ISO 12251:2026

International
Standard
ISO 12251
Third edition
Diesel engines — Clamp mounted
2026-01
CR fuel injectors — Mounting
dimensions
Moteurs diesels — Injecteurs de combustible pour rampe
commune fixée par patte — Dimensions de montage
Reference number
© ISO 2026
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 .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
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
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
International Standard ISO 12251:2026(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° concave 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:
d injector upper guide diameter
d injector shaft diameter
d capnut diameter
d injector lower guide diameter
d 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
D bore for nozzle shaft
l overall injector length
l distance between sealing face and upper guide diameter of inje
...