ISO 6621-2:2003

Internal combustion engines — Piston rings — Part 2: Inspection measuring principles

ISO 6621-2:2003 specifies the principles to be used in the measuring for inspection purposes of piston rings for both reciprocating internal combustion engines and compressors working under analogous conditions. It is applicable to all such rings of a diameter greater than or equal to 200 mm.

Moteurs à combustion interne — Segments de piston — Partie 2: Principes de mesure pour inspection

General Information

• Status: Withdrawn
• Publication Date: 09-Nov-2003
• Withdrawal Date: 09-Nov-2003
• 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/WG 4 - Piston rings
• Current Stage: 9599 - Withdrawal of International Standard
• Completion Date: 03-Mar-2020

Relations

• Revised: ISO 6621-2:2020 - Internal combustion engines — Piston rings — Part 2: Inspection measuring principles
• Effective Date: 21-Nov-2015
• Revises: ISO 6621-2:1984 - Internal combustion engines — Piston rings — Part 2: Inspection measuring principles
• Effective Date: 15-Apr-2008

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 6621-2
Second edition
2003-11-15

Internal combustion engines —
Piston rings —
Part 2:
Inspection measuring principles
Moteurs à combustion interne — Segments de piston —
Partie 2: Principes de mesure pour inspection




Reference number
ISO 6621-2:2003(E)
©
ISO 2003

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ISO 6621-2:2003(E)
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ii © ISO 2003 — All rights reserved

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ISO 6621-2:2003(E)
Contents Page
Foreword. iv
Introduction . v
1 Scope. 1
2 Normative references . 1
3 Terms and definitions. 1
4 Measuring principles . 1
4.1 General measuring conditions . 1
4.2 Ring characteristics and their measurement. 2
4.2.1 Ring width (in millimetres). 2

4.2.2 Radial wall thickness, a (in millimetres) . 4

1
4.2.3 Total free gap m, p (in millimetres). 5

4.2.4 Closed gap, s (in millimetres). 6

1
4.2.5 Tangential force, F (in newtons). 7
t
4.2.6 Diametral force, F (in newtons). 13

d
4.2.7 Ovality, U (in millimetres) . 13
4.2.8 Point deflection, W (in millimetres). 14
4.2.9 Light tightness (percentage of ring circumference) . 14
4.2.10 Taper on peripheral surface (in micrometres or degrees) . 15
4.2.11 Barrel on peripheral surface, t , t (in millimetres) . 16
2 3
4.2.12 Land width, h , h (in millimetres). 18
4 5
4.2.13 Land offset (in millimetres) . 18
4.2.14 Plating/coating thickness (in millimetres) . 19
4.2.15 Nitrided case depth (in millimetres) . 20
4.2.16 Keystone angle (in degrees) . 21
4.2.17 Obliqueness (in degrees) . 24
4.2.18 Twist (in millimetres). 24
4.2.19 Unevenness T , T . 25
er eu
4.2.20 Helix (axial displacement of gap ends) (in millimetres) . 27
4.2.21 Free flatness (in millimetres). 27
4.2.22 Surface roughness Ra, Rz (in micrometres). 28
Bibliography . 29

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ISO 6621-2:2003(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies
(ISO member bodies). The work of preparing International Standards is normally carried out through ISO
technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the
International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
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 6621-2 was prepared by Technical Committee ISO/TC 22, Road vehicles.
This second edition cancels and replaces the first edition (ISO 6621-2:1984), 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

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ISO 6621-2:2003(E)
Introduction
ISO 6621 is one of a series of International Standards dealing with piston rings for reciprocating internal
[4], [5] [6] [7], [8], [9], [10] [11]
combustion engines. Others are ISO 6622 , ISO 6623 , ISO 6624 , ISO 6625 ,
[12], [13] [14]
ISO 6626 and ISO 6627 .
The common features and dimensional tables presented in this part of ISO 6621 constitute a broad range of
variables, and the designer selecting a particular ring type must bear in mind the conditions under which it will
be required to operate. It is also essential that the designer refer to the specifications and requirements of
ISO 6621-3 and ISO 6621-4 before completing a selection.

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INTERNATIONAL STANDARD ISO 6621-2:2003(E)

Internal combustion engines — Piston rings —
Part 2:
Inspection measuring principles
1 Scope
This part of ISO 6621 specifies the principles to be used in the measuring for inspection purposes of piston
rings for both reciprocating internal combustion engines and compressors working under analogous conditions.
It is applicable to all such rings of a diameter u 200 mm.
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 4287-1:1984, Surface roughness — Terminology — Part 1: Surface and its parameters
ISO 4287:1997, Geometrical Product Specifications (GPS) — Surface texture: Profile method — Terms,
definitions and surface texture parameters
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
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 6621-1 and in 4.2 apply.
4 Measuring principles
4.1 General measuring conditions
The following general conditions are applicable to all measuring principles, unless otherwise specified.
a) The ring shall rest on the reference plane in the free or open condition. No additional force shall be
applied to load the ring on the reference plane, except when measuring unevenness in accordance
with 4.2.19 or helix in accordance with 4.2.20.
b) Certain measurements are made with the ring in the closed condition in a gauge of nominal cylinder bore
diameter. When orientated rings are measured in this way, they shall be so placed that the top side of the
ring is towards the reference plane.
c) Measurements shall be made using instruments with a resolution not exceeding 10 % of the tolerance of
the dimension being measured.
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ISO 6621-2:2003(E)
4.2 Ring characteristics and their measurement
Characteristic/Definition Measuring principle/method
4.2.1 Ring width (in millimetres)

4.2.1.1 Parallel-sided rings, h
1
distance between the sides at Measure with spherical measuring probes, each of radius
any particular point perpendicular
1,5 mm ± 0,05 mm, exerting a measuring force of approximately 1 N (see
to the reference plane measured
Figure 3).
in millimetres (see Figures 1 and
2)
In the case of slotted oil rings, the measurement shall be made between
the slots and not across them, i.e. across a solid portion of the ring cross
section (see Figure 2).

Figure 1

Figure 2

Figure 3
4.2.1.2 Keystone rings, half-keystone rings, h
3
distance between the sides at a Method A
distance a from the peripheral
6
surface (see Figure 4), or, This method determines h for a specified value of a (see Figure 4).

3 6
alternatively, width controlled by
a at a specified width h (see
6 3
Figure 6)

Figure 4
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ISO 6621-2:2003(E)
Characteristic/Definition Measuring principle/method
Measure with spherical measuring probes, each of radius
1,5 mm ± 0,05 mm, exerting a measuring force of approximately 1 N (see
Figure 5).
If the measuring equipment is set up with parallel gauges instead of
keystone gauges, the use of spherical measuring probes will give rise to
an error, as follows:
 for 6° keystone angle: 0,004 mm (ring types: T, TB, TBA, and TM);
 for 7° keystone angle: 0,012 mm (ring types: HK, and HKB);
 for 15° keystone angle: 0,026 mm (ring types: K, KB, KBA, and KM).
To obtain the correct measured width of the keystone ring, the above
values shall be deducted from the measured values.
NOTE Values of a are given in ISO 6624-1 to ISO 6624-4.
6
Dimensions in millimetres

Figure 5
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ISO 6621-2:2003(E)
Characteristic/Definition Measuring principle/method
Method B
This method determines a for a specified value of h (see Figure 4).
6 3
Measure with a flat face probe exerting a measuring force of
approximately 1 N. The ring shall be placed between two sharp edged
(radius u 0,01 mm) circular discs which are spaced apart at the specified
gauge width h (see Figure 6).
3
NOTE Values of h are given in ISO 6624-1 to ISO 6624-4.
3
Dimensions in millimetres

Figure 6
4.2.2 Radial wall thickness, a (in millimetres)

1
a) Measure radially between a flat measuring surface on the peripheral
radial distance between the
surface and a special measuring surface with a radius of
peripheral surface and the inside
approximately 4 mm on the bore using a measuring force of 3 N to
surface of the ring measured in
10 N (see Figure 8).
millimetres (see Figure 7)

Figure 7
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ISO 6621-2:2003(E)
Characteristic/Definition Measuring principle/method


Figure 8
b) Measure radially between cylindrical inserts or rollers of radius
approximately 4 mm with a measuring force of 3 N to 10 N. The
peripheral surface of the rollers shall be perpendicular to the
reference plane.
The length of the rollers shall be greater than the axial ring width (see
Figure 9).

Figure 9
4.2.3 Total free gap m, p (in millimetres)

chordal distance between the gap Measure with a steel rule to the nearest 0,25 mm.
ends of the ring in a free
Optionally, this feature may be measured with callipers.
unstressed state, measured at
the centre line of the radial wall
thickness measured in millimetres
(see Figure 10); for rings with an
internal notch for a peg —
chordal distance marked as p in
Figure 11

Figure 10
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ISO 6621-2:2003(E)
Characteristic/Definition Measuring principle/method


Figure 11
4.2.4 Closed gap, s (in millimetres)

1
distance between the gap ends of
Measure in a bore gauge of nominal diameter using a wedge gauge or
the ring measured at the
feeler gauges and using a measuring force of approximately 1 N (see
narrowest point, which the ring
Figure 12).
would have when fitted in a
The diameter of the bore gauge shall comply with
gauge of nominal cylinder bore
size (see Figure 12) +0,001d
1
d
1
0
NOTE The closed gap s is
1
related to the nominal diameter d .
Correction shall be made for any deviation of the bore gauge from the

1
nominal ring diameter.

Figure 12
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ISO 6621-2:2003(E)
Characteristic/Definition Measuring principle/method
4.2.5 Tangential force, F (in newtons)
t
4.2.5.1 Single-piece rings (not recommended for rings d < 50 mm — see 4.2.6.)
1
Tape method (see Figure 13)
force necessary to maintain the
ring at the closed gap condition
Carry the encircling steel tape of thickness 0,08 mm to 0,10 mm around
by means of a tangential pull on
10 mm diameter rollers set 20 mm apart (see Figure 13). In tightening the
the ends of a circumferential
tape, close the ring to the point where the gap ends touch and then open
metal tape or hoop (see
to the closed-gap dimension previously measured. Then read off the ring
Figures 13 and 14)
force from the precision measuring scale. The gap of the ring shall be
symmetrically disposed between the rollers.
An alternative method to set up the tangential loading of the force
measuring instrument is using a solid disc of nominal bore diameter
± 0,005 mm to set up the length of the tape. Insert the gauge disc into the
tape and adjust the tape length until the specified mean limit tangential
force is indicated.

Key
1 measuring scale
2 diameter of roller = 10 mm
Figure 13
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ISO 6621-2:2003(E)
Characteristic/Definition Measuring principle/method
Hoop method (see Figure 14)

Place the ring in a correctly sized hoop with its gap aligned to the gap of
the hoop. Then close the hoop in a precision loading machine until the
loading pins are at a predetermined distance apart at which point the
hoop is precisely at the cylinder bore diameter appropriate to the ring
(see Figure 14). Read the force from the display.

Key
1 measuring scale
a
Loading-pin spacing to suit machine.
Figure 14
Encircling tape method
A steel tape 0,08 mm to 0,1 mm in thickness encircles the ring crossing
at the gap (see Figure 13).
The tape is tightened until the ring is closed to the closed gap previously
measured. The ring force is then read off the precision measuring scale.
NOTE No vibration when measuring single-piece rings according to the
three methods.
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ISO 6621-2:2003(E)
Characteristic/Definition Measuring principle/method
4.2.5.2 Multi-piece rings
force which is necessary to For the measurement of coil-spring-loaded rings or similar rings where
maintain the ring at the closed the spring is supported in the bore of the ring, the gap of the spring shall
gap condition by means of a be positioned at 180° to the gap of the ring.
tangential pull on the ends of a
circumferential metal tape or For the measurement of expander/segment oil-control rings, the ring
hoop (see Figure 15)
assembly shall be mounted in a carrier simulating the piston-ring groove.
The gap of the spring element shall be placed at 180° to the gap of the
NOTE Vibration is used to
rails, both of which shall be in line. Choice of ring carrier type (see
reduce friction during or prior to
Table 1) to be decided between manufacturer and client.
measurement.
For the measurement of a ring provided with a wavy spring, or other
spring which is groove-root supported, the ring assembly shall be
mounted in a carrier simulating the groove, the root diameter of which is
equal to the mean diameter of the piston ring groove in which the ring will
be used.
Tolerance on carrier root diameter ± 0,02 mm. The gap of the wavy
spring shall be at 180° to the gap of the ring.

Key
1 measuring scale
Figure 15
Table 1 — Alternative ring carriers

Carrier groove

mm
Type
Width Tolerance

I h + 0,01
1
+0,02

II h + 0,02
1 0
III h + 0,03
1
where h = nominal ring width.
1


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ISO 6621-2:2003(E)
Characteristic/Definition Measuring principle/method

Tape method with circumferential vibration
Procedures are identical to those used for single piece rings but an
appropriate vibration shall be applied to the tape-loading mechanism to
relieve forces of friction (see Figure 16). A suitable level is 40 Hz to 50 Hz
at an amplitude of ± 0,15 mm.

Key
1 measuring scale
a
Vibration.
Figure 16
Encircling tape method with axial vibration

Procedures are identical to those used for the single-piece rings
(encircling tape method) except that a carrier may be used and vibration
(slapping) is applied to the encircled ring or encircled ring with carrier to
reduce friction (see Figure 17). A suitable level of slapping is 1 to 3
times/s.

a
Slapping.
Figure 17
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ISO 6621-2:2003(E)
Characteristic/Definition Measuring principle/method
Hoop method with circumferential vibration

Procedures are identical to those used for single-piece rings but an
appropriate vibration shall be applied to the hoop loading mechanism to
relieve all forces of friction (see Figure 18).

Key
1 measuring scale
a
Vibration.
b
Loading-pin spacing to suit machine.
Figure 18

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ISO 6621-2:2003(E)
Characteristic/Definition Measuring principle/method
Tape or hoop method with axial vibration

Procedures are identical to those used for single-piece rings but an
appropriate axial vibration shall be applied to the carrier which is
simulating the ring groove to relieve forces of friction.
A suitable level of vibration is 420 Hz (≅ 25 000 cycles/min). The axial
vibration shall have such an amplitude that the exerted force on the
carrier will reach approximately ± 18 N (see Figure 19 — hoop method
shown).

Key
1 measuring scale
a
Axial vibration.
b
Loading-pin spacing to suit machine.
Figure 19
Before tangential force measurements are made, rings shall be

degreased and optionally lightly coated with thin machine oil.
Closed-gap measurements should be made immediately prior to
measuring tangential force.
In order to improve consistency of measurement and particularly with
coil-spring-loaded rings which have been oxided or phosphated, it is
permissible to rotate the spring forwards and backwards to smooth the
surface before carrying out measurements.
The manufacturer and client should agree on a suitable factor to take
account of different machines, different locations and different operators.
NOTE The reproducibility of tangential-force measurements has not been
high in the past but current machines using encircling tape and hoop methods
give an overall reproducibility of the order of 10 %.
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ISO 6621-2:2003(E)
Characteristic/Definition Measuring principle/method
4.2.6 Diametral force, F (in newtons)

d
NOTE This method is applicable only to single-piece rings.
force, acting diametrically at 90°
to the gap, necessary to maintain
Measure in purpose-built machines which incorporate flat plates for
the ring at the nominal diameter
closing the rings (see Figure 20).
condition measured in the
direction of the force (see
Figure 20)

Figure 20
4.2.7 Ovality, U (in millimetres)
difference between the mutually NOTE This method is applicable only to single-piece rings.
perpendicular diameters d and
3
d when the ring is drawn to Measure with the ring drawn to its closed gap in a flexible steel tension
4
tape or band of thickness 0,08 mm to 0,10 mm using a diametral
closed gap within a flexible tape
measuring device exerting a measuring force of u 1 N (see Figure 21).
NOTE It may be either positive
(d > d ) or negative (d < d ) (see
With the ring closed within the tape, it is an acceptable alternative to
3 4 3 4
Figure 21). clamp it between plates and then remove the tape prior to measuring the
diameters d and d . However, clamping of the ring between plates is not
3 4
applicable to oil control rings with slots.

Figure 21
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ISO 6621-2:2003(E)
Characteristic/Definition Measuring principle/method
4.2.8 Point deflection, W (in millimetres)
deviation of the butt ends from
Measure with a probe of spherical radius 1,5 mm ± 0,05 mm using a
the true circle when restrained in
measuring force of approximately 1 N, with the ring mounted in a gauge
a gauge of nominal cylinder-bore
of nominal cylinder-bore diameter relieved over the gauge angle, θ, (see
diameter (see Figure 22)
Figure 22).
Point deflection W and the gauge angle θ shall be agreed between
manufacturer and client.
The following gauge tolerances apply for this test:
 angle θ : ± 1°
+0,001d
1
 diameter: d
1
0
 circularity: u 0,0001d
1

Key
1 gauge
Figure 22
4.2.9 Light tightness (percentage of ring circumference)
ability of the peripheral surface of Measure in a gauge equipped with a suitable light source and determine
a ring when mounted in a gauge the percentage of the ring circumference which will allow light to pass
of nominal cylinder-bore diameter (see Figure 23).
to exclude the passage of light
(see Figure 23) It is permissible to rotate the ring in the gauge to remove any slight
surface roughness on the peripheral surface. Unless otherwise specified,
NOTE Areas of the ring showing examination and measurement should be made without magnification
pinpoint, burry or fuzzy light are and with normal eyesight. It is important to avoid errors of parallax and to
protect the viewer against stray light penetration.
considered light-tight.

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ISO 6621-2:2003(E)
Characteristic/Definition Measuring principle/method

Luminance behind the ring shall be 400 lx to 1 500 lx above the ambient
conditions. The following gauge tolerances apply for this test:
+0,001d
1
 diameter: d
1
0
 circularity: u 0,0001d
1

Key
1 lamp
Figure 23
4.2.10 Taper on peripheral surface (in micrometres or degrees)
intentional angular deviation of Method A
the peripheral surface from a line
Measure at the back of the ring perpendicular to the reference plane
perpendicular to the reference
using flat faced probes exerting a force of approximately 1 N (see
plane (see Figure 24)
Figure 25).
NOTE In the case of the taper
The measurement recorded is the difference in radial dimension of the
faced peripheral surface with partly
ring peripheral surface between two points, near the top and near the
cylindrical area, both measuring
bottom, distance H apart. H shall be approximately two-thirds of the total
points have to be placed on the taper
axial width of the ring's peripheral surface and the recorded measurement
area.
may be converted to the taper angle in degrees or minutes.

Figure 24
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ISO 6621-2:2003(E)
Characteristic/Definition Measuring principle/method


Figure 25
Method B
Place the ring on a reference plane and graph the peripheral surface of
the back of the ring, perpendicular to the reference plane, using a profile
recorder.
The magnification used shall be clearly indicated.
NOTE The same methods can be used to determine the unintentional taper
which could be present on, for example, a nominally straight faced rectangular
ring.
4.2.11 Barrel on peripheral surface, t , t (in millimetres)
2 3
intentional convex deviation of Method A
the peripheral surface from a line
Measure at the back of the ring perpendicular to the reference plane
perpendicular to the reference
using flat-faced probes exerting a force of approximately 1 N (see
plane (see Figure 26 for
Figure 27).
symmetrical barrel and Figure 28
for asymmetrical barrel)
The measurement recorded is the difference in radial dimension of the
ring peripheral surface between two points — one at the peak of the
barrel (at or near the centre line of the ring), and the second at a distance
h /2 from the centreline of the nominal ring width h .
8 1

a
Reference plane.
Figure 26
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ISO 6621-2:2003(E)
Characteristic/Definition Measuring principle/method


Figure 27
Method B
Mount the ring on a reference plane and graph the peripheral surface of
the back of the ring, perpendicular to the reference plane, using a profile
recorder.
The magnification used shall be clearly indicated (recommended ratio
between vertical and horizontal magnifications: 10 or 25).
NOTE The same methods can be used to determine the unintentional barrel
which could be present on, for example, a nominally straight-faced rectangular
ring.

Gauge width (h ) is for information only.
8
Key
1 mark
a
Reference plane.
Figure 28
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ISO 6621-2:2003(E)
Characteristic/Definition Measuring principle/method
4.2.12 Land width, h , h (in millimetres)
4 5
width of the land which Method A
theoretically should be in contact
For all forms of land (sharp edge, chamfered or radiused), measure with
with the cylinder bore (see
a measuring microscope or on a projector. The measurement shall be
Figure 29)
made only on the peripheral surface of the lands (see Figure 30).


Figure 29 Figure 30
Method B
For all forms of land, place the ring on a reference plane and graph the
lands on a profile recorder.
The magnification used shall be cl
...