ISO 8642:2025

Aerospace - Self-locking nuts with maximum operating temperature greater than 425 °C - Test methods

This document specifies test methods for metric self-locking nuts with MJ threads intended for use in aerospace applications at maximum operating temperature greater than 425 °C. It describes the test device and the method for each test. It applies to self-locking nuts as defined above, provided that the relevant documents (dimensional standard, product standard, drawing, procurement specification, etc.) refer to this document.

Aéronautique et espace — Écrous à freinage interne dont la température maximale d'utilisation est supérieure à 425 °C — Méthodes d'essai

General Information

• Status: Published
• Publication Date: 06-Jul-2025
• ICS: 49.030.30 - Nuts
• Technical Committee: ISO/TC 20/SC 4 - Aerospace fastener systems
• Drafting Committee: ISO/TC 20/SC 4 - Aerospace fastener systems
• Current Stage: 6060 - International Standard published
• Start Date: 07-Jul-2025
• Due Date: 02-Nov-2025
• Completion Date: 07-Jul-2025

Relations

• Revises: ISO 8642:2008 - Aerospace - Self-locking nuts with maximum operating temperature greater than 425 degrees C - Test methods
• Effective Date: 05-Nov-2022

Overview

ISO 8642:2025 - "Aerospace - Self-locking nuts with maximum operating temperature greater than 425 °C - Test methods" specifies standardized test methods for metric self-locking nuts with MJ threads intended for high‑temperature aerospace service (>425 °C). The standard describes required test devices and step‑by‑step methods for each inspection and applies when referenced by dimensional standards, product specifications, drawings or procurement documents. This is the third edition (replacing ISO 8642:2008).

Key topics and technical requirements

The document defines a comprehensive test suite used to verify mechanical, dimensional and locking performance of high‑temperature self‑locking nuts. Key test topics include:

• Hardness testing - Rockwell, Rockwell superficial and Vickers (HV 5 to HV 100) according to ISO 6508‑1 and ISO 6507‑1; measurement zones, minimum specimen thickness (≥10× penetration depth) and parallelism (≤3°). Note: nuts tested for hardness shall not be reused.
• Bearing surface squareness - mandrel, sliding collar and feeler gauge method to assess out‑of‑squareness against dimensional tolerances.
• Axial load and push‑out tests - verification of retention and axial strength under load.
• Wrenching feature and torque‑out tests - evaluation of wrench fit, drive features and torque required to overcome self‑locking.
• Self‑locking torque - ambient and after heat‑soak at maximum operating temperature to confirm locking performance after thermal exposure.
• Permanent set and vibration tests - assessment of irreversible deformation and resistance to vibration‑induced loosening.
• No‑rotation (captive washer) and swaging tests - tests for specific nut configurations and attachment methods.

Each test section identifies the test device, lubrication and environmental conditions (ambient or heat‑soak), and acceptance checks.

Applications and who uses it

ISO 8642:2025 is essential for:

• Aerospace fastener manufacturers qualifying high‑temperature self‑locking nuts
• Quality assurance and metallurgical test laboratories performing acceptance and lot tests
• Design and mechanical engineers specifying fasteners for high‑temperature engine, exhaust or hot‑structure applications
• Procurement/specification writers and certification bodies ensuring traceable test methods
• MRO and airworthiness teams validating repair or replacement parts

Using ISO 8642 helps ensure consistent test methods, reduces risk of fastener failure in extreme thermal environments, and supports supplier conformity and procurement clarity.

Related standards

Normative references cited include:

• ISO 5855‑2 (MJ thread limit dimensions)
• ISO 6507‑1 (Vickers hardness test)
• ISO 6508‑1 (Rockwell hardness test)
• ISO 691 (wrench and socket opening tolerances)
• ISO 7403 (spline drives - wrenching configuration)

Keywords: ISO 8642:2025, self‑locking nuts, aerospace test methods, high‑temperature fasteners, MJ threads, hardness test, torque‑out test, vibration test.

Standards Content (Sample)

International
Standard
ISO 8642
Third edition
Aerospace — Self-locking nuts with
2025-07
maximum operating temperature
greater than 425 °C — Test methods
Aéronautique et espace — Écrous à freinage interne dont la
température maximale d'utilisation est supérieure à 425 °C —
Méthodes d'essai
Reference number
© ISO 2025
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: [phone]
Email: [e-mail]
Website: www.iso.org
Published in Switzerland
ii
Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Inspections and tests . 1
4.1 Hardness test .1
4.1.1 Procedure .1
4.1.2 Method . .2
4.2 Bearing surface squareness test .2
4.2.1 Test device .2
4.2.2 Method . .3
4.3 Axial load test .4
4.3.1 Test device .4
4.3.2 Method . .4
4.4 Wrenching feature test .5
4.4.1 General .5
4.4.2 Test device .5
4.4.3 Method . .6
4.5 Torque-out test .6
4.5.1 General .6
4.5.2 Test device .7
4.5.3 Method . .7
4.6 Test of no rotation of the captive washer.8
4.6.1 General .8
4.6.2 Test device .8
4.6.3 Method . .8
4.7 Push-out test .8
4.7.1 General .8
4.7.2 Test device .9
4.7.3 Method . .9
4.8 Self-locking torque at ambient temperature .10
4.8.1 Test device .10
4.8.2 Method . .11
4.9 Self-locking torque at ambient temperature after heat soak at maximum operating
temperature . 12
4.9.1 Test device . 12
4.9.2 Method . .14
4.10 Permanent set test . 15
4.10.1 General . 15
4.10.2 Test device . 15
4.10.3 Method . .16
4.11 Vibration test .17
4.11.1 General .17
4.11.2 Test device .17
4.11.3 Method . .19
4.12 Swaging test . 20
4.12.1 General . 20
4.12.2 Visual examination . 20
Bibliography .21

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,
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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).
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 Technical Committee ISO/TC 20, Aircraft and space vehicles,
Subcommittee SC 4, Aerospace fastener systems.
This third edition cancels and replaces the second edition (ISO 8642:2008), which has been technically
revised.
The main changes are as follows:
— several references to figures and tables have been corrected;
— designation of figures and tables has been complemented.
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
International Standard ISO 8642:2025(en)
Aerospace — Self-locking nuts with maximum operating
temperature greater than 425 °C — Test methods
1 Scope
This document specifies test methods for metric self-locking nuts with MJ threads intended for use in
aerospace applications at maximum operating temperature greater than 425 °C. It describes the test device
and the method for each test.
It applies to self-locking nuts as defined above, provided that the relevant documents (dimensional standard,
product standard, drawing, procurement specification, etc.) refer to this document.
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 691, Assembly tools for screws and nuts — Wrench and socket openings — Tolerances for general use
ISO 5855-2, Aerospace — MJ threads — Part 2: Limit dimensions for bolts and nuts
ISO 6507-1, Metallic materials — Vickers hardness test — Part 1: Test method
ISO 6508-1, Metallic materials — Rockwell hardness test — Part 1: Test method
ISO 7403, Aerospace — Spline drives — Wrenching configuration — Metric series
3 Terms and definitions
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/
4 Inspections and tests
4.1 Hardness test
4.1.1 Procedure
The following procedures shall be followed:
— Rockwell hardness in accordance with ISO 6508-1;
— Vickers hardness HV 5 to HV 100 in accordance with ISO 6507-1;
— Rockwell superficial hardness in accordance with ISO 6508-1;
— microhardness.
The method corresponding to the hardness unit indicated should be used. Conversion charts may be used if
other test methods have been used to obtain the results; given their inaccuracy, the results obtained can be
inaccurate. In the event of a dispute, the results obtained using the method corresponding to the hardness
unit indicated shall take precedence.
4.1.2 Method
This test shall be carried out at ambient temperature.
The measurement zone (bearing surface, across flats, underside of anchor nut lugs, etc.) shall satisfy the
following conditions:
a) thickness at least equal to 10 × the penetration depth;
b) parallelism with respect to bearing surface no greater than 3°.
If the above requirements cannot be met, carry out this test on a cut section after moulding the nut into a
resin capable of maintaining it in the correct orientation.
Remove all possible coating (protection, lubrication, paint, etc.) in the measurement zone. Align the bearing
surface to obtain the required relationship. These two operations shall not generate any heat liable to modify
the characteristics of the material constituting the nut being tested.
Carry out the test and check the conformity with the requirements of the dimensional standard, product
standard or drawing.
Nuts subjected to this test shall not be reused.
4.2 Bearing surface squareness test
4.2.1 Test device
The test device is illustrated in Figure 1 and shall include the following elements:
a) a threaded mandrel with end in accordance with ISO 5855-2, with the exception of the pitch diameter
which shall be in accordance with the values specified in Table 4 for the maximum mandrel;
b) a collar sliding on the plain portion of the threaded mandrel whose external diameter B is at least equal
to reference dimension A for type I, III and VI nuts in Figure 2 and equal to reference dimension A for
type II, IV and V nuts in Figure 2;
c) an appropriate feeler gauge.

Key
1 sliding collar
2 threaded mandrel
3 feeler gauge
a
Pitch diameter.
Figure 1 — Bearing surface squareness test
a) Type I b) Type II c) Type III d) Type IV e) Type V f) Type VI
NOTE Type I are wrench nuts; type II and type III are clinch nuts; type IV and type V are fixed anchor nus; and
type VI are floating anchor nuts and gang channel nuts.
Figure 2 — Nut configurations
4.2.2 Method
The test shall be carried out at ambient temperature.
For floating nuts, extract the nut from the cage or channel.
Lubricate the mandrel and nut threads (or threaded part) as stated in Table 1 (if necessary). Install, with or
without using a spanner, the threaded mandrel into the nut or threaded part until it engages with the self-
locking zone.
Move the collar into contact with the bearing surface.
Evaluate the out-of-squareness by means of a feeler gauge whose thickness corresponds to the permissible
squareness error permitted by the dimensional standard, the drawing or the procurement specification.
For clinch nuts, the sliding collar shall have a counterbore to accommodate the shank.

Table 1 — Test bolt and lubrication
Nut to be tested Test bolt
Additional lubrica-
tion
Material Coating Material Coating
Steel or alloy steel Any Alloy steel None
Silver or MoS None Synthetic oil
Stainless steel Stainless steel
None Silver
4.3 Axial load test
4.3.1 Test device
The test device is illustrated in Figure 3 and shall include the following elements:
a) a steel bearing plate, heat-treated to a hardness HRC ≥ 40;
b) a bolt with a rolled thread and the following characteristics:
1) threads in accordance with ISO 5855-2;
2) tensile strength class greater than that of the nut under test;
3) material and coating: no specific requirement;
c) a torque wrench.
Dimensions in millimetres
a) Wrench nuts and non-floating b) Clinch nuts c) Floating anchor nuts and gang
anchor nuts channel nuts
a
Maximum shank diameter.
b
Chamfer to suit the nut radius.
c
Hole to allow the specified float.
d
Loading direction.
Figure 3 — Axial load test fixture
4.3.2 Method
4.3.2.1 Principle
The axial load is transmitted to the nut by the bolt, the nut resting on the bearing plate.

4.3.2.2 80 % test
This test shall be carried out at ambient temperature.
Lubricate the bolt and nut threads as stated in Table 1 (if necessary). Assemble the bearing plate on the
bolt. Assemble the nut and measure the locking torque, using a torque wrench, when the protrusion is two
pitches minimum (including chamfer).
Position the assembly on the tensile machine. Apply the load slowly and progressively. Reduce the load
slowly and progressively when the value quoted in the procurement specification has been reached.
Remove the assembly from the tensile machine. Unscrew the nut a half-turn and cease movement, then again
unscrew and measure the breakaway torque, using the torque wrench.
Remove the nut, then submit it to a visual examination and, if necessary, an examination at a magnification
of × 10 after sectioning, to check conformity with the requirements of the procurement specification.
4.3.2.3 100 % test
This test shall be carried out at ambient temperature.
If the test includes a heat soak, then heat the nut and maintain it at the temperature quoted in the
procurement specification. Take the nut from the oven and allow it to cool slowly to ambient temperature,
then, in all cases, proceed as follows.
Lubricate the bolt and nut threads as specified in Table 1 (if necessary), assemble the bearing plate on the
bolt. Assemble the nut with a protrusion of two bolt pitches minimum (including chamfer).
Position the assembly on the tensile machine and apply the load slowly and progressively. Reduce the load
slowly and progressively when the value quoted in the procurement specification has been reached.
Remove the assembly from the tensile machine. Remove the nut, then submit it to a visual examination,
and if necessary, an examination at a magnification of × 10 after sectioning to check conformity with the
requirements of the procurement specification.
Nuts subjected to this test shall not be reused.
4.4 Wrenching feature test
4.4.1 General
This test applies only to wrenchable nuts.
4.4.2 Test device
The test device is illustrated in Figure 4 and shall include the following elements:
a) a steel block, heat-treated to a hardness of HRC ≥ 40;
b) a bolt with a rolled thread and the following characteristics:
1) threads in accordance with ISO 5855-2;
2) tensile strength class: no specific requirement;
3) material and coating: no specific requirement;
c) a torque wrench.
NOTE Any other device that prevents the rotation of the nut and allows the specified torque to be applied is
acceptable; for example:
— a nut welded on a block of the same material, the assembly being heat-treated to the correct level;
— nuts mounted in counter-rotation on a threaded rod of strength class appropriate to hold the required torques
without deformation;
— a nut mounted on a bolt of strength class appropriate to hold the required torques without deformation as a spacer
is placed between the nut and the bolt head.
Dimensions in millimetres
a
Width of slot equal to diameter of circle circumscribing the wrenching feature.
b
Chamfer to suit underhead radius.
c
Including chamfer, where P is the pitch.
d
Depth of slot equal to flange height of nut under test.
Figure 4 — Wrenching feature test
...