ISO 12251
(Main)Diesel engines — Clamp mounted CR fuel injectors — Mounting dimensions
Diesel engines — Clamp mounted CR fuel injectors — Mounting dimensions
ISO 12251 specifies dimensional requirements for clamp mounted CR fuel injectors and their corresponding cylinder head features in diesel engines. According to the injector nozzle capnut, the clamp mounted CR fuel injectors can be divided into two types: a) Type A, where the nozzle capnut is straight (see Figure 4); b) Type B, where the nozzle capnut is stepped (see Figure 5). Each clamp mounted CR fuel injector type is subdivided by a series of different sizes. NOTE This differentiation corresponds to the relationship between the two guide diameters of the clamp mounted CR fuel injectors.
Moteurs diesels — Injecteurs de combustible pour rampe commune fixée par patte — Dimensions de montage
General Information
Relations
Standards Content (Sample)
FINAL DRAFT
International
Standard
ISO/FDIS 12251
ISO/TC 22/SC 34
Diesel engines — Clamp mounted
Secretariat: JISC
CR fuel injectors — Mounting
Voting begins on:
dimensions
2025-10-10
Moteurs diesels — Injecteurs de combustible pour rampe
Voting terminates on:
commune fixée par patte — Dimensions de montage
2025-12-05
RECIPIENTS OF THIS DRAFT ARE INVITED TO SUBMIT,
WITH THEIR COMMENTS, NOTIFICATION OF ANY
RELEVANT PATENT RIGHTS OF WHICH THEY ARE AWARE
AND TO PROVIDE SUPPOR TING DOCUMENTATION.
IN ADDITION TO THEIR EVALUATION AS
BEING ACCEPTABLE FOR INDUSTRIAL, TECHNO
LOGICAL, COMMERCIAL AND USER PURPOSES, DRAFT
INTERNATIONAL STANDARDS MAY ON OCCASION HAVE
TO BE CONSIDERED IN THE LIGHT OF THEIR POTENTIAL
TO BECOME STAN DARDS TO WHICH REFERENCE MAY BE
MADE IN NATIONAL REGULATIONS.
Reference number
ISO/FDIS 12251:2025(en) © ISO 2025
FINAL DRAFT
ISO/FDIS 12251:2025(en)
International
Standard
ISO/FDIS 12251
ISO/TC 22/SC 34
Diesel engines — Clamp mounted
Secretariat: JISC
CR fuel injectors — Mounting
Voting begins on:
dimensions
Moteurs diesels — Injecteurs de combustible pour rampe
Voting terminates on:
commune fixée par patte — Dimensions de montage
RECIPIENTS OF THIS DRAFT ARE INVITED TO SUBMIT,
WITH THEIR COMMENTS, NOTIFICATION OF ANY
RELEVANT PATENT RIGHTS OF WHICH THEY ARE AWARE
AND TO PROVIDE SUPPOR TING DOCUMENTATION.
© ISO 2025
IN ADDITION TO THEIR EVALUATION AS
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
BEING ACCEPTABLE FOR INDUSTRIAL, TECHNO
LOGICAL, COMMERCIAL AND USER PURPOSES, DRAFT
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
INTERNATIONAL STANDARDS MAY ON OCCASION HAVE
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
TO BE CONSIDERED IN THE LIGHT OF THEIR POTENTIAL
or ISO’s member body in the country of the requester.
TO BECOME STAN DARDS TO WHICH REFERENCE MAY BE
MADE IN NATIONAL REGULATIONS.
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 Reference number
ISO/FDIS 12251:2025(en) © ISO 2025
ii
ISO/FDIS 12251:2025(en)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms, definitions and symbols . 1
4 General requirements . 2
4.1 Clamp mounting arrangement .2
4.2 Injector guiding dimensions .3
4.3 Concentricity of the nozzle shaft .4
4.4 High pressure connector .5
5 Dimensions and tolerances . 5
5.1 Type A, clamp mounted CR fuel injectors with straight capnut .5
5.2 Type B, clamp mounted CR fuel injectors with stepped capnut .8
6 Other specifications . 10
7 Designation .10
Bibliography .12
iii
ISO/FDIS 12251:2025(en)
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 ISO/TC 22 Road vehicles, Subcommittee SC 34 Propulsion, powertrain, and
powertrain fluids.
This third edition cancels and replaces (ISO 12251:2017) which has been technically revised.
The main changes are as follows:
— the reference to ISO 1302 has been replaced with ISO 21920-1;
— d outer diameter of sealing washer has been added;
— R radius/transition from D to sealing face on cylinder head has been added;
1 4
— s thickness of sealing washer has been added;
— Figure 4 and Figure 5 have been updated with d , s and R ;
6 1
— Table 1 and Table 3 have been updated with d and s;
— Table 2 and Table 4 have been updated with R ;
— the bibliography has been modified.
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.
iv
ISO/FDIS 12251:2025(en)
Introduction
The mainstream of fuel injection system for diesel engine has shifted from P-L-N (pump-line-nozzle) and
unit injectors to CR fuel injection system.
The injector for CR fuel injection system has most of the functions for delicately injecting high pressure fuel
inside the injector, and since they are easily affected by distortion or deformation of the injector body, it is
necessary to pay close attention to the installation on the cylinder head.
This document provides design engineers with standard dimensions for installing the CR injector on the
cylinder head.
v
FINAL DRAFT International Standard ISO/FDIS 12251:2025(en)
Diesel engines — Clamp mounted CR fuel injectors —
Mounting dimensions
1 Scope
This document specifies dimensional requirements for clamp mounted CR fuel injectors and their
corresponding cylinder head features in diesel engines.
According to the injector nozzle capnut, the clamp mounted CR fuel injectors can be divided into two types:
a) Type A, where the nozzle capnut is straight (see Figure 4);
b) Type B, where the nozzle capnut is stepped (see Figure 5).
Each clamp mounted CR fuel injector type is subdivided by a series of different sizes.
NOTE This differentiation corresponds to the relationship between the two guide diameters of the clamp
mounted CR fuel injectors.
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 2974, Diesel engines — 60° female cones for high-pressure fuel injection components
3 Terms, definitions and symbols
No terms and definitions are listed in this document.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online brow
...
ISO/FDIS 12251
ISO/TC 22/SC 34
Secretariat: JISC
Date: 2025-09-25
Diesel engines — Clamp mounted CR fuel injectors — Mounting
dimensions
Moteurs diesels — Injecteurs de combustible pour rampe commune fixée par patte — Dimensions de montage
FDIS stage
CONFIDENTIAL
ISO/FDIS 12251:2025(en)
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
E-mail: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii
Template for comments and secretariat observations
ISO/FDIS 12251:2025(en)
Contents
Foreword . iv
Introduction . v
1 Scope . 6
2 Normative references . 6
3 Terms, definitions and symbols . 6
4 General requirements . 8
4.1 Clamp mounting arrangement . 8
4.2 Injector guiding dimensions . 10
4.3 Concentricity of the nozzle shaft . 12
4.4 High pressure connector . 14
5 Dimensions and tolerances . 14
5.1 Type A, clamp mounted CR fuel injectors with straight capnut . 14
5.2 Type B, clamp mounted CR fuel injectors with stepped capnut . 27
6 Other specifications . 31
7 Designation . 1
Bibliography . 3
1 MB = Member body / NC = National Committee (enter the ISO 3166 two-letter country code, e.g. CN for China; comments from the
2 Type of comment: ge = general te = technical ed =
ISO/CS editing unit are identified by **)
Page iii
editorial
of 5
iii
CONFIDENTIAL
ISO/FDIS 12251:2025(en)
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 ISO/TC 22 Road vehicles, Subcommittee SC 34 Propulsion, powertrain, and
powertrain fluids.
This third edition cancels and replaces (ISO 12251:2017) which has been technically revised.
The main changes are as follows:
— the reference to ISO 1302 has been replaced with ISO 21920-1;
— d6 outer diameter of sealing washer has been added;
— R1 radius/transition from D4 to sealing face on cylinder head has been added;
— s thickness of sealing washer has been added;
— Figure 4 and Figure 5 have been updated with d , s and R ;
6 1
— Table 1 and Table 3 have been updated with d and s;
— Table 2 and Table 4 have been updated with R ;
— the bibliography has been modified.
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.
iv
Template for comments and secretariat observations
ISO/FDIS 12251:2025(en)
Introduction
The mainstream of fuel injection system for diesel engine has shifted from P-L-N (pump-line-nozzle) and unit
injectors to CR fuel injection system.
The injector for CR fuel injection system has most of the functions for delicately injecting high pressure fuel
inside the injector, and since they are easily affected by distortion or deformation of the injector body, it is
necessary to pay close attention to the installation on the cylinder head.
This document provides design engineers with standard dimensions for installing the CR injector on the
cylinder head.
1 MB = Member body / NC = National Committee (enter the ISO 3166 two-letter country code, e.g. CN for China; comments from the
2 Type of comment: ge = general te = technical ed =
ISO/CS editing unit are identified by **)
Page v of
editorial
v
CONFIDENTIAL
ISO/FDIS 12251:2025(en)
Diesel engines — Clamp mounted CR fuel injectors — Mounting dimensions
1 Scope
This document specifies dimensional requirements for clamp mounted CR fuel injectors and their corresponding cylinder head features in diesel engines.
According to the injector nozzle capnut, the clamp mounted CR fuel injectors can be divided into two types:
a) Type A, where the nozzle capnut is straight (see Figure 4);
b) Type B, where the nozzle capnut is stepped (see Figure 5).
Each clamp mounted CR fuel injector type is subdivided by a series of different sizes.
NOTE This differentiation corresponds to the relationship between the two guide diameters of the clamp mounted CR fuel injectors.
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 2974, Diesel engines — 60° female cones for high-pressure fuel injection components
3 Terms, definitions and symbols
No terms and definitions are listed in this document.
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/
For the purposes of this document, the following symbols apply:
ISO/FDIS 12251:2025(en)
Template for comments and secretariat observations
Date:2 Project: ISO/DIS 12251
Document: N1086 DIS commentsinjector upper guide diameter
Deleted Cells
025-
04-
05d
d injector shaft diameter
d3 capnut diameter
d injector lower guide diameter
d5 injector nozzle shaft diameter
d sealing washer outer diameter
D bore for upper guide diameter
D bore for capnut space
D bore for lower guide diameter
D5 bore for nozzle shaft
l overall injector length
l2 distance between sealing face and upper guide diameter of injector
l overlapping (upper guide diameter/cylinder head)
l injector clamping width
l clamp width
l injector clamping height
l distance between sealing face and datum B on injector
l distance between sealing face and datum C on injector
R1 radius/transition D4 to sealing face cylinder head
s sealing washer thickness
t concentricity of nozzle shaft
1 MB = Member body / NC = National Committee (enter the ISO 3166 two-letter coun7 try code, e.g. CN for China; comments from the
2 Type of comment: ge = general te = technical ed =
ISO/CS editing unit are identified by **)
Page 7 of
editorial
CONFIDENTIAL
ISO/FDIS 12251:2025(en)
NOTE d6, s, R1, l7 and l8 are dependent on the cylinder head design as agreed between injector manufacturer and customer.
4 General requirements
4.1 Clamp mounting arrangement
A typical clamp mounting arrangement is shown in Figure 1 (schematically).
ISO/FDIS 12251:2025(en)
Template for comments and secretariat observations
1 MB = Member body / NC = National Committee (enter the ISO 3166 two-letter coun9 try code, e.g. CN for China; comments from the
2 Type of comment: ge = general te = technical ed =
ISO/CS editing unit are identified by **)
Page 9 of
editorial
CONFIDENTIAL
ISO/FDIS 12251:2025(en)
Key
1 top of injector (e.g. hydraulic connection)
2 injector sealing face
a
Overall injector length.
b
Injector clamping height.
NOTE The injector clamping height varies depending on engine application and is agreed between manufacturer and customer.
Figure 1 — Typical injector clamp mounting arrangement
4.2 Injector guiding dimensions
The important injector guiding dimensions are shown in Figure 2 (schematically).
ISO/FDIS 12251:2025(en)
Template for comments and secretariat observations
Key
1 upper guide
2 lower guide
a
See Figures 4 and 5 for details.
b
Overlapping at upper guide (upper guide diameter/cylinder head).
Figure 2 — Typical injector guiding arrangement
1 MB = Member body / NC = National Committee (enter the ISO 3166 two-letter coun11 try code, e.g. CN for China; comments from the
2 Type of comment: ge = general te = technical ed =
ISO/CS editing unit are identified by **)
Page 11
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of 5
CONFIDENTIAL
ISO/FDIS 12251:2025(en)
4.3 Concentricity of the nozzle shaft
The determination of the concentricity, ϕt, of the nozzle shaft relative to the axis of the two guide diameters, ϕd and ϕd , is shown in Figure 3 (schematically).
1 4
The concentricity, ϕt, is the result of the form and position tolerances of the two guide diameters, based on the production conditions, and is specified by the injector
manufacturer by simultaneously taking into consideration the compatibility with the dimensional structure of the cylinder head.
The purpose of this concentricity is to determine the position of the nozzle shaft (centre axis of ϕd ) related to the cylinder head bore, ϕD ; this is important in order to ensure
5 5
that the nozzle shaft does not interfere with the bore in the cylinder head (see Tables 2 and 4, table footnote c, and Figure 3, key item a).
Requirements for measurement of the concentricity, ϕt:
— the datum B position (l ) shall be agreed between injector manufacturer and customer,
— the datum C position (l ) depends on the distance between the two guides and the overlapping (see Figure 2: l and l ) and shall be agreed between injector manufacturer
8 2 3
and customer.
ISO/FDIS 12251:2025(en)
Template for comments and secretariat observations
Key
1 datum positions (at the two guide diameters)
2 injector sealing face
a
Required gap between the nozzle shaft and the bore in the cylinder head (no interference).
b
For single guide diameter injector types.
Figure 3 — Concentricity 𝜙𝑑 of the nozzle shaft to the guide diameters
1 MB = Member body / NC = National Committee (enter the ISO 3166 two-letter coun13 try code, e.g. CN for China; comments from the
2 Type of comment: ge = general te = technical ed =
ISO/CS editing unit are identified by **)
Page 13
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of 5
CONFIDENTIAL
ISO/FDIS 12251:2025(en)
4.4 High pressure connector
The high-pressure connector shall comply with the requirements of ISO 2974.
5 Dimensions and tolerances
5.1 Type A, clamp mounted CR fuel injectors with straight capnut
Figure 4 shows the injector and nozzle dimensions and the corresponding cylinder head dimensions.
ISO/FDIS 12251:2025(en)
Template for comments and secretariat observations
Dimensions in millimetres
Surface roughness in micrometres, as defined in ISO 21920-1
a
Sealing washer thickness s and outer diameter d6 may vary depending on engine manufacturer’s requirements.
b
d is valid for the overlapping section.
c
d is valid above the overlapping section.
1 MB = Member body / NC = National Committee (enter the ISO 3166 two-letter coun15 try code, e.g. CN for China; comments from the
2 Type of comment: ge = general te = technical ed =
ISO/CS editing unit are identified by **)
Page 15
editorial
of 5
CONFIDENTIAL
ISO/FDIS 12251:2025(en)
Figure 4 — Injector and cylinder head dimensions, Type A (detail X of Figure 2)
The specified clamp mounted CR fuel injector and nozzle dimensions and the corresponding cylinder head bore dimensions are given in Table 1 and Table 2.
Table 1 — Clamp mounted CR fuel injector dimensions, Type A
Dimensions in millimetres
a d c b a b
d2 d1 d3 d4 d5 d6 s l1 l2 l4
Size
no ref no m
m. . m. in.
+0,
1 7 1
16 4
-
A.1 6, to 0,
-
,9 0
0,1
9 2 8
0,1
+0,
1 7
5 1
-
A.2 17 7, to
-
+0 0 3
0,1
5 2
0,1
0 0 ,2
5 11,
0 0
1 1 5 1,
- -
7 7 to 5
0,1 0, - -0,10
3 11 0,
+0, 1
10 5
1 1
-
9, to
A.3 19
- or
0,2
5 2
0,1 1
5 7
+0,
2,
- 8
A.4 19 9,
- 0
0,1 or
0,1 to
5 1
5 2
ISO/FDIS 12251:2025(en)
Template for comments and secretariat observations
a d c b a b
d2 d1 d3 d4 d5 d6 s l1 l2 l4
Size
no ref no m
m. . m. in.
0 or
0 1
or
or
or
0 0 0 0
2 2
2 9 4
4, 4 5
- - - -
A.5 21 4, , to
or
2 , 5
0,1 0,1 0, 0,
1 2 2
5 1
0 3 13 10
or
M L Cla Para Ty C P
Deleted Cells
o r
B i use/ graph/ pe
m o
a See Figure 2. Deleted Cells
/ n Subc Figure of
m p
b See Figure 1Observations of the secretariat.
N e lause /Table com e o
Deleted Cells
n s
C n ment c The nominal dimension for the sealing washer thickness s is defined before assembly to the
1 t e Deleted Cells
engine.
u
s d
m d Maximum value of d shall be considered to decrease when R has a large value to avoid
6 1 Deleted Cells
c
interference between the radius R and
h 1
b
Deleted Cells
a sealing washer.
e
n
r Deleted Cells
g
e
1 MB = Member body / NC = National Committee (enter the ISO 3166 two-letter coun17 try code, e.g. CN for China; comments from the
2 Type of comment: ge = general te = technical ed =
ISO/CS editing unit are identified by **)
Page 17
editorial
of 5
CONFIDENTIAL
ISO/FDIS 12251:2025(en)
Table 2 — Cylinder head bore dimensions, Type A
Dimensions in millimetres
Apply
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