ISO 6621-1:2007

Internal combustion engines - Piston rings - Part 1: Vocabulary

ISO 6621-1:2007 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 ISO 6621-1:2007 apply to piston rings for reciprocating internal combustion engines. They may also be used for piston rings of compressors working under analogous conditions.

Moteurs à combustion interne — Segments de piston — Partie 1: Vocabulaire

General Information

• Status: Withdrawn
• Publication Date: 29-Oct-2007
• Withdrawal Date: 29-Oct-2007
• ICS: 01.040.43 - Road vehicle engineering (Vocabularies)
• 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 - Propulsion, powertrain and powertrain fluids
• Current Stage: 9599 - Withdrawal of International Standard
• Start Date: 05-Mar-2018
• Completion Date: 13-Dec-2025

Relations

• Revised: ISO 6621-1:2018 - Internal combustion engines - Piston rings - Part 1: Vocabulary
• Effective Date: 05-Nov-2015
• Revises: ISO 6621-1:1986 - Internal combustion engines - Piston rings - Part 1: Vocabulary
• Effective Date: 15-Apr-2008

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 6621-1
Second edition
2007-11-01
Internal combustion engines — Piston
rings —
Part 1:
Vocabulary
Moteurs à combustion interne — Segments de piston —
Partie 1: Vocabulaire
Reference number
©
ISO 2007
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©  ISO 2007
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ii © ISO 2007 – All rights reserved

Contents Page
Foreword. iv
Introduction . v
1 Scope . 1
2 Normative references . 1
3 Piston ring classification . 2
4 Piston ring types. 3
4.1 Cross section configuration . 3
4.2 Peripheral surface configuration . 4
4.3 Edge configuration . 5
4.4 Coated, plated and nitrided surfaces configuration . 6
4.5 Joint configuration . 6
5 Piston ring nomenclature . 7
5.1 Free (unstressed) ring. 7
5.2 Closed ring . 7
5.3 Assembled ring in closed condition. 8
5.4 Edges, surfaces and faces. 9
5.5 Chamfered edges. 10
5.6 Scraper ring. 10
5.7 Oil control ring . 11
6 Terms and definitions. 13
6.1 Types of piston ring. 13
6.2 Physical characteristics of rings. 14
6.3 Piston part . 15
6.4 Measuring devices. 15
Annex A (informative) List of equivalent terms in English, French, Russian, German, Spanish,
Portuguese, Italian and Japanese. 16
Alphabetical index . 22
Bibliography . 29

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.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
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.
ISO 6621-1 was prepared by Technical Committee ISO/TC 22, Road vehicles.
This second edition cancels and replaces the first edition (ISO 6621-1:1986) which has been technically
revised.
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
iv © ISO 2007 – All rights reserved

Introduction
ISO 6621 is one of a series of International Standards dealing with piston rings for reciprocating internal
[1],[2] [3] [4],[5],[6],[7] [8]
combustion engines. Others are ISO 6622 , ISO 6623 , ISO 6624 , ISO 6625 ,
[9],[10],[11] [12]
ISO 6626 and ISO 6627 .
INTERNATIONAL STANDARD ISO 6621-1:2007(E)

Internal combustion engines — Piston rings —
Part 1:
Vocabulary
1 Scope
This part of ISO 6621 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 part of ISO 6621 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.
NOTE 2 In addition to terms given in the three official ISO languages (English, French and Russian), this part of
ISO 6621 gives the equivalent terms in the German, Spanish, Portuguese, Italian and Japanese languages. However, only
the terms given in the official languages can be considered as ISO terms.
These have been included at the request of Technical Committee ISO/TC 22 and are published under the responsibility of
the member bodies for Germany (DIN), Spain (AENOR), Portugal (IPQ), Italy (UNI) and Japan (JIS).
2 Normative references
The following referenced documents are indispensable for the application 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 286-1, Geometrical product specifications (GPS) — ISO code system for tolerances of linear sizes —
Part 1: Basis of tolerances, deviations and fits
3 Piston ring classification
2 © ISO 2007 – All rights reserved

4 Piston ring types
4.1 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 ISO Standard referenced in each table.
Table 1 — Cross section configuration
Type Cross section Relevant
International Standard
6622-1
Rectangular ring
6622-2
6624-1
Keystone ring
6624-3
6624-2
Half keystone ring
6624-4
Scraper ring (stepped) 6623
Napier ring (undercut stepped) 6623

Slotted oil control ring 6625
Double-bevelled-edge oil control ring 6625

Bevelled-edge oil control ring 6625

Coil-spring-loaded slotted oil control ring
6626-2
Coil-spring-loaded double-bevelled-edge oil control ring
6626-2
Table 1 (continued)
Type Cross section Relevant
International Standard
Coil-spring-loaded bevelled-edge oil control ring
6626-2
Steel oil control ring with V-groove 6626-3

Expander/segment oil control ring 6627

4.2 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
International Standard
6622-1/6622-2
Peripheral surface straight faced
6624-1/6624-2
6624-3/6624-4
6622-1/6622-2
Peripheral surface barrel faced 6624-1/6624-2

6624-3/6624-4
6622-1/6622-2
Peripheral surface asymmetrical barrel faced
6624-1/6624-3
6622-1/6622-2
Peripheral surface taper faced 6623/6624-1
6624-3
6622-1/6622-2
Peripheral surface taper faced partially cylindrical machined or
6623/6624-1
lapped
6624-3
4 © ISO 2007 – All rights reserved

4.3 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
6622-1/6622-2
Internal bevel top side (positive twist type)
6624-1/6624-3
6622-1
Internal step top side (positive twist type)
6624-1
6622-1
Internal bevel bottom side (negative twist type)
6622-2
Internal step bottom side (negative twist type) 6622-1

6622-1/6623
Inside edges chamfered 6624-1/6624-2

6622-2
Inside edges rounded
6624-3/6624-4
6622-1
Outside edges chamfered
6624-2
6622-2
Outside edges rounded
6624-3/6624-4
4.4 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 Standard
6622-1/6623
6624-1/6624-2
Peripheral surface plain, i.e., uncoated, unplated and not nitrided
6626/6626-2
6622-1/6622-2
6623/6624-1
Peripheral surface coated or plated “fully-faced configuration” 6624-2/6624-3

6624-4/6626
6626-2/6627
6622-1
Peripheral surface coated or plated “semi-inlaid configuration”
6624-1/6624-2
6622-1/6622-2
6623/6624-1
Peripheral surface coated or plated “inlaid configuration”
6624-2/6624-3
6624-4
6622-2/6624-3
Piston ring nitrided on all surfaces
6624-4/6627/6626-3
Piston ring nitrided on specific surfaces only (e.g., “peripheral
6624-3/6624-4
surface only”)
4.5 Joint configuration
The more common joint configurations in general use are shown in Table 5.
6 © ISO 2007 – All rights reserved

Table 5 — Joint configuration
Configuration Joint configuration Relevant International Standard
Joint with side notch 6621-4
Joint with internal notch 6621-4

5 Piston ring nomenclature
5.1 Free (unstressed) ring
Terms commonly used to describe free (unstressed) rings are shown in Figure 1.

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 1 — (Unstressed) ring
5.2 Closed ring
Additional terms commonly used to describe closed rings are shown in Figure 2.
Key
1 closed gap, s
2 cylinder bore/nominal ring diameter, d
Figure 2 — Closed ring
5.3 Assembled ring in closed condition
Terms commonly used to describe piston rings assembled in the piston groove are shown in Figure 3.

a)  rectangular ring b)  keystone ring
Key
1 groove root radius 4 nominal diameter, d 7 groove width
2 groove root diameter 5 radial wall thickness, a 8 ring width, h
1 1
3 radial clearance 6 side clearance 9 nominal ring width, h
Method A: a ref…h measured
6 3
Method B: h ref…a measured
3 6
Figure 3 — Ring clearances
8 © ISO 2007 – All rights reserved

5.4 Edges, surfaces and faces
Terms commonly used to describe edges, surfaces and faces on the rings are shown in Figure 4.

Key
1 inside edges 6 opposite gap corners 11 inside gap corners
2 inside surface 7 inside edge of slot 12 side face
3 inside edge of gap 8 gap face 13 outside groove face
4 peripheral surface 9 outside gap corners
5 peripheral edges 10 slot face
Figure 4 — Edges, surfaces and faces
5.5 Chamfered edges
Terms commonly used to describe edge chamfers on rings are shown in Figure 5.

Key
1 outside chamfered edge
2 peripheral edge
3 side face edge
4 inside chamfered edge
Figure 5 — Chamfered edges
5.6 Scraper ring
Terms commonly used to describe scraper and Napier rings are shown in Figure 6.

Key
1 Napier angle
2 step width, h
3 undercut radius, r
4 Napier undercut and step
5 step depth, a
Figure 6 — Section of Napier ring
10 © ISO 2007 – All rights reserved

5.7 Oil control ring
Terms commonly used to describe coil-spring-loaded slotted oil control rings are shown in Figure 7.

Key
1 angular slot spacing 7 bridge 13 land width, h
2 angular pitch at gap 8 radial thickness with coil spring, a 14 land spacing, B
12 3
3 bridge length, w 9 slot width, c 15 external land depth, a
2 1 17
4 slot spacing, w 10 coil spring diameter, d 16 groove depth, a
3 1 4
5 slot length, w 11 coil spring groove diameter, d 17 groove depth and bridge, a
1 14 13
6 slot 12 coil spring 18 radial wall thickness, a
Figure 7 — Coil-spring-loaded slotted oil control ring
Terms commonly used to describe expander/segment oil control rings are shown in Figure 8.

assembly
expander-spacer segment
Key
1 seating tab 7 assembly radial thickness, a 13 spacer width, h
11 13
2 nominal ring assembly diameter, d 8 expander-spacer 14 seating tab angle, θ
3 segment closed gap, s 9 segment 15 expander width, h
1 9
4 expander-spacer ends 10 spacer radial thickness, a 16 segment width, h
8 12
5 peripheral surface 11 expander radial thickness, a 17 segment radial wall thickness, a
9 1
6 nominal assembly width, h 12 seating tab thickness, a
1 14
a
Stagger segment gaps and expander ends (all three components).
Figure 8 — Expander/segment oil control ring
12 © ISO 2007 – All rights reserved

6 Terms and definitions
For the purpose of this document the following terms and definitions apply.
6.1 Types of piston ring
6.1.1
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
6.1.2
single-piece ring
piston ring formed from only one part that is intended for installation in a single ring groove
6.1.3
multi-piece ring
piston ring comprising two or more component parts that are intended for installation in a single ring groove
6.1.4
compression ring
piston ring whose primary purpose is to prevent leakage of gas past the piston
6.1.5
oil control ring
piston ring with oil return slots or an equivalent whose primary purpose is to scrape oil from the cylinder wall
6.1.6
rectangular ring
compression ring with a rectangular cross section whose geometrically simple form provides an adequate seal
under normal engine operating conditions
6.1.7
keystone ring
compression ring with both sides tapered
NOTE The keystone ring is used in those cases when 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.
6.1.8
half keystone ring
compression ring with one side face tapered
NOTE Usually the tapered side face is the one that faces the combustion chamber.
6.1.9
scraper ring (stepped)
ring with a rectangular shaped step on the lower peripheral edge to scrape oil from the cylinder wall
NOTE It can also act as a lower compression ring.
6.1.10
Napier ring (undercut step)
scraper ring with a radiused undercut step
6.1.11
slotted oil control ring
slotted oil control ring with parallel side faces and two contact lands
NOTE Due to the narrow lands of this type of ring, a high unit pressure is achieved.
6.1.12
bevelled-edge oil control ring
slotted oil control ring with lands that are chamfered on their outer edges
NOTE 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.
6.1.13
double-bevelled oil control ring
slotted oil control ring with lands that are chamfered on their upward facing edges
NOTE By chamfering the edges of both lands in the same direction the oil scraping effect is even further improved.
6.1.14
coil-spring-loaded slotted oil control ring
slotted oil control ring whose radial pressure is increased by means of a cylindrical coil spring
NOTE This spring acts equally in all directions against the inside of the ring.
6.1.15
coil-spring-loaded bevelled-edge oil control ring
coil-spring-loaded slotted oil control ring with lands that are chamfered on their outer edges
6.1.16
coil-spring-loaded double-bevelled oil control ring
coil-spring-loaded slotted oil control ring with lands that are chamfered on their upward facing edges
6.1.17
coil-spring-loaded bevelled-edge chromium plated oil control ring
coil-spring-loaded slotted oil control ring with lands that are chromium plated and chamfered on their inner and
outer edges
NOTE May or may not be profile ground.
6.1.18
expander/segment oil control ring
multi-piece oil control ring comprised of an expander-spacer and two segments
NOTE Expander-spacer design will vary with manufacturer.
6.2 Physical characteristics of rings
6.2.1
nominal ring diameter
nominal diameter, d , identical to the nominal cylinder bore, in accordance with ISO 286
6.2.2
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
6.2.3
joint
location on a piston ring where the butt ends come together
6.2.4
butting
touching of the two ring gap faces
14 © ISO 2007 – All rights reserved

6.2.5
effective free gap
free gap, m, minus the measured closed gap, s
See Figure 1 for m; see Figure 2 for s .
NOTE Free gap used in the formulae for calculation of E value, tangential force, F , diametral force, F , and stresses.
t d
6.2.6
pressure pattern
contact pressure distribution around the circumference of the ring when closed in its nominal cylinder bore
6.2.7
contact pressure
pressure that a ring exerts radially against the cylinder wall
NOTE Pressure is expressed in N/mm .
6.2.8
pin point or burry light
intermittent pinpoints of bright light or hazy light, but not bright direct light, observed in the test for
light-tightness
6.3 Piston part
6.3.1
ring groove
groove in the piston in which the piston ring is fitted
6.4 Measuring devices
6.4.1
ring gauge
solid annular gauge having an inside diameter of the nominal cylinder bore
6.4.2
reference plane (datum surface)
plane on which the piston ring is placed for measurements, except where otherwise specified
16 © ISO 2007 – All rights reserved

Annex A
(informative)
List of equivalent terms in English, French, Russian, German, Spanish, Portuguese, Italian and Japanese

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
поверхностью
barrel on peripheral bombé sur la бочкообразность по ballige Lauffläche periferia bombeada Abaulamento na face Bombatura sul
外周面バレル
surface périphérie рабочей поверхности de contato diametro esterno
bevelled-edge oil segment racleur маслосъёмное кольцо Dachfasen- segmento de engrase Anel de óleo de Anello raccogliolio
ベベルオイルコントロ
control ring régulateur d’huile со встречной фаской Ölabstreifring de patines chanfros simétricos co
...