ISO 6621-4:2024

International
Standard
ISO 6621-4
Fourth edition
Internal combustion engines —
2024-05
Piston rings —
Part 4:
General specifications
Moteurs à combustion interne — Segments de piston —
Partie 4: Spécifications générales
Reference number
© ISO 2024
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
ii
Contents Page
Foreword .v
Introduction .vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Piston ring codes . 1
5 Designation of piston rings . 4
5.1 Designation elements and order .4
5.1.1 General .4
5.1.2 Mandatory elements .4
5.1.3 Measurement principles .5
5.1.4 Additional elements .5
5.1.5 Elements for additional marking .5
5.2 Designation examples .5
5.2.1 Designation example of a piston ring in accordance with ISO 6622-1 .5
5.2.2 Designation example of a piston ring in accordance with ISO 6624-1 .5
5.2.3 Designation example of a piston ring in accordance with ISO 6626-1 .6
6 Designation of piston rings . 6
6.1 General .6
6.2 Mandatory topside identification marking .6
6.3 Additional marking.7
7 General characteristics . 7
7.1 Ring shape .7
7.2 Light tightness .8
7.3 Closed gap .8
7.4 Tangential force, F , and diametral force, F , of single piece piston rings .8
t d
7.4.1 Calculation of F , and F values in dimension tables of dimensional standards .8
t d
7.4.2 Correction of F , and F values . . .9
t d
7.4.3 Examples for correction of F and F . .10
t d
7.5 Tangential force F of multipiece oil control rings as specified in ISO 6626-1, ISO 6626-
t
2, and ISO 6626-3 . 12
7.5.1 General . 12
7.5.2 Rounding of values . 12
7.5.3 Examples for calculating tangential force F . 12
t
7.6 Tangential force F of expander/rail oil control rings as specified in ISO 6627 . 13
t
7.6.1 General . 13
7.6.2 Example for calculating the tangential force F — Selected type of piston ring:
t
ISO 6627 –ES3 – 85 × 3 – MC67 MC68/CR1 PNH . 13
8 Notches for preventing ring rotation .13
8.1 Ring joint with internal notch (only for compression rings as specified in ISO 6622 and
ISO 6624) . 13
8.2 Ring joint with side notch (only for compression rings as specified in ISO 6622) . 15
9 Machining of surfaces . 16
9.1 Peripheral surfaces .16
9.2 Side faces .16
9.3 Other surfaces .16
10 Plated, coated, and treated surfaces . . 17
10.1 Chromium plating on peripheral and side surfaces .17
10.1.1 General .17
10.1.2 Chromium plating thickness.17
10.1.3 Chromium plated rings of fully faced design .17

iii
10.1.4 Chromium plated rings of semi-inlaid design .17
10.1.5 Chromium plated rings of inlaid design .18
10.1.6 Side Chromium plated rings design .19
10.1.7 Radius, chamfer and dimensions of peripheral edges of chromium plated rings .19
10.1.8 Peripheral edges at the gap of chromium plated rings and rails. 20
10.1.9 Hardness of chromium plating .21
10.2 Spray-coated peripheral surfaces .21
10.2.1 Codes .21
10.2.2 Spray-coating thickness .21
10.2.3 Spray-coated rings of fully faced design .21
10.2.4 Spray-coated rings of semi-inlaid design .21
10.2.5 Spray-coated rings of inlaid design . 22
10.2.6 Radius, chamfer of peripheral edges of spray-coated rings . 23
10.2.7 Peripheral edges at gap of spray-coated rings . 23
10.2.8 Hardness of spray-coating .24
10.3 Nitride surfaces.24
10.3.1 Codes .24
10.3.2 Nitride case depth .24
10.3.3 Radius and dimensions of outside and inside edges of nitride steel rings . 25
10.3.4 Peripheral edges at the gap of nitride steel rings and rails . 25
10.4 Treated surfaces . 26
10.4.1 Ferro-oxidized all over — Code FE — Coating thickness (0,001 to 0,005) mm . 26
10.4.2 Phosphated all over — Code PO — Coating thickness 0,002 mm min. . 26
10.4.3 Phosphated all over — Code PR — Coating thickness 0,002 mm max. . 26
10.5 Physical vapour deposition coating (PVD) . 26
10.5.1 Codes . 26
10.5.2 PVD coating . 26
10.5.3 PVD coating thickness . 26
10.5.4 Hardness of PVD coating . 26
10.5.5 Peripheral edges at the gap of PVD-coated rings and rails .27
11 Miscellaneous .27
11.1 Cleanliness .27
11.2 Corrosion protection .27
11.3 Packaging .27
12 Traceability (optional) .27
Bibliography .28

iv
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 document 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 6621-4:2015), which has been technically
revised.
The main changes are as follows:
— harmonization of the nomenclature with revised standards;
— side notch dimensioning revised.
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.

v
Introduction
This document is one of a series of International Standards dealing with piston rings for reciprocating
internal combustion engines. Others are 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, and ISO 6627.

vi
International Standard ISO 6621-4:2024(en)
Internal combustion engines — Piston rings —
Part 4:
General specifications
1 Scope
This document specifies the general characteristics of piston rings for reciprocating internal combustion
engines for road vehicles and other applications (the individual dimensional criteria for these rings are given
in the relevant International Standards). It also provides a system for ring coding, designation, and marking.
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 are referred to in the text in such a way that some or all of their content constitutes
requirements of this document. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
ISO 6507-3, Metallic materials — Vickers hardness test — Part 3: Calibration of reference blocks
ISO 6621-1, Internal combustion engines — Piston rings — Part 1: Vocabulary
ISO 6621-2, Internal combustion engines — Piston rings — Part 2: Inspection measuring principles
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 6621-1 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/
4 Piston ring codes
Codes used for piston rings shall be as given in Table 1, corresponding to their explanatory descriptions.
Table 1 — Codes and descriptions in alphabetical order
Code Description Relevant International Stand-
ard
B Symmetrical barrel-faced rectangular ring ISO 6622-1, ISO 6622-2
BA Asymmetrical barrel-faced rectangular ring ISO 6622-1, ISO 6622-2
CR1E . CR2E Peripheral surface chromium plated semi-inlaid design ISO 6621-4
CR1F . CR2F Peripheral surface chromium plated inlaid design ISO 6621-4
a
Material mark (for alternative materials) at the discretion of the manufacturer.
b
Any other additional marking on customer’s request, which shall be quoted clearly in the order, shall be agreed upon between
the manufacturer and customer.

TTabablele 1 1 ((ccoonnttiinnueuedd))
Code Description Relevant International Stand-
ard
CRF . CR4 Peripheral surface chromium plated fully-faced design ISO 6621-4
CRS5 … CRS10 Chromium plating thickness for side faces ISO 6621-4
CSN, CSG, CSE Type of coil spring ISO 6626-1, ISO 6626-2
D Bevelled edge oil control ring ISO 6625
D/22 Radial wall thickness for “d /22” ISO 6622-1, ISO 6623
DSF Coil-spring loaded bevelled edge oil control ring ISO 6626-1, ISO 6626-2
DSF-C Coil-spring loaded bevelled edge oil control ring, ISO 6626-1, ISO 6626-2
chromium plated, and profile ground
DSF-CNP Coil-spring loaded bevelled edge oil control ring, ISO 6626-1
chromium plated, not profile ground
DSF-NG Coil-spring loaded bevelled edge oil control ring (face ISO 6626-1
geometry like type DSF-C or DSF-CNP)
DV Bevelled edge V-groove oil control ring ISO 6625
E Scraper ring (stepped) ISO 6623
EM2 . EM4 Scraper ring (stepped), taper-faced ISO 6623
ES1 . ES3 Expander/rail oil control rings ISO 6627
FE Ferro-oxidized on all sides ISO 6621-4
G Double bevelled oil control ring ISO 6625
GSF Coil-spring loaded double bevelled oil control ring ISO 6626-1, ISO 6626-2
HK Straight faced half keystone ring 7° ISO 6624-2, ISO 6624-4
HKB Symmetrical barrel-faced half keystone ring 7° ISO 6624-2, ISO 6624-4
HKBA Asymmetrical barrel-faced half keystone ring 7° ISO 6624-2, ISO 6624-4
IF Internal bevel top side ISO 6622-1, ISO 6622-2, ISO 6624-
1, ISO 6624-3
IFU Internal bevel bottom side ISO 6622-1, ISO 6622-2
IFV Variable internal bevel on the top side ISO 6622-1
IFVU Variable internal bevel on the bottom side ISO 6622-1
IW Internal step top side ISO 6622-1, ISO 6624-1
IWU Internal step bottom side ISO 6622-1
K Straight-faced keystone ring 15° ISO 6624-1, ISO 6624-3
KA Outside chamfered edges ISO 6622-1
KB Symmetrical barrel-faced keystone ring 15° ISO 6624-1, ISO 6624-3
KBA Asymmetrical barrel-faced keystone ring 15° ISO 6624-1, ISO 6624-3
KG Reduced size of peripheral edges at the gap of chromi- ISO 6621-4
um plated/spray coated/nitride/PVD coated rings
KI Inside chamfered edges ISO 6622-1
KM1 . KM5 Taper-faced keystone ring 15° ISO 6624-1, ISO 6624-3
KU Reduced peripheral bottom edge chromium plated fully ISO 6621-4
faced design
LF Uncoated ring peripheral surface or uncoated land pe- ISO 6621-4
ripheral surface, fully lapped
LM Taper-faced piston ring with partly cylindrical ma- ISO 6621-4
chined peripheral surface
a
Material mark (for alternative materials) at the discretion of the manufacturer.
b
Any other additional marking on customer’s request, which shall be quoted clearly in the order, shall be agreed upon between
the manufacturer and customer.

TTabablele 1 1 ((ccoonnttiinnueuedd))
Code Description Relevant International Stand-
ard
LP Taper-faced piston ring with lapped land over the whole ISO 6621-4
circumference but not over the whole width of the pe-
ripheral surface
M1 .M6 Taper-faced rectangular ring ISO 6622-1, ISO 6622-2
MC11 . MC69 Material subclasses ISO 6621-3
MM Manufacturer's mark ISO 6621-4
MR ISO 6621-4
Reduced ratio m ()da− for reduced load
b
MU Any other additional mark ISO 6621-4
a
MX Material mark ISO 6621-4
MY Mark for required ring shape “negative ovality” ISO 6621-4
MZ Mark for required ring shape “round” ISO 6621-4
N Napier ring (undercut step) ISO 6623
NB030 . NB130 Nitride surface, case depth specified on peripheral sur- ISO 6621-4
face and bottom side
NE1 . NE3 Ring joint with lateral stop ISO 6621-4
NH1 . NH3 Ring joint with internal stop ISO 6621-4
NM2 . NM4 Napier ring (undercut step), taper-faced ISO 6623
NP030 . NP130 Nitride surface, case depth specified on peripheral ISO 6621-4
surface only
NS 010 . NS 050 Nitride surface, case depth on rails ISO 6627
NT010 . NT130 Nitride surface, case depth specified on peripheral sur- ISO 6621-4, ISO 6626-3
face and side faces
NX003 … NX025 Nitride surface of expanders ISO 6627
PC001…PC050 Physical vapour deposition coating (PVD) thickness ISO 6621-4
PN 0.5…PN 2.5 Nominal unit pressure classes ISO 6626-3
PNE, PNL, PNR, Classes of nominal unit pressure ISO 6626-1, ISO 6626-2, ISO 6627
PNM, PNH, PNV
PO Phosphated on all sides (max. value specified) ISO 6621-4
PR Phosphated on all sides (minimum value specified) ISO 6621-4
R Straight-faced rectangular ring ISO 6622-1, ISO 6622-2
RU Napier or scraper ring with reduced undercut or step ISO 6623
(mini napier/stepped)
S Slotted oil control ring ISO 6625
S005 . S100 Closed gap (minimum values) ISO 6621-4
SC1 . SC4 Peripheral surface spray coated fully faced design ISO 6621-4
SC1E . SC4E Peripheral surface spray coated semi-inlaid design ISO 6621-4
SC1F . SC4F Peripheral surface spray coated inlaid design ISO 6621-4
SOR-L Steel oil control ring with R-shaped groove (radial wall ISO 6626-3
thickness large)
SOR-S Steel oil control ring with R-shaped groove (radial wall ISO 6626-3
thickness small)
SOV-L Steel oil control ring with V-shaped groove (radial wall ISO 6626-3
thickness large)
a
Material mark (for alternative materials) at the discretion of the manufacturer.
b
Any other additional marking on customer’s request, which shall be quoted clearly in the order, shall be agreed upon between
the manufacturer and customer.

TTabablele 1 1 ((ccoonnttiinnueuedd))
Code Description Relevant International Stand-
ard
SOV-S Steel oil control ring with V-shaped groove (radial wall ISO 6626-3
thickness small)
SSF Coil-spring loaded slotted oil control ring with rectan- ISO 6626-1, ISO 6626-2
gular groove
SSF-L Coil-spring loaded slotted oil control ring with rectan- ISO 6626-1, ISO 6626-2
gular groove at periphery and V-shaped groove beneath
T Straight-faced keystone ring 6° ISO 6624-1, ISO 6624-3
TB Symmetrical barrel-faced keystone ring 6° ISO 6624-1, ISO 6624-3
TBA Asymmetrical barrel-faced keystone ring 6° ISO 6624-1, ISO 6624-3
TM1 . TM5 Taper-faced keystone ring 6° ISO 6624-1, ISO 6624-3
TT00 … TT30 Seating tab angle θ ISO 6627
WF Reduced heat set ISO 6621-5, ISO 6626-1, ISO 6626-
WK Reduced slot length ISO 6626-1, ISO 6626-2
Y Ring shape negative ovality ISO 6621-4
Z Ring shape round ISO 6621-4
a
Material mark (for alternative materials) at the discretion of the manufacturer.
b
Any other additional marking on customer’s request, which shall be quoted clearly in the order, shall be agreed upon between
the manufacturer and customer.
5 Designation of piston rings
5.1 Designation elements and order
5.1.1 General
When designating piston rings complying with the relevant International Standards, the following details
shall be provided in the order given, using the codes according to Table 1.
5.1.2 Mandatory elements
The following mandatory elements shall constitute the designation of a piston ring:
— designation (i.e. piston ring);
— number of International Standard;
— type of piston ring (e.g. R);
— hyphen;
— size of piston ring, d × h ;
1 1
— radial wall thickness “regular” without code;
— code D/22 if the selected wall thickness, in accordance with ISO 6622-1 and ISO 6623, is d /22;
— hyphen;
— material code (e.g. MC11).
5.1.3 Measurement principles
Measurements shall be made according to ISO 6621-2.
5.1.4 Additional elements
The following optional elements may be added to the designation of a piston ring and, if so added, shall be
positioned on a second line beneath or separated by a slash (/) from the mandatory elements given in 5.1.2:
— reduced ratio md()−a , MR;
— ring shape (e.g. Z);
— selected nominal closed gap if it differs from the closed gap specified in the dimension tables (e.g. S05);
— the selected coating (e.g. CR3);
— uncoated rings with fully lapped peripheral surface [e.g. for LF taper faced rings with partly cylindrical
peripheral surface, LM (machined), or LP (lapped)];
— selected surface treatment (e.g. PO);
— selected inside edge feature (e.g. KI);
— inside step of bevel (e.g. IWU);
— selected notch to prevent ring rotation (e.g. NH1);
— reduced slot length, if required (e.g. WK);
— coil spring with reduced heat set, if required (e.g. WF);
— selected type of coil spring (e.g. CSG);
— selected pressure class (e.g. PNM).
5.1.5 Elements for additional marking
Any additional marking shall be the following with the additional elements of 5.1.3:
— manufacturer's mark, if required (e.g. MM);
— marking of required ring shape (MY or MZ);
— material, MX (see Table 1, footnote a);
— code for any other marking, MU (see Table 1, footnote b).
5.2 Designation examples
5.2.1 Designation example of a piston ring in accordance with ISO 6622-1
For a straight-faced rectangular ring (R) of nominal diameter d = 90 mm (90) and nominal ring width
h = 2,5 mm (2,5), made of grey cast iron, non-heat-treated material subclass 11 (MC11):
Piston ring ISO 6622-1 R - 90 × 2,5 - MC11
Parameters in parentheses are used in the ISO ring designation.
5.2.2 Designation example of a piston ring in accordance with ISO 6624-1
For a keystone ring 6°, taper-faced 60′ (TM3) of nominal diameter d = 105 mm (105) and nominal ring width
h = 2,5 mm (2,5) made of spheroidal graphite cast iron, martensitic type, material subclass 51 (MC51), ring
shape round (Z) with a selected closed gap of 0,3 mm (S030), inside edges chamfered (KI), and peripheral
surface chromium plated, with plating thickness 0,1 mm minimum (CR2):
Piston ring ISO 6624-1 TM3 – 105 × 2,5 – MC51 / Z S030 KI CR2
Parameters in parentheses are used in the ISO ring designation.
5.2.3 Designation example of a piston ring in accordance with ISO 6626-1
For a coil-spring loaded, bevelled edge oil control ring, chromium plated and profile ground (DSF-C), with
nominal diameter d = 125 mm (125) and nominal ring width h = 5 mm (5) made of grey cast iron, not
1 1
heat-treated, material subclass 11 (MC11) with a selected closed gap of 0,2 mm (S020) having a chromium
plating thickness on the lands of 0,15 mm minimum (CR3), phosphated on all cast iron surfaces to a depth
of 0,002 mm minimum (PO) with reduced slot length (WK), a coil-spring with reduced heat set (WF) having
a variable pitch with coil diameter, d ground (CSE), tangential force, F , according to the medium nominal
7 t
unit pressure class (PNM) marked with manufacturer's mark (MM):
Piston ring ISO 6626-1 DSF-C – 125 × 5
...