ISO 28741:2013
(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
ISO 28741:2013 specifies the main properties and dimensions of spark-plugs, including the terminals and the dimensions of their cylinder head housings, for use with spark-ignition engines. ISO 28741:2013 does not cover screened and waterproof spark-plugs.
Véhicules routiers — Bougies d'allumage et leur logement dans la culasse — Caractéristiques élémentaires et dimensions
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INTERNATIONAL ISO
STANDARD 28741
Second edition
2013-11-15
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 28741:2013(E)
©
ISO 2013
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ISO 28741:2013(E)
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© ISO 2013
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ii © ISO 2013 – All rights reserved
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ISO 28741:2013(E)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Terminals . 3
4.1 Solid post terminal dimensions . 3
4.2 Threaded terminal dimensions . 4
5 Dimensions, threads, and related items . 4
5.1 Spark-plug reach . 4
5.2 Gasket . 5
5.3 Threads, limiting dimensions, and tolerances . 5
6 Other dimensions of spark-plugs and their cylinder head housings .6
7 Installation tightening torque . 6
8 Spark-plug dimensions . 7
8.1 Spark-plugs with flat seating . 7
8.2 Spark-plugs with conical seating .14
9 Cylinder head housings .19
9.1 Cylinder head housing for spark-plugs with flat seating .19
9.2 Cylinder head housing for spark-plugs with conical seating .21
Annex A (normative) Spark-plugs with half thread and their cylinder head housings .22
Annex B (informative) Optional terminals for compact spark-plugs .24
Annex C (informative) Optional combination cylinder head housing for spark-plugs with conical
or flat seating .26
Bibliography .27
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ISO 28741:2013(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 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).
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. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation on the meaning of ISO specific terms and expressions related to conformity
assessment, as well as information about ISO’s adherence to the WTO principles in the Technical Barriers
to Trade (TBT) see the following URL: Foreword - Supplementary information
The committee responsible for this document is ISO/TC 22, Road vehicles, Subcommittee SC 1,
Ignition equipment.
This second edition cancels and replaces the first edition (ISO 28741:2009), which has been technically
revised. It also incorporates the Technical Corrigendum ISO 28741:2009/Cor1:2009.
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ISO 28741:2013(E)
Introduction
The purpose of this International Standard is to provide a compact and concise specification on
spark-plugs and their cylinder head housings, which replaces 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 28741:2013(E)
Road vehicles — Spark-plugs and their cylinder head
housings — Basic characteristics and dimensions
1 Scope
This International Standard specifies the main properties and dimensions of spark-plugs, including the
terminals and the dimensions of their cylinder head housings, for use with spark-ignition engines.
This International Standard does not cover screened and waterproof spark-plugs (see ISO 3412,
ISO 3895, and ISO 3896).
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are
indispensable for its application. 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: Deviations for constructional
screw threads
ISO 4095, Aerospace — Bihexagonal drives — Wrenching configuration — Metric series
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.
3.1
installed height
l
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, 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, with the exception of the M14 × 1,25 thread.
3.3
hexagon/bi-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
Note 1 to entry: A bi-hexagon is a 12-point installation feature, which requires that a 12-point socket wrench be
used to install the spark-plug.
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ISO 28741:2013(E)
3.4
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.
3.5
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.
3.6
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).
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 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
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.
3.10
spark-plug projection
b-a
distance that any part of the spark-plug projects past the spark-plug reach 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.
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ISO 28741:2013(E)
4 Terminals
4.1 Solid post terminal dimensions
The dimensions of solid post 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
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.
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ISO 28741:2013(E)
Dimensions in millimetres
Key
1 reference plane
Figure 2 — Solid post terminal for M10 × 1 bi-hex 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.
4.2 Threaded terminal dimensions
The dimensions of threaded terminals shall be in accordance with Figure 3.
Dimensions in millimetres
Key
a
Length of usable thread.
b
Cylindrical part.
c
Depending on manufacturing process, tolerance class 7e is acceptable on finished product.
Figure 3 — Threaded terminal
5 Dimensions, threads, and related items
5.1 Spark-plug reach
The spark-plug reach shall be in accordance with Table 5 or 7 (see also Figures 4 to 15).
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ISO 28741:2013(E)
The following basic types of spark-plug reach are defined:
— Short: S
— Medium: M
— Long: L
— Extended long: EL
— Extra long: XL
— Extended extra long: EXL
5.2 Gasket
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 thickness shall be as specified
in Table 3. Non-captive gaskets may be used in special cases.
5.3 Threads, limiting dimensions, and tolerances
The threads of spark-plugs and the corresponding tapped holes in the cylinder heads shall conform to
ISO 68-1, ISO 261, ISO 965-1, and ISO 965-3. Their limiting dimensions, minor diameters, basic profiles,
and initial clearances are specified in Tables 1 and 2, respectively.
Table 1 — Limiting dimensions
Dimensions in millimetres
Toler- Major diameter Pitch diameter Minor diameter
Thread
ance Dimension
size
max. min. max. min. max. min.
class
Plug thread
a
6e 17,933 17,697 16,959 16,819 16,092 15,845
(on finished plug)
M18 × 1,5
Tapped hole in the not
6H 18,000 17,216 17,026 16,676 16,376
cylinder head specified
Plug thread
b
6e 13,937 13,725 13,125 12,993 12,404 12,181
(on finished plug)
M14 × 1,25
Tapped hole in the not
6H 14,000 13,368 13,188 12,912 12,647
cylinder head specified
Plug thread
b
6e 11,937 11,725 11,125 10,993 10,404 10,181
(on finished plug)
M12 × 1,25
Tapped hole in the not
6H 12,000 11,368 11,188 10,912 10,647
cylinder head specified
Plug thread
c
6g 9,974 9,794 9,324 9,212 8,747 8,563
(on finished plug)
M10 × 1
Tapped hole in the not
6H 10,000 9,500 9,350 9,153 8,917
cylinder head specified
a
With a root radius ≥ 0,150 mm (0,1 P).
b
With a root radius ≥ 0,125 mm (0,1 P).
c
With a root radius ≥ 0,1 mm (0,1 P).
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ISO 28741:2013(E)
Table 2 — Minor diameters, basic profiles, and initial clearances for threads used
Dimensions in millimetres
a b
Minor diameter Fundamental deviation
Thread size
d es
3max
M18 × 1,5 − 6e d = (16,376 − 0,067 – 0, 217) = 16,092 0,067
3max
M14 × 1,25 − 6e d = (12,647 − 0,063 − 0,180) = 12,404 0,063
3max
M12 × 1,25 − 6e d = (10,647 − 0,063 − 0,180) = 10,404 0,063
3max
M10 × 1 − 6g d = (8,917 − 0,026 − 0,144) = 8,747 0,026
3max
a
The maximum value of the minor diameter, d , is calculated according to
3max
ISO 965-1:1998, Clause 11 with a truncation of H/6, in accordance with the following
equation: d = D − es − 2(H/4 − H/6).
3max 1
b
The fundamental deviation (term used in ISO 965-1 instead of “initial clearance”),
es, between the pitch diameters of the thread and of the tapped hole is intended to
prevent the possibility of seizure, as a result of combustion deposits on the bare
thre
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