercut 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 an angled undercut step on the lower peripheral edge

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

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

coil-spring loaded oil control ring with R-shaped groove

coil-spring loaded oil control ring (3.1.14) with a groove for the coil-spring that has a defined diameter

3.1.19

coil-spring loaded oil control ring with V-shaped groove

coil-spring loaded oil control ring (3.1.14) with a groove for the coil-spring that is V-shaped

3.1.20

expander/rail oil control ring

multi-piece (3.1.3) oil control ring comprised of an expander and two rails

Note 1 to entry: Expander design will vary with manufacturer.

3.1.1 Physical characteristics of rings

3.2.1

nominal ring diameter

nominal diameter, d₁, 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, s₁

Note 1 to entry: See Figure 2 for m; see Figure 3 for s₁.

Note 2 to entry: Free gap used in the formulae for calculation of E value, tangential force, F_t, diametral force, F_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/mm².

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

3.1.2 Piston part

3.3.1

ring groove

groove in the piston in which the piston ring (3.1.1) is fitted

3.1.3 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

4.0 Piston ring classification

See Figure 1 for piston ring classification.

Key

<graphic></graphic> construction

<graphic></graphic> function

<graphic></graphic> cross-section configuration

<graphic></graphic> peripheral surface configuration

<graphic></graphic> edge configuration

<graphic></graphic> surface layer configuration

<graphic></graphic> spring type

Figure 1 — Piston ring classification

5.0 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 Table 2 to Table 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
Rectangular ring		ISO 6622‑1 ISO 6622‑2
Keystone ring		ISO 6624‑1 ISO 6624‑3
Half keystone ring		ISO 6624‑2 ISO 6624‑4
Scraper ring (undercut step)		ISO 6623
Napier ring (angled undercut step)		ISO 6623
Slotted oil control ring		ISO 6625
Double bevelled-edge oil control ring		ISO 6625
Bevelled-edge oil control ring		ISO 6625
Coil-spring loaded slotted oil control ring		ISO 6626-1 ISO 6626-2
Coil-spring loaded double bevelled-edge oil control ring		ISO 6626-1 ISO 6626-2
Coil-spring loaded bevelled-edge oil control ring		ISO 6626-1 ISO 6626-2
Coil-spring loaded oil control ring with R-shaped groove		ISO 6626‑3
Coil-spring loaded oil control ring with V-shaped groove		ISO 6626‑3
Expander/rail oil control ring		ISO 6627

5.1.1 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 International Standards
Peripheral surface straight-faced		ISO 6622‑1 ISO 6622‑2 ISO 6623 ISO 6624‑1 ISO 6624‑2 ISO 6624‑3 ISO 6624‑4
Peripheral surface symmetric barrel-faced		ISO 6622‑1 ISO 6622‑2 ISO 6624‑1 ISO 6624‑2 ISO 6624‑3 ISO 6624‑4
Peripheral surface asymmetric barrel-faced		ISO 6622‑1 ISO 6622‑2 ISO 6624‑1 ISO 6624-2 ISO 6624‑3 ISO 6624-4
Peripheral surface taper-faced		ISO 6622‑1 ISO 6622‑2 ISO 6623 ISO 6624‑1 ISO 6624‑3
Peripheral surface taper-faced partially cylindrical machined or lapped		ISO 6622‑1 ISO 6622‑2 ISO 6623 ISO 6624‑1 ISO 6624‑3

5.1.2 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
Internal bevel top side (positive twist type)		ISO 6622‑1 ISO 6622‑2 ISO 6624‑1 ISO 6624‑3
Internal step top side (positive twist type)		ISO 6622‑1 ISO 6624‑1
Internal bevel bottom side (negative twist type)		ISO 6622‑1 ISO 6622‑2
Internal step bottom side (negative twist type)		ISO 6622‑1
Inside edges chamfered		ISO 6622‑1 ISO 6623 ISO 6624‑1 ISO 6624‑2 ISO 6625
Inside edges rounded		ISO 6622‑2 ISO 6624‑3 ISO 6624‑4
Outside edges chamfered		ISO 6622‑1 ISO 6624‑2
Outside edges rounded		ISO 6622‑2 ISO 6624‑3 ISO 6624‑4

5.1.3 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
Peripheral surface plain, i.e. uncoated, unplated and not nitrided		ISO 6622‑1 ISO 6622-2 ISO 6623 ISO 6624‑1 ISO 6624‑2 ISO 6625 ISO 6626-1 ISO 6626-2
Peripheral surface coated or plated “fully-faced configuration”		ISO 6622‑1 ISO 6622‑2 ISO 6623 ISO 6624‑1 ISO 6624‑2 ISO 6624‑3 ISO 6624‑4 ISO 6626-1 ISO 6626-2 ISO 6626-3 ISO 6627
Peripheral surface coated or plated “semi-inlaid configuration”		ISO 6622‑1 ISO 6624‑1 ISO 6624‑2
Peripheral surface coated or plated “inlaid configuration”		ISO 6622‑1 ISO 6622‑2 ISO 6623 ISO 6624‑1 ISO 6624‑2 ISO 6624‑3 ISO 6624‑4 ISO 6626-3
Piston ring nitrided on all surfaces		ISO 6622-1 ISO 6622‑2 ISO 6624‑3 ISO 6624‑4 ISO 6626‑3 ISO 6627
Piston ring nitrided on specific surfaces only (e.g. “peripheral surface only”)		ISO 6624‑3 ISO 6624‑4

5.1.4 Joint configuration

The more common joint configurations in general use are shown in Table 5.

Table 5 — Joint configuration

Configuration	Joint configuration	Relevant International Standards
Joint with side notch		ISO 6621-4
Joint with internal notch		ISO 6621-4

6.0 Piston ring nomenclature

6.1 Free (unstressed) ring

Terms commonly used to describe free (unstressed) rings are shown in Figure 2.

Key

1 peripheral surface

2 back of the ring

3 free gap, m

4 butt ends

5 radial wall thickness, a₁

6 inside surface

7 ring width, h₁

8 side faces

9 peripheral edges

10 coil spring excursion, f₁

Figure 2 — (Unstressed) ring

6.1.1 Closed ring

Additional terms commonly used to describe closed rings are shown in Figure 3.

Key

1 closed gap, s₁

2 cylinder bore/nominal ring diameter, d₁

Figure 3 — Closed ring

6.1.2 Assembled ring in closed condition

Terms commonly used to describe piston rings assembled in the piston groove are shown in Figure 4.


a) Rectangular ring	b) Keystone ring

Key

1 groove root radius

2 groove root diameter

3 radial clearance

4 nominal diameter, d₁

5 radial wall thickness, a₁

6 side clearance

7 groove width

8 ring width, h₁

9 nominal ring width, h₁

10 included angle

11 keystone ring width, h₃^a

12 keystone ring depth, a₆^b

13 radial thickness of flat (facet) on keystone ring side, a₇

^a Method A: a₆ ref … h₃ measured.

^b Method B: h₃ ref … a₆ measured.

Figure 4 — Ring clearances

6.1.3 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

2 inside surface

3 inside edge of gap

4 peripheral surface

5 peripheral edges

6 opposite gap corners

7 inside edge of slot

8 gap face

9 outside gap corners

10 slot face

11 inside gap corners

12 side face

13 outside groove face

Figure 5 — Edges, surfaces and faces

6.1.4 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

6.1.5 Scraper and Napier ring

Terms commonly used to describe scraper and Napier rings are shown in Figure 7.

Key

1 Napier angle

2 step width, h₂

3 undercut radius, r₂

4 napier undercut and step

5 step depth, a₂

Figure 7 — Section of Napier ring

6.1.6 Oil control ring

Terms commonly used to describe coil-spring loaded slotted oil control rings are shown in Figure 8.

Terms commonly used to describe expander/rail oil control rings are shown in Figure 9.

Key

1 angular slot spacing

2 angular pitch at gap

3 bridge length, w₂

4 slot spacing, w₃

5 slot length, w₁

6 slot

7 bridge

8 radial thickness over coil spring, a₁₂

9 slot width, c₁

10 coil spring diameter, d₇

11 coil spring groove diameter, d₁₄

12 coil spring

13 land width, h₅

14 land spacing, B₃

15 external land depth, a₁₇

16 groove depth, a₄

17 groove depth and bridge, a₁₃

18 radial wall thickness, a₁

Figure 8 — Coil-spring loaded slotted oil control ring

Key

1 tab

2 expander ends

3 peripheral surfaces

4 rail closed gap, s₁

5 nominal ring assembly diameter, d₁

6 nominal assembly axial width, h₁

7 assembly radial thickness, a₁₁

8 expander

9 rail

10 nominal rail axial width, h₁₂

11 rail radial wall thickness, a₁

12 expander radial thickness, a₉

13 tab radial thickness, a₁₄

14 expander radial thickness excluding tab, a₈

15 pad radial thickness, a₁₅

16 expander axial width over pads, h₁₃

17 axial distance between height of expander tab and height of expander pad, h₂₈

18 pad height, h₁₄

19 tab angle, θ

20 expander axial width, h₉

21 rail axial width near inside diameter (ID), after coiling, h₁₀

22 rail axial width near outside diameter (OD), after coiling and surface treatment or plating, h₁₁

^a Stagger rail gaps and expander ends (all three components).

Figure 9 — Expander/rail oil control ring

(informative)

List of equivalent terms in English, French, Russian, German, Spanish, Portuguese, Italian, Japanese and Chinese

Table 6— Table A.1

English	French	Russian	German	Spanish	Portuguese	Italian	Japanese	Chinese
barrel-faced	face bombée	с бочкообразной рабочей поверхностью	Balligkeit	bombeado	Face abaulada	Bombatura	バレルフェース	桶面
barrel on peripheral surface	bombé sur la périphérie	бочкообразность по рабочей поверхности	ballige Lauffläche	periferia bombeada	Abaulado na face de contato	Bombatura sul diametro esterno	外周面バレル	外圆面桶面度
bevelled-edge oil control ring	segment racleur régulateur d’huile chanfreiné symétrique	маслосъёмное кольцо со встречной фаской	Dachfasen-Ölabstreifring	segmento de engrase de patines achaflanados simétricos	Anel de óleo de chanfros simétricos	Anello raccogliolio con smussi convergenti	ベベルオイルコントロールリング	异向倒角油环
butting	arc-boutement	смыкание замка	Berührung der Stoßflächen	contacto de las puntas	Toque das pontas	Contatto delle estremità dell’anello	合い口突き当たり	抵头
cam turned	tourné en forme suivant une came	обточенный по копиру	formgedreht	torneado de forma	Torneamento de forma	Tornito di forma (con camma)	カム旋削加工	凸轮仿形车削
closed gap	jeu à la coupe	тепловой зазор	Stoßspiel	ajuste de puntas	Folga entre pontas	Gioco al taglio	合い口隙間	闭口间隙
coating layer thickness	épaisseur du revêtement	толщина покрытия	Schichtdicke	espesor del recubrimiento	Espessura da cobertura	Spessore rivestimento	コーティング厚さ	镀层厚度
coil-spring loaded oil control ring	segment racleur d’huile à ressort hélicoïdale	маслосъёмное кольцо с витым расширителем	Ölabstreifring mit Schlauchfeder	segmento de engrase con expansor helicoidal	Anel de óleo com mola helicoidal	Anello raccogliolio caricato con molla elicoidale	コイルスプリング付きオイルコントロールリング	螺旋撑簧油环
compression ring	segment de compression	компрессионное кольцо	Verdichtungsring	segmento de compresión	Anel de compressão	Anello di compressione	コンプレッションリング	气环
Chromium plating	revêtu de chrome	Хромирование	Chromschicht	Capa de Cromo/ Cromación	Camada de cromo	rivestimento di cromo	クロムめっき	镀铬层
datum surface (reference plane)	voir plan de référence	базовая поверхность (для измерений)	Messebene	(superficie de referencia) ver: plano de referencia	(Superfície de referência) Veja plano de referência	Piano di riferimento	データム面	基准面
diametral force	force diamétrale	диаметральная сила	Diametralkraft	carga diametral	Força diametral	Forza diametrale	直径張力	径向弹力
double-bevelled oil control ring	segment racleur régulateur d’huile chanfreiné parallèle	маслосъёмное кольцо с параллельными фасками	Gleichfasen-Ölabstreifring	segmento de engrase de patines achaflanados paralelos	Anel de óleo de chanfros paralelos	Anello raccogliolio con smussi paralleli	ダブルベベルオイルコントロールリング	同向倒角油环
effective free gap	ouverture libre effective	раствор замка минус тепловой зазор	tatsächliche Maulweite	abertura libre efectiva	Abertura livre efetiva	Apertura libera efficace	有効自由合い口隙間	有效自由开口尺寸
free flatness	planéité à l’état libre	отклонение от плскости кольца в свободном состоянии	Ebenheit im unbelasteten Zustand	planicidad en estado libre	Planicidade no estado livre	Planarità dell’anello libero	平面ひずみ	挠曲度
fully-faced	sur toute la face	с покрытием рабочей поверчности	Laufflächen-beschichtung	totalmente recubierto	Face de contato totalmente revestida	Superficie periferica interamente rivestita	外周全面	全镀层
half keystone ring	segment semi-trapèze	кольцо трапецевидное одностороннее	einseitiger Trapezring	segmento semitrapecial	Anel semi-trapezoidal	Anello semi-trapezoidale	ハーフキーストンリング	楔形环
heat-formed	mis en forme à chaud	с горячей формовкой	thermisch gespannt	abertura térmica	Conformação térmica	Formatura a caldo	ヒート・フォーム	热成型
helix	voilage	смещение стыкования	Stoßversatz	desalineación axial de las puntas	Desalinhamento axial das pontas	Elica / ondulazione	ヘリックス	翘度
inlaid	encastré	с заполненной канавкой	Füllung	inserto/cajetin	Embutido	Rivestimento esterno in cava	インレイド	镶嵌
inside edges chamfered	arêtes intérieures chanfreinées	с внутренними фасками	Innenkantenbruch	cantos interiores biselados	Arestas internas chanfradas	Spigoli interni smussati	内周面取り	内棱边倒角
inside and peripheral edges chamfered	arêtes intérieures et périphériques chanfreinées	с фасками на внутренней и рабочей поверхностях	Innen- und Außenkantenbruch	cantos interiores y exteriores biselados	Arestas internas e externas chanfradas	Spigoli interni ed esterni smussati	内外周面取り	内、外圆面棱边倒角
internal bevel bottom (negative twist type)	chanfrein sur le diamètre intérieur, flanc inférieur (type torsion négative)	кольцо с внутренней нижней фаской (обратное скручивание)	Innenfase an Unterflanke (negative Vertwistung)	chaflán interior, inferior (torsional negativo)	Chanfro interno inferior (torção negativa)	Smusso sul diametro interno inferiore (tipo a torsione negativa)	下面インターナルベベル(逆ねじれタイプ)	下侧面内倒角 (反扭曲)
internal bevel top (positive twist type)	chanfrein sur le diamètre intérieur, flanc supérieur (type torsion positive)	кольцо с внутренней верхней фаской (прямое скручивание)	Innenfase an Oberflanke (positive Vertwistung)	chaflán interior, superior (torsional positivo)	Chanfro interno superior (torção positiva)	Smusso sul diametro interno superiore (tipo a torsione positiva)	上面インターナルベベル(正ねじれタイプ)	上侧面内倒角 (正扭曲)
internal step bottom (negative twist type)	embrèvement sur le diamètre intérieur, flanc inférieur (type torsion négative)	кольцо с внутренней нижней выточкой (обратное скручивание)	Innenwinkel an Unterflanke (negative Vertwistung)	escalón interior, inferior (torsional negativo)	Rebaixo interno inferior (torção negativa)	Gradino sul diametro interno inferiore (tipo a torsione negativa)	下面インターナルステップ(逆ねじれタイプ)	下侧面内切台 (反扭曲)
internal step top (positive twist type)	embrèvement sur le diamètre intérieur, flanc supérieur (type torsion positive)	кольцо с внутренней верхней выточкой (прямое скручивание)	Innenwinkel an Oberflanke (positive Vertwistung)	escalón interior, superior (torsional positivo)	Rebaixo interno superior (torção positiva)	Gradino sul diametro interno superiore (tipo a torsione positiva)	上面インターナルステップ(正ねじれタイプ)	上侧面内切台 (正扭曲)
joint	coupe	замок	Stoß	corte	Configuração da folga entre pontas	Estremità	合い口部形状	定位切口
joint with internal notch	coupe avec encoche intérieure	замок с внутренней фиксацией	Stoß mit Innensicherung	entalla para fijo interior	Folga entre pontas com fenda interna	Gioco al taglio con arresto interno	内周回り止め	内圆面定位切口
joint with side notch	coupe avec encoche frontale	замок с боковой фиксацией	Stoß mit Flankensicherung	entalla para fijo lateral	Folga entre pontas com fenda lateral	Gioco al taglio con arresto frontale	側面回り止め	侧面定位切口
keystone angle	angle du trapèze	угол трапеции (трапецивидности)	Trapezwinkel	ángulo trapecial	Ângulo do trapezoidal	Angolo del trapezio	キーストン角度	梯形角
keystone ring	segment trapèze	кольцо трапецевидное двустороннее	Trapezring	segmento trapecial	Anel trapezodial	Anello trapezoidale	キーストンリング	梯形环
land offset	décalage des lèvres	смещение перемычки	Laufstegversatz	desalineación de los patines	Desalinhamento das faces	Disassamento pattino	当たり面段差	刮油边径向偏移度
land width	hauteur de lèvre	высота перемычки	Laufsteghöhe	altura del patin de roce	Largura da face de contato	Altezza pattino	当たり幅	刮油边宽度或刮油边轴向高度
light tightness	étanchéité à la boite lumière	плотность прилегания	Lichtspaltdichtheit	estanqueidad a la luz	Vedação à luz	Tenuta alla luce	ライトタイトネス	光密封度
modulus of elasticity	module d’élasticité	модуль упругости	Elastizitätsmodul	módulo de elasticidad	Módulo de elasticidade	Modulo di elasticità	縦弾性係数	弹性模量
Mini Napier							ミニナピア
multi-piece ring	segment multi-pièces	составное кольцо (многоелементное)	mehrteiliger Ring	segmento de varias piezas	Anel de múltiplas peças	Anello multipezzo	組合せリング	组合环
Napier ring (angled undercut step)	segment bec d’aigle	скребковое кольцо с подвнутренней канавкой	Abstreifring mit hinterstochener Eindrehung	segmento rascador de uña	Anel raspador (“Napier”)	Anello raschiaolio (Napier)	ナピアリング	鼻形环
Napier ring, taper-faced	segment bec d’aigle, face conique	скребковое кольцо с подвнутренней канавкой и конической рабочей поверхностью	Abstreifring mit konischer Lauffläche und hinterstochener Eindrehung	Segmento rascador de uña con periferia cónica	Anel raspador (“Napier”) com face de contato cônica	Anello raschiaolio (Napier) con superficie periferica conica	テーパフェースナピアリング	锥面鼻形环
Nitrided surface	surface nitrurée	Aзотированный слой	Nitrierschicht	Superficie Nitrurada	Superfície nitretada	superficie nitrurata	窒化面	渗氮层
nominal ring diameter	diamètre nominal du segment	номинальный диаметр кольца	Nenndurchmesser	Diametro nominal del segmento	Diâmetro nominal do anel	Diametro nominale dell’anello	リング呼び径	环的公称直径
obliqueness	obliquité	коробление	Schieflage	Inclinación	Obliquidade	Obliquità	傾き	偏移角
oil-control ring	segment racleur d’huile	маслосъёмное кольцо	Ölabstreifring	Segmento control de aceite	Anel de óleo	Anello raccogliolio	オイルコントロールリング	油环
ovality	ovalité	овальность	Ovalität oder Kreisförmigkeit	Ovalidad	Ovalização	Ovalità	オーバリティ	椭圆度
peripheral edges chamfered	arêtes périphériques chanfreinées	с наружными фасками	Laufflächenkanten-bruch	Canto exteriores biselados	Arestas externas chanfradas	Spigoli esterni smussati	外周面取り	外圆面倒角
peripheral surface (ring face)	surface périphériques (portée du segment)	рабочая поверхность кольца	Lauffläche	Face de Contacto	Face de contato	Superficie di scorrimento	外周面	外圆面
pin point or burry light	pointe d’épingle ou lumière irisée	мерцающий просвет	Lichtpunkte oder Lichtschimmer	Luz difusa	Passagem de luz intermitente ou luz “salpicada”	Trafilamento luce puntinato o sfumato	かすかな断続的な光	点状或毛刺漏光
piston ring	segment de piston	поршневое кольцо	Kolbenring	Segmento de pistón	Anel de pistão	Anello (segmento)	ピストンリング	活塞环
point deflection	protusion	прогиб стыкования	Stoßeinfall	Punto de flexion	Deflexão das pontas	Rientranza punte	ポイントデフレクション	开口端内沉量
pressure pattern	répartition de pression	эпюра рапределения давления	Radialdruckverteilung	Distribucioón de la presión	Distribuição da pressão	Distribuzione della pressione	面圧分布	压力分布图形
PVD (physical vapour deposition) coating	revêtement PVD	Вакуумное напыление	PVD (physical vapour deposition) Schicht	Recobrimiento PVD (de vapor fisico)	Camada PVD (deposição física à vapor)	riporto PVD	PVD(物理蒸着)皮膜	物理气相沉积层
radial wall thickness	épaisseur radiale	радиальная толщина	radiale Wanddicke	Espesor radial	Espessura radial	Spessore radiale	厚さ	径向厚度
rectangular ring	segment rectangulaire	прямоугольное кольцо	Rechteckring	Segmento rectangular	Anel retangular	Anello rettangolare	レクタンギュラリング	矩形环
reference plane	plan de référence	базовая поверхность	Messebene	Plano de referencia	Plano de referência	Piano di riferimento	基準面	基准面
ring gauge	bague de contrôle	кольцевой калибр	Kontrollring	Calibre para segmento	Calibre do anel	Calibro circolare	リングゲージ	环规
ring groove	gorge de segment	поршневая канавка	Ringnut	Ranura de pistón	Canaleta / canalete	Cava del pistone	リング溝	环槽
ring width	hauteur du segment	высота кольца	Ringhöhe	Alture de segmento	Altura do anel	Altezza dell’anello	リング幅	环高
scraper ring (undercut step)	segment racleur mixte (épaulé droit)	скребковое кольцо (с канавкой)	Abstreifring	Segmento rascador	Anel raspador (“Stepped”)	Anello raschiaolio (con gradino)	スクレーパリング	外切扭曲环
scraper ring (stepped), taper- faced	segment racleur mixte (épaulé droit) à portée conique	скребковое кольцо (с канавкой) с конической рабочей поверхностью	Abstreifring mit konischer Lauffläche	Segmento cónico rascador	Anel raspador (“Stepped”) com face de contato cônica	Anello raschiaolio (con gradino) a superficie periferica conica	テーパフェーススクレーパリング	锥面外切扭曲环
semi-inlaid	semi-encastré	с полузаполненной канавкой	einseitige Füllung	Semi inserto	Semi embutido	Riporto esterno in semi-cava	セミインレイド	半镶嵌
single-piece ring	segment mono-pièce	одноэлементное кольцо	einteiliger Ring	Segmento de una sola pieza	Anel de uma só peça	Anello monopezzo	シングルピースリング	单片式环
slotted oil control ring	segment racleur d’huile avec lumières	маслосъёмное кольцо с прорезями	Ölschlitz-Ölabstreifring	Segmento de engrase con ventanas	Anel de óleo com fendas	Anello raccogliolio con feritoie scarico olio	窓付きオイルコントロールリング	回油孔式油环
straight-faced	cylindrique	С цилиндрической рабочей поверхностью	zylindrische Lauffläche	Cara recta	Face de contato plana	Superficie di contatto cilindrica	ストレートフェース	柱面
tangential force	force tangentielle	тангенциальная (касательная) сила	Tangentialkraft	Carga tangencial	Força tangencial	Carico tangenziale	接線張力	切向弹力
taper-faced keystone ring	segment trapèze à face conique	кольцо трапецевидное двустороннее с конической рабочей поверхностью	Trapezring mit konischer Lauffläche	Segmento cónico trapecial	Anel trapezoidal com face de contacto cônica	Anello trapezoidale a superficie periferica conica	テーパフェースキーストンリング	锥面梯形环
taper-faced ring	segment conique	кольцо с конической рабочей поверхностью	Rechteckring mit konischer Lauffläche (Minutenring)	Segmento cónico	Anel com face de contato cônica	Anello con conicità periferica	テーパフェースリング	锥面环
taper on periphery	cône sur la périphérie	конусность по рабочей поверхности	Winkligkeit (Zylindrizität, Konizität)	Perfil cónico	Ângulo da face de contato	Conicità sulla periferia	外周面テーパ	外圆面斜度
total free gap	ouverture libre totale	размер замка кольца в свободном состоянии	Maulweite	Abertura libre	Abertura livre total	Apertura totale anello libero	自由合い口隙間	自由开口尺寸
twist	torsion	скручивание	Vertwistung	Torsión / torsional	Torção	Torsionale	ツイスト	扭曲度
uncoated ring	segment non-revêtue	кольцо без покрытия	unbeschichteter Ring	Segmento sin recubrimiento	Anel sem cobertura	Anello non rivestito	無処理リング	无镀层环
unevenness	inégalité	неровность	Unebenheit	Ondulación	Empenamento	Ondulazione	側面うねり	平度
witness line	ligne témoin	линия контакта	Tragspiegel	Linea de testimonio	Linha testemunha	Testimonio di contatto	当たり確認線	珩磨带

(informative)

Alphabetical Index

Characteristic/Term	Symbol	ISO Reference	Clause/Subclause
Assembly radial thickness	a₁₁	ISO 6621-1 ISO 6627	6.7 4, 6.3
Alpha (inner angle of oil control ring lands)	α	ISO 6626‑3	-
Asymmetric barrel-faced rings (see rectangular and keystone rings)	—	ISO 6621-1 ISO 6622-1 ISO 6622-2 ISO 6624-1 ISO 6624-2 ISO 6624-3 ISO 6624-4	5.2 5.4 4.3, 5.3 5.3, 5.7 4.3 5.3, 5.7 4.3
Axial distance between height of expander tab and height of expander pad	h₂₈	ISO 6621-1 ISO 6627	6.7 4, 5.2
Barrel (barrel drop on peripheral surface)	t₂, t₃	ISO 6621-2 ISO 6622‑1 ISO 6622‑2 ISO 6624‑1 ISO 6624-2 ISO 6624-3 ISO 6624-4 ISO 6627	4.2.11 5.3.1, 5.4.1 4.2.1, 4.3.1 5.2.1, 5.3.1 4.2.1, 4.3.1 5.2.1, 5.3.1 4.2.1, 4.3.1 4, 6.2.4
Barrel-faced rings, see rectangular, keystone and half keystone rings	—	ISO 6621-1 ISO 6622‑1 ISO 6622‑2 ISO 6624‑1, 6624-3 ISO 6624-2, 6624‑4	5.3 5.3 5.2 5.2, 5.6 4.2
Barrel peak axial position	h₂₆	ISO 6621‑2	4.2.11
Beta (outer angle of oil control ring lands)	β	6626‑3	3.1, 5.3
Bevelled-edge oil control ring	—	ISO 6621-1 ISO 6625 ISO 6626	3.1.12, 5.1 3.3, 3.4 -
Bridge length	w₂	ISO 6621-1 ISO 6625 ISO 6626-1 ISO 6626-2	6.7 4.1 6.1.2.1 5.1.1
Burrs	—	ISO 6621-5	4.4.1.1
Casting skin	—	ISO 6621-5	4.5.2
Cavities	—	ISO 6621-5	4.2
Chamfering	—	ISO 6621-4	10.1.7
Chipping	—	ISO 6621-5	4.4.2
Chromium beads (nodules)	—	ISO 6621-5	4.5.4
Chromium plating	—	ISO 6621-4	10.1
Chromium plating hardness	—	ISO 6621-4	10.1.9
Classification of piston rings	—	ISO 6621-1	4
Closed gap (see Rail closed gap)	s₁	ISO 6621-1 ISO 6621-2 ISO 6621-4 ISO 6622‑1 ISO 6622‑2 ISO 6623 ISO 6624‑1 ISO 6624-2 ISO 6624-3 ISO 6624-4 ISO 6625 ISO 6626-1 ISO 6626-2 ISO 6626-3	6.2 3.5, 4.2.4 7.3 8 7 7 8 7 7 7 6 9 9 3.1, 9
Coating configurations	—	ISO 6621-1	5.4
Coating thickness	—	ISO 6621-4 ISO 6622-1 ISO 6622‑2 ISO 6623 ISO 6624‑1 ISO 6624-2 ISO 6624-3 ISO 6624-4 ISO 6626-1 ISO 6626-2 ISO 6626-3 ISO 6627	10 6.5 5.8 5.3 6.4 5.3 6.4 5.3 6.2 5.2 5.7 6.2.1, 6.2.3
Coating thickness tolerance	—	ISO 6621-4	10
Coil spring diameter	d₇	ISO 6621-1 ISO 6626-1 ISO 6626-2 ISO 6626-3	6.7 7.1, 9 7.1.9 3.1, 6, 7, 8.2
Coil spring excursion	f₁	ISO 6621-1 ISO 6626-1 ISO 6626-2 ISO 6626-3	6.1 7.2 6.2 6.2
Coil spring groove diameter	d₁₄	ISO 6621-1 ISO 6626-1 ISO 6626-2 ISO 6626-3	6.7 9 8 3.1, 7
Coil-spring loaded bevelled-edge oil control rings	—	ISO 6621-1 ISO 6626-1 ISO 6626-2	3.1.15, 3.1.17, 5.1 5.2, 5.3, 5.6, 5.7 4.2, 4.3, 4.6, 4.7
Coil-spring loaded double-bevelled oil control rings	—	ISO 6621-1 ISO 6626-1 ISO 6626-2	3.1.16, 5.1 5.5 4.5
Coil-spring loaded oil control ring	—	ISO 6626-1 ISO 6626-2 ISO 6626-3	- - -
Coil-spring loaded oil control ring with R-shaped groove		ISO 6621-1 ISO 6626-3	3.1.18 4.1
Coil-spring loaded oil control ring with V-shaped groove		ISO 6621-1 ISO 6626-3	3.1.19 4.2
Coil-spring loaded slotted oil control rings	—	ISO 6621-1 ISO 6626-1 ISO 6626-2	3.1.14, 5.1 5.4, 5.8 4.4, 4.8
Coil spring types	—	ISO 6626-1 ISO 6626-2, ISO 6626-3	7.1 6.1
Coil spring wire diameter	d₈	ISO 6626-1 ISO 6626-2, ISO 6626-3	7.1 6.1
Contact pressure	P_o	ISO 6626-1 ISO 6626-2 ISO 6626-3 ISO 6627	8.1.4, 8.2, Annex A 7.4 3.1, 5.8.1 4
Contact pressure classes	—	ISO 6626-1 ISO 6626-2 ISO 6627	Annex A 7.4 9.3
Correction of F_t and F_d values	—	ISO 6621-4	7.4
Cracks	—	ISO 6621-5	4.3.3
Cross-sectional configuration	—	ISO 6621-1	5.1
Cutter diameter	d₅	ISO 6625 ISO 6626-1 ISO 6626-2	4.1 6.1.1 5.1.1
Cylinder bore	—	ISO 6621-1	6.2
Datum surface (reference plane)	—	ISO 6621-1	3.4.2
Deburring	—	ISO 6621-5	4.4.1
Definitions (terms and definitions)	—	ISO 6621-1	3
Deposits (dirt)	—	ISO 6621-5	4.5.2
Depressions (dents)	—	ISO 6621-5	4.3.2
Designation of piston rings	—	ISO 6621-4	5
Diametral force	F_d	ISO 6621-2 ISO 6621-4 ISO 6622‑1 ISO 6622-2 ISO 6623 ISO 6624-1 ISO 6624-2 to ISO 6624-4 ISO 6625	4.2.6 7.4 8 7 7 8 7 6
Discoloration	—	ISO 6621-5	4.5.1
DLC coating (see PVD coating)	—	-	-
Double-bevelled oil control ring	—	ISO 6621-1 ISO 6625	3.1.13, 5.1 3.2
Tool marks	—	ISO 6621-5	4.3.1
Edge chamfers (KA, KI)	—	ISO 6621-1 ISO 6622-1 ISO 6623 ISO 6624‑1 ISO 6624‑2 ISO 6625	5.3, 6.5 6.4 5.1 6.1 5.2 4.2
Edge features/configurations	—	ISO 6621-1	5.3
Edge radius outside, inside	h_x, h_y	ISO 6622‑2 ISO 6624‑3 ISO 6624‑4	5.7 6.1 5.2
Edges (CR, SC, nitrided)	—	ISO 6621-4	10
Edges (defects)	—	ISO 6621-5	4.4
Expander axial width	h₉	ISO 6621-1 ISO 6627	6.7 4, 5.2
Expander axial width over pads	h₁₃	ISO 6621-1 ISO 6627	6.7 4, 5.2
Expander/rail oil control rings	—	ISO 6621-1 ISO 6627	3.1.20 -
Expander radial thickness	a₉	ISO 6621-1 ISO 6627	6.7 4, 5.2
Expander radial thickness excluding tab	a₈	ISO 6621-1 ISO 6627	6.7 4, 5.2
External land depth	a₁₇	ISO 6621-2 ISO 6626-3	6.7 3.1, 7, 8.2
Ferro-oxide treatment	—	ISO 6621-4	10.4.1
Flatness of side faces (unevenness)	—	ISO 6621-2 ISO 6621-5	4.2 7.3
Force correction factors	—	ISO 6621-4 ISO 6622‑1 ISO 6622-2 ISO 6623 ISO 6624-1 ISO 6624-2 ISO 6624-3 ISO 6624-4 ISO 6625 ISO 6626-1 ISO 6626-2	7.4 7 6 6 7 6 7 6 5 8.1.1 7.2
Free gap (effective)	—	ISO 6621-1	3.2.5
Free gap (total)	m	ISO 6621-1 ISO 6621-2	6.1 4.2.3
Gauge width	h₈	ISO 6622-1 ISO 6622-2 ISO 6624‑1, 6624-3 ISO 6624-2, 6624-4	5.3.1 4.2.1 5.2.1 4.2.1
Gap corners	—	ISO 6621-5	4.4
Gap edges chamfered	—	ISO 6621-4	10
Gas porosity	—	ISO 6621-5	4.2
Groove angle for type SOV	θ	ISO 6626-3	3.1, 4.2.1, 8
Groove depth	a₄	ISO 6621-1 ISO 6625 ISO 6626-1 ISO 6626-2 ISO 6626‑3	6.7 6 9 9 3.1, 7, 8.2
Groove depth and bridge	a₁₃	ISO 6621-1 ISO 6626-1 ISO 6626-2 ISO 6626‑3	6.7 9 9 3.1, 7, 8.2
Groove root diameter (piston)	—	ISO 6621-1	6.3
Groove root radius (piston)	—	ISO 6621-1	6.3
Groove width (piston)	—	ISO 6621-1	6.3
Half keystone rings	—	ISO 6621-1 ISO 6624‑2 ISO 6624‑4	3.1, 8, 5.1 - -
Hardness (material)	—	ISO 6621-3	4
Hardness test indentations	—	ISO 6621-5	4.3.2
Heat set behaviour	—	ISO 6621-5	5
Heat set resistance	—	ISO 6621-5 ISO 6626-1 ISO 6626-2 ISO 6626-3	5 7.5 7.5 6.4
Helix	—	ISO 6621-2 ISO 6621-5	4.2.20 7.4
Inside chamfered edges, KI	—	ISO 6621-1 ISO 6622-1 ISO 6623 ISO 6624‑1 ISO 6624‑2 ISO 6625	5.3, 6.5 6.4 5.1 6.1 5.2 4.2
Inside rounded edges	h_y	ISO 6621-1 ISO 6622‑2 ISO 6624‑3 ISO 6624‑4	5.3 5.7 6.1 5.2
Inside surfaces	—	ISO 6621-1	6.1, 6.4
Internal bevel, IF/IFU	—	ISO 6622‑1 ISO 6622‑2 ISO 6624‑1 ISO 6624‑3	6.3 5.5, 5.6 6.2 6.2
Internal notch	—	ISO 6621-1 ISO 6621-4	5.5 8.1
Internal step, IW/IWU	—	ISO 6622‑1 ISO 6624‑1	6.3 6.2
Joint configuration	—	ISO 6621-1 ISO 6621-4	5.5 8.1
Keystone angle	—	ISO 6621-2 ISO 6624‑1 ISO 6624‑2 ISO 6624‑3 ISO 6624‑4	4.2.16 5 4 5 4
Keystone rings	—	ISO 6621-1 ISO 6624‑1 ISO 6624‑3	3.1.7, 5.1 - -
Land angle inside (see Alpha)	α	-	-
Land angle outside (see Beta)	β	-	-
Land offset	—	ISO 6621-2 ISO 6626-1 ISO 6626-2	4.2.13 6.4 6.4
Land spacing	B₃	ISO 6621-1 ISO 6626-1 ISO 6626-2 ISO 6626‑3	6.7 9 9 3.1, 5.4
Land width	h₄, h₅	ISO 6621-1 ISO 6621-2 ISO 6625 ISO 6626-1 ISO 6626-2 ISO 6626‑3	6.7 3.13, 4.2.12 6 9 9 3.1, 5.2
Light tightness	—	ISO 6621-2 ISO 6621-4	4.2.9 7.2
Lower barrel measurement position	h₂₅	ISO 6621-2	4.2.11
Macrocracks (chromium)	—	ISO 6621-5	4.5.4
Macrocracks (nitrided surface)	—	ISO 6621-5	4.5.6
Marking	—	ISO 6621-4	5.1.5, 6
Material removal	—	ISO 6621-5	4.4.1.2
Materials	—	ISO 6621-3 ISO 6626-1 ISO 6626-2 ISO 6626‑3 ISO 6627	4 7.5 7.5 6.4 8
Measuring principles (inspection)	—	ISO 6621-2	4
Mini Napier rings (see scraper rings), RU		ISO 6623	5.4
Modulus of elasticity, E	—	ISO 6621-3 ISO 6621-4	4 7.4
Napier rings (see scraper rings)	—	-	-
Negative twist	—	ISO 6621-1 ISO 6621-2 ISO 6622‑1 ISO 6622‑2	5.3 4.2.18 6.3 5.6
Nitrided case depth	—	ISO 6621-2 ISO 6621-4 ISO 6622-1 ISO 6622‑2 ISO 6624‑3 ISO 6624‑4 ISO 6626‑3 ISO 6627	4.2.15 10.3.2 6.6 5.9 6.4.2 5.4 5.6 6.1.3, 6.2.2
Nitrided surfaces	—	ISO 6621-4 ISO 6624‑3	10.3 6.4.2
Nomenclature piston rings	—	ISO 6621-1	6
Nominal assembly axial width (see Ring width)	h₁	ISO 6627	4, 5.2, 6.2.4, 7
Nominal diameter (Nominal ring assembly diameter)	d₁	ISO 6621-1 ISO 6627	3.2.1, 6.2, 6.7 4
Nominal rail axial width	h₁₂	ISO 6621-1 ISO 6627	6.7 4, 6.2.4
Nominal ring width (see Ring width)	h₁	ISO 6624-1 ISO 6624-2 ISO 6624-3 ISO 6624-4	8 7 7 7
Nominal unit pressure (see Contact pressure)	p_o	ISO 6627	4, 9.2, 9.3
Normalized tangential force	F_N	ISO 6626-1, 6626-2 ISO 6626‑3	8.1.4 5.8.3
Notch against ring rotation	—	ISO 6621-4	8
Obliqueness	—	ISO 6621-2	4.2.17
Oil control rings	—	ISO 6621-1 ISO 6625 to ISO 6627	6.7 -
Outside chamfered edges, KA	—	ISO 6621-1 ISO 6622‑1 ISO 6624‑2	5.3, 6.5 6.4 5.2
Outside rounded edges	h_x	ISO 6621-1 ISO 6622‑2 ISO 6624‑3 ISO 6624‑4	5.3 5.7 6.1 5.2
Ovality (see ring shape)	U	ISO 6621-2 ISO 6621-4	4.2.7 7.1
Packaging	—	ISO 6621-4	11.3
Pad height	h₁₄	ISO 6621-1 ISO 6627	6.7 4, 5.2
Pad radial thickness	a₁₅	ISO 6621-1 ISO 6627	6.7 4, 5.2
Peripheral edges	—	ISO 6621-1 ISO 6621-4	6.1, 6.4, 6.5 10
Peripheral edges at gap	—	ISO 6621-1 ISO 6621-4 ISO 6621-5	6.4 10 4.4.2
Peripheral surface	—	ISO 6621-1	6.1, 6.4
Peripheral surface coatings	—	ISO 6621-4	10
Peripheral surface configurations	—	ISO 6621-1	5.2
Peripheral surface machining	—	ISO 6621-4 ISO 6621-5	9.1 7
Peripheral surface profile (deviations)	—	ISO 6621-5	7.2
Phosphate treatment	—	ISO 6621-4	10.4.2, 10.4.3
Pin point or burry light	—	ISO 6621-1 ISO 6621-2	3.2.8 3.10, 4.2.9
Piston ring classifications	—	ISO 6621-1	4
Piston ring codes and descriptions	—	ISO 6621-4	4
Piston ring types	—	ISO 6621-1	5
Plain	—	ISO 6621-1	5.4
Point deflection	—	ISO 6621-2	4.2.8
Pores/porosities	—	ISO 6621-5	4.2, 4.5
Positive twist	—	ISO 6621-1 ISO 6621-2 ISO 6622‑1 ISO 6622‑2 ISO 6624‑1 ISO 6624‑3	5.3 4.2.18 6.3 5.5 6.2 6.2
Pressure pattern	—	ISO 6621-1	3.2.6
PVD coating	—	ISO 6621-4	10.5
Quality requirements (visual features)	—	ISO 6621-5	4
Radial thickness over coil spring	a₁₂	ISO 6621-1 ISO 6626-1 ISO 6626-2 ISO 6626‑3	6.7 9 9 7, 8.2
Radial wall thickness (see Rail radial wall thickness)	a₁	ISO 6621-1 ISO 6621-2 ISO 6622-1 ISO 6622-2 ISO 6623 ISO 6624-1 ISO 6624-2 ISO 6624-3 ISO 6624-4 ISO 6625 ISO 6626-1 ISO 6626-2 ISO 6626-3	6.1, 6.3, 6.7 3, 4.2.2 8 7 7 8 7 7 7 6 9 9 3.1, 5.1, 9
Radial width of flat (facet) on keystone ring side	a₇	ISO 6624-2/ ISO 6624-4	4
Rail	—	ISO 6621-1 ISO 6627	6.7 6.2
Rail axial width near inside diameter (ID), after coiling	h₁₀	ISO 6621-1 ISO 6627	6.7 4, 6.2.4
Rail axial width near outside diameter (OD), after coiling and surface treatment or plating	h₁₁	ISO 6621-1 ISO 6627	6.7 4, 6.2.4
Rail closed gap (see Closed gap)	s₁	ISO 6627	4, 5.2, 7
Rail radial wall thickness (see Radial wall thickness)	a₁	ISO 6627	4, 6.2
Raised material (marking)	—	ISO 6621-5	6
Rectangular rings	—	ISO 6621-1 ISO 6622‑1/ISO 6622‑2	3.1.6, 5.1 -
Reduced m/d1 ratio	—	ISO 6621-4	7.4
Reference plane (see Datum surface)	—	ISO 6621-1	3.4.2
Ring back (back of the ring)	—	ISO 6621-1	6.1
Ring gap (see Closed gap)	s₁	-	-
Ring gauge	—	ISO 6621-1	3.4.1
Ring groove (piston)	—	ISO 6621-1	3.3.1, 6.3
Ring rotation (prevention of)	—	ISO 6621-4	8
Ring shape (see Ovality)	—	-	-
Ring width (see Nominal assembly axial width and Nominal ring width)	h₁	ISO 6621-1 ISO 6621-2 ISO 6622‑1 ISO 6622-2 ISO 6623 ISO 6625 ISO 6626-1 ISO 6626-2 ISO 6626-3	6.1, 6.3 4.2.1.1 8 7 7 6 9 9 3.1, 5.1, 9
R-shaped groove (steel oil control ring)	—	ISO 6626-3	4.1
Roughness (see Surface roughness)	—	-	-
Sand inclusions	—	ISO 6621-5	4.2
Scraper rings (see Napier rings)	—	ISO 6621-1 ISO 6623	3.1.9, 5.1, 6.6 -
Scratches	—	ISO 6621-5	4.3.1
Side faces	—	ISO 6621-1 ISO 6621-4	6.1, 6.4 9.2
Side machining	—	ISO 6621-4	9.2
Side notch	—	ISO 6621-1 ISO 6621-4	5.5 8.2
Slot arrangement	—	ISO 6625 ISO 6626-1 ISO 6626-2	4.1 6.1 6.1
Slot length	w₁	ISO 6621-1 ISO 6625 ISO 6626-1 ISO 6626-2 ISO 6626‑3	6.7 4.1 6.1.2 6.1.2 3.1, 5.5
Slot spacing	w₃	ISO 6621-1 ISO 6626‑3	6.7 3.1, 5.5
Slot width	c₁	ISO 6621-1 ISO 6625 ISO 6626-1/ ISO 6626-2 ISO 6626‑3	6.7 6 5.2 – 5.8 3.1, 5.5
Slotted oil control rings	—	ISO 6621-1 ISO 6625 ISO 6626-1 / ISO 6626-2	3.1.11, 5.1 3.1 5.4, 5.8
Specific tangential force	F_tc	ISO 6627	4, 7
Spray coating	—	ISO 6621-4 ISO 6622‑1 ISO 6622‑2 ISO 6623 ISO 6624‑1 ISO 6624‑2 ISO 6624‑3 ISO 6624‑4	10.2 6.5 5.8 5.3 6.4.2 5.3 6.4.1 5.3
Spray coating (defects)	—	ISO 6621-5	4.4.2.2, 4.5.5
Spray coating hardness	—	ISO 6621-4	10.2.8
Spring excursion	—	ISO 6621-1 ISO 6626-1 ISO 6626-2 ISO 6626‑3	6.1 7.3 7.3 3.1, 6.2
Spring groove offset	v	ISO 6626-2	6.4
Spring slot offset	u	ISO 6626-2	6.4
Steel rails (see rail)	—	—	—
Step depth	a₂	ISO 6621-1 ISO 6623	6.6 7
Step width	h₂	ISO 6621-1 ISO 6623	6.6 7
Strength properties (mechanical properties)	—	ISO 6621-3	4
Surface machining, coating	—	ISO 6621-4	9
Surface roughness	—	ISO 6621-2 ISO 6621-4	4.2.22 9
Surface treatment	—	ISO 6621-4	10
Tab angle	θ	ISO 6621-1 ISO 6627	6.7 4, 6.1.4
Tab radial thickness	a₁₄	ISO 6621-1 ISO 6627	6.7 4, 5.2
Tangential force	F_t	ISO 6621-2 ISO 6621-4 ISO 6622‑1 ISO 6622-2 ISO 6623 ISO 6624-1 ISO 6624-2 ISO 6624-3 ISO 6624-4 ISO 6625 ISO 6626-1 ISO 6626-2 ISO 6626-3 ISO 6627	4.2.5 7.4, 7.5, 7.6 8 7 7 8 7 7 7 6 8.1 8.1 3.1, 5.8.2 4, 7, 9.1
Tangential force loss (heat set)	—	ISO 6621-5	5
Tangential force, normalized	F_N	ISO 6626-1 ISO 6626-2 ISO 6626‑3	8.1.4 8.1.4 3.1, 5.8
Taper-faced rings	—	ISO 6621-1 ISO 6621-2 ISO 6621-4 ISO 6622-1 ISO 6622-2 ISO 6623	5.2 3.11, 4.2.10 4 5.5, 6.2, 6.3.1 4.4, 5.4, 5.6 4.3, 4.5
Tapered peripheral surface	—	ISO 6621-1 ISO 6621-2 ISO 6621-4 ISO 6622-1 ISO 6622-2 ISO 6623	5.2 3.11, 4.2.10 4 5.5, 6.2, 6.3.1 4.4, 5.4, 5.6 4.3, 4.5
Temperature influence (heat set)	—	ISO 6621-5	5
Torsion/twist	—	ISO 6621-1 ISO 6621-2 ISO 6622-1 ISO 6622‑2 ISO 6624‑1 ISO 6624‑3	5.3 4.2.18 6.3 5.5, 5.6 6.2 6.2
Total free gap (see free gap)	m	ISO 6621-2	3.4, 4.2.3
Upper barrel measurement position	h₂₇	ISO 6621-2	4.2.11
Undercut radius	r₂	ISO 6621-1 ISO 6623	6.6 4
V-shaped groove (steel oil control ring)	—	ISO 6621-1 ISO 6626‑3	5.1 4.2
Visible defects (features)	—	ISO 6621-5	4
Wall thickness (see Radial wall thickness)	a₁	-	-
Witness line	—	ISO 6621-1	3.2.2

Bibliography

[1] ISO 286‑1, Geometrical product specifications (GPS) — ISO code system for tolerances on linear sizes — Part 1: Basis of tolerances, deviations and fits

[2] ISO 6621‑2:2020, Internal combustion engines — Piston rings — Part 2: Inspection measuring principles

[3] ISO 6621‑3:2000, Internal combustion engines — Piston rings — Part 3: Material specifications

[4] ISO 6621‑4:2015, Internal combustion engines — Piston rings — Part 4: General specifications

[5] ISO 6621‑5:2013, Internal combustion engines — Piston rings — Part 5: Quality requirements

[6] ISO 6622‑1:2003, Internal combustion engines — Piston rings — Part 1: Rectangular rings made of cast iron

[7] ISO 6622‑2:2013, Internal combustion engines — Piston rings — Part 2: Rectangular rings made of steel

[8] ISO 6623:2013, Internal combustion engines — Piston rings — Scraper rings made of cast iron

[9] ISO 6624‑1:2017, Internal combustion engines — Piston rings — Part 1: Keystone rings made of cast iron

[10] ISO 6624‑2:2016, Internal combustion engines — Piston rings — Part 2: Half keystone rings made of cast iron

[11] ISO 6624‑3:2017, Internal combustion engines — Piston rings — Part 3: Keystone rings made of steel

[12] ISO 6624‑4:2016, Internal combustion engines — Piston rings — Part 4: Half keystone rings made of steel

[13] ISO 6625:1986, Internal combustion engines — Piston rings — Oil control rings

[14] ISO 6626‑1, Internal combustion engines — Piston rings — Part 1: Coil spring loaded oil control rings made of cast iron

[15] ISO 6626‑2:2013, Internal combustion engines — Piston rings — Part 2: Coil-spring loaded oil control rings of narrow width made of cast iron

[16] ISO 6626‑3:2019, Internal combustion engines — Piston rings — Part 3: Coil-spring-loaded oil control rings made of steel

[17] ISO 6627:2022, Internal combustion engines — Piston rings — Expander/rail oil-control rings

Table of Contents