ISO 28741:2023
(Main)Road vehicles — Spark-plugs and their cylinder head housings — Basic characteristics and dimensions
Road vehicles — Spark-plugs and their cylinder head housings — Basic characteristics and dimensions
This document specifies the main properties and dimensions of spark plugs, including the terminals and the dimensions of their cylinder head housings, for use with any spark-ignited engines. The usage of spark plugs is not restricted to road vehicles only. This document does not cover screened and waterproof spark plugs (see ISO 3412, ISO 3895 and ISO 3896).
Véhicules routiers — Bougies d'allumage et leur logement dans la culasse — Caractéristiques élémentaires et dimensions
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INTERNATIONAL ISO
STANDARD 28741
Third edition
2023-08
Road vehicles — Spark-plugs and
their cylinder head housings — Basic
characteristics and dimensions
Véhicules routiers — Bougies d'allumage et leur logement dans la
culasse — Caractéristiques élémentaires et dimensions
Reference number
ISO 28741:2023(E)
© ISO 2023
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ISO 28741:2023(E)
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© ISO 2023
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Published in Switzerland
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ISO 28741:2023(E)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 High voltage terminals . 3
4.1 General . 3
4.2 SAE terminals . 3
4.3 Cup terminals . 4
4.4 Threaded terminal dimensions . 5
5 Dimensions, threads and related items . 5
5.1 Spark plug reach . 5
5.2 Gasket . 6
5.3 Threads, limiting dimensions and tolerances . 6
6 Other dimensions of spark plugs and their cylinder head housings .7
7 Installation torque .7
8 Spark plug dimensions .8
8.1 Spark plugs with flat seating . 8
8.2 Spark plugs with conical seating . 13
9 Cylinder head housings .16
9.1 Cylinder head housing for spark plugs with flat seating. 16
9.2 Cylinder head housing for spark plugs with conical seating . 18
Annex A (normative) Spark plugs with half thread and their cylinder head housings .19
Annex B (informative) Optional terminals for compact spark plugs .21
Annex C (normative) Optional combination cylinder head housing for park plugs with
conical or flat seating . .23
Bibliography .24
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ISO 28741:2023(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.
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 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.
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constitute an endorsement.
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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 32,
Electrical and electronic components and general system aspects.
This third edition cancels and replaces the second edition (ISO 28741:2013), which has been technically
revised.
The main changes are as follows:
— subclause 4.3: new subclause for additional terminal options;
— implementation of M18 spark plugs.
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.
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ISO 28741:2023(E)
Introduction
The purpose of this document is to provide a compact and concise specification on spark plugs and
their cylinder head housings, which has replaced the large number of existing individual International
Standards on each type of spark plug.
It is intended to specify the main properties, the design requirements, and the dimensions of most of
the existing types of spark plugs and their cylinder head housings. In this way, the user can work with
one comprehensive International Standard valid for most types of spark plugs, instead of a number of
International Standards, each of which is specified for one type only.
The testing of spark plugs is covered in ISO 11565.
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INTERNATIONAL STANDARD ISO 28741:2023(E)
Road vehicles — Spark-plugs and their cylinder head
housings — Basic characteristics and dimensions
1 Scope
This document specifies the main properties and dimensions of spark plugs, including the terminals
and the dimensions of their cylinder head housings, for use with any spark-ignited engines. The usage
of spark plugs is not restricted to road vehicles only.
This document does not cover screened and waterproof spark plugs (see ISO 3412, ISO 3895 and
ISO 3896).
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 68-1, ISO general purpose screw threads — Basic profile — Part 1: Metric screw threads
ISO 261, ISO general purpose metric screw threads — General plan
ISO 965-1, ISO general purpose metric screw threads — Tolerances — Part 1: Principles and basic data
ISO 965-3, ISO general purpose metric screw threads — Tolerances — Part 3: Limit deviations for screw
threads
ISO 6518-1, Road vehicles — Ignition systems — Part 1: Vocabulary
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 6518-1 and the following
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
installed height
distance from the contact point of the cylinder head to the top of the spark-plug terminal, including the
compressed gasket thickness with the spark plug installed at the specified installation torque
Note 1 to entry: For conical seating (3.5) the contact point is defined from the gauge point of the seat.
3.2
spark plug thread size
nominal size of the spark-plug thread used to interface between the spark plug and the cylinder head
thread
Note 1 to entry: These are standard metric threads according to the ISO 965 series, with the exception of the
M14 × 1,25 thread.
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ISO 28741:2023(E)
3.3
hexagon
feature of the spark plug shell that is used to install the spark plug into the cylinder head, interfacing
with the installation socket while the spark plug is installed into the cylinder head
3.4
bi-hexagon
12-point installation feature in which a 12-point socket wrench is used to install the spark plug
3.5
conical seating
conical section of the spark plug shell on some spark plug types, which is used for the seal interface
between the spark plug and the cylinder head
Note 1 to entry: There is typically no gasket used between the conical mating surfaces.
3.6
flat seating
flat surface of some spark plug types which is perpendicular to the spark plug axis and is used for the
seal interface between the spark plug and the cylinder head
Note 1 to entry: This seal typically uses a gasket between the flat seat of the spark plug and the mating flat
surface of the cylinder head.
3.7
high voltage terminal
part of the spark plug that is used as the contact point between the high-voltage ignition source and the
spark plug
Note 1 to entry: The connection between the high-voltage ignition source and the spark plug terminal can be
made with a threaded fastener, with a snap clip that interfaces with the solid terminal or by spring-loaded
mechanical contact.
3.8
installation tightening torque
rotational force applied to the spark plug hexagon (3.3) to ensure proper seating and sealing of the
spark plug to the cylinder head
Note 1 to entry: The value of the correct installation tightening torque can vary from conditions that affect the
friction between the spark plug threads and the cylinder head threads. These include cylinder head material,
spark plug shell plating, thread lubrication, and contamination from combustion deposits. It is advisable to
ensure that spark plugs are not over-torqued during installation, as this can damage spark plug integrity and can
result in engine damage. Spark plugs with smaller thread sizes require a lower installation tightening torque.
3.9
insulator diameter
e
nominal diameter of the insulator in a defined region of the insulator between the top of the shell and
the terminal of the spark plug, which interfaces with a corresponding region of the high-voltage boot of
the ignition lead or ignition coil
Note 1 to entry: The fit is the key to suppression of high-voltage leakage around the spark plug insulator
(flashover).
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ISO 28741:2023(E)
3.10
spark plug reach
a
distance from the spark-plug seating surface (flat seat) or from the gauge diameter (conical seat) to the
point on the shell designed to be aligned with the combustion chamber surface on the cylinder head
with the spark plug properly installed
Note 1 to entry: It is advisable to design the spark plug reach and the cylinder head housing in such a way that
they match, so as to ensure correct fit of the spark plug into the combustion chamber.
3.11
spark plug projection
b-a
distance that any part of the spark plug projects past the spark plug reach (3.10) into the combustion
chamber
Note 1 to entry: It is important to consider this dimension for possible interference with the engine piston at top
dead centre.
4 High voltage terminals
4.1 General
The type of terminal to be used shall be agreed between spark plug manufacturer and customer.
4.2 SAE terminals
The dimensions of so-called solid post SAE terminals shall be in accordance with Figures 1 and 2.
Nuts for use with threaded terminals shall have the same external dimensions as those of the solid post
terminal and shall have internal threads to 6H tolerance prior to assembly on the threaded terminals.
Dimensions in millimetres
Key
1 reference plane
a
For existing products, values between 7° and 30° are allowed.
Figure 1 — Solid post terminal
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ISO 28741:2023(E)
The measurement of the minimum diameter of 6,3 mm shall be taken at any or all points around the
post circumference. A ring gauge shall be used for measuring the maximum diameter of 6,4 mm.
Dimensions in millimetres
Key
1 reference plane
Figure 2 — Solid post terminal for M10 x 1 bi-hexagon 12 mm spark plugs
The measurement of the minimum diameter of 5,3 mm shall be taken at any or all points around the
post circumference. A ring gauge shall be used for measuring the maximum diameter of 5,4 mm.
4.3 Cup terminals
Alternatively, especially for engines with high ignition voltage requirement, a so-called cup terminal in
accordance with Figure 3 shall be used.
Dimensions in millimetres
a
Intention: no sharp edges, chamfer is allowed too.
b
Bottom surface is allowed to have cone shape for manufacturing reasons.
Figure 3 — Cup terminal
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ISO 28741:2023(E)
A variant of the cup terminal, which does not include the inner geometry is allowed too as shown in
Figure 4. The outer dimensions are identical.
Dimensions in millimetres
a
Intention: no sharp edges, chamfer is allowed too.
Figure 4 — Flat top terminal
4.4 Threaded terminal dimensions
The dimensions of threaded terminals shall be in accordance with Figure 5.
Dimensions in millimetres
a
Length of usable thread.
b
Cylindrical part.
c
Depending on manufacturing process, tolerance class 7e is acceptable on finished product.
Figure 5 — Threaded terminal
5 Dimensions, threads and related items
5.1 Spark plug reach
The plug reach shall be in accordance with Table 5 or 7 (see also Figures 6 to 11).
The following basic types of spark plug reach are defined:
— short: S
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ISO 28741:2023(E)
— medium: M
— long: L
— extended long: EL
— extra long: XL
— extended extra long: EXL
5.2 Gasket
The task of the gasket shall be to ensure a gas tight interface between the cylinder head and the
spark plug under all operating conditions possible. The corresponding test conditions are defined in
ISO 11565.
When unused spark plugs with flat seating have been tightened once with a torque, as specified
in Clause 7 and Table 3, on threads that are clean, smooth, and dry, the gasket thicknes
...
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