kSIST FprEN 4048:2026

• Abstract:

  This document specifies the characteristics, qualification and acceptance requirements for self-locking nuts with MJ threads in NI-PH2601, MoS2 coated, for aerospace applications.; Classification: 1 550 MPa /425 °C .; It is applicable whenever referenced.

• Status: Not Published
• Public Enquiry End Date: 30-Jan-2025
• ICS: 49.030.30 - Nuts
• Technical Committee: I13 - Imaginarni 13
• Current Stage: 5020 - Formal vote (FV) (Adopted Project)
• Start Date: 02-Jun-2026
• Due Date: 21-Jul-2026
• Completion Date: 09-Jun-2026
• Ref Project: FprEN 4048 - Aerospace series - Nuts, self-locking, MJ threads, in heat-resisting nickel base alloy NI-PH2601 (Inconel 718), MoS2 coated - Classification: 1 550 MPa (at ambient temperature)/425 °C - Technical specification

• Revises: SIST EN 4048:2005 - Aerospace series - Nuts, self-locking, MJ threads, in heat resisting nickel base alloy NI-PH2601 (Inconel 718), MoS2 coated - Classification: 1 550 MPa (at ambient temperature) / 425 °C - Technical specification
• Effective Date: 09-Jul-2024

SIST publishes kSIST FprEN 4048:2026 for aerospace self-locking nuts with MJ threads in NI-PH2601 (Inconel 718), MoS2 coated. The document specifies the characteristics, qualification requirements, and acceptance requirements for these nuts, classified at 1 550 MPa at ambient temperature and 425 °C. It is used whenever a product standard or definition document calls it up.

What does kSIST FprEN 4048:2026 specify?

kSIST FprEN 4048:2026 specifies aerospace series self-locking nuts with MJ threads in heat-resisting nickel base alloy NI-PH2601, with MoS2 coating. It sets the technical basis for how these nuts are made, tested, accepted, marked, packed, and identified for use in aerospace applications.

The document is organized into:

• Clause 1 - Scope
• Clause 2 - Normative references
• Clause 3 - Terms and definitions
• Clause 4 - Quality assurance, including qualification and acceptance
• Clause 5 - Requirements

The technical content is supported by Tables 1 to 8, which tie each requirement to a test method, sample size, defect class, or limiting value. For a reader, that means the document is not only a description of the part, but also a working rule set for inspection and release.

What are the key requirements of kSIST FprEN 4048:2026?

kSIST FprEN 4048:2026 combines product definition, process controls, and performance tests. In practice, it tells a manufacturer what must be controlled during manufacture and what must be demonstrated before the nuts can be accepted for aerospace use.

Materials, dimensions, and manufacture

Clause 5.1 requires the material to match the product standard or definition document, with confirmation by chemical analysis or a manufacturer’s certificate of conformity for the semi-finished product. Clause 5.2 requires dimensions, tolerances, and form and position tolerances to meet the product standard, with standard gauging. This matters because the nut must fit the MJ thread system and assemble correctly with the mating bolt.

Clause 5.3 controls how the nut is made. Nuts may be hot-formed or cold-formed, hot forming is limited to 1 150 °C, and the parts must be air cooled or faster. Heat treatment is tightly defined, including solution treatment when applicable and a precipitation treatment cycle, so the finished nut reaches the intended strength and temperature capability.

Surface condition and coating

Clause 5.3.6 covers the MoS2 coating. Appearance follows EN 2491, thickness is measured by ISO 1463 or ISO 3497, and EN 2491 also governs adhesion. In practice, this means the coating must be present, measurable, and well attached because it affects friction, installation torque, and service behavior.

Clause 5.5 deals with surface and metallurgical quality before coating. Cracks are not permitted, laps, seams, and inclusions are checked, and surface contamination on machined surfaces is not allowed. Grain size and microstructure are also controlled, which matters because aerospace fasteners depend on stable material properties, not only on shape and size.

Mechanical performance

Clause 5.4 is the core performance section. It covers axial load at ambient temperature, axial load after 425 °C baking, wrench feature torque, vibration performance, permanent set, reusability, flareability for clinch nuts, push out, torque out, and hardness. These tests show whether the nut keeps its locking function, strength, and mechanical integrity after loading and heat exposure.

Several of these tests are tied to Table 5 and Table 7 values by thread size. For a buyer or quality manager, this means the acceptance criteria are not generic: they depend on the MJ thread size and the specific performance case, such as ambient use or after thermal exposure.

Qualification and acceptance

Clause 4 separates qualification from acceptance. Qualification applies to each type and diameter and uses 46 nuts selected by simple random sampling from a single inspection lot. Acceptance applies to each production batch or inspection lot, and the manufacturer performs it or has it performed under its responsibility. This matters because it distinguishes type approval from routine batch release.

Clause 4 also allows some qualification tests to be reused for acceptance when a stricter inspection is needed, provided the same sample size is kept. Table 2 shows how qualification samples are allocated across non-destructive and destructive tests.

Marking, packaging, and traceability

Clauses 5.6 to 5.8 cover product marking, packaging, and labelling. Each basic package must contain only one part number and one batch number, and the label must show the designation, quantity, batch number, and the manufacturer’s name and trade mark. In practice, this supports traceability, storage control, and segregation of mixed lots.

What terms does kSIST FprEN 4048:2026 define?

• Batch - the quantity of finished parts made from the same material, melt, production cycle, manufacturing route, and heat and surface treatment history.
• Inspection lot - a set of parts from one production batch with the same part number that fully defines the part.
• Self-locking torque - the torque needed to keep rotation moving between the nut and the mating part when the locking system is fully engaged and the assembly has no axial load.
• Seating torque - the tightening torque that introduces or increases axial load in the assembly.
• Breakaway torque - the torque needed to start unscrewing after the axial load has been removed.
• AQL (acceptance quality limit) - the quality limit used in a sampling plan for a specified, relatively high probability of acceptance.
• LQ (limiting quality) - the quality limit in a sampling plan corresponding to a specified 10 % probability of acceptance.

Who uses kSIST FprEN 4048:2026?

kSIST FprEN 4048:2026 is used by aerospace fastener manufacturers, subcontractors, and inspection laboratories that make or test self-locking nuts. It is also used by quality assurance staff, receiving inspectors, and buyers who need to specify the exact nut type, test regime, and release criteria.

The document is especially relevant when a supplier must demonstrate qualification, run batch acceptance testing, or document coating thickness, locking torque, heat treatment, and traceability. It gives all of these groups the same reference point for what counts as acceptable product.

What changed in kSIST FprEN 4048:2026 from the previous edition?

kSIST FprEN 4048:2026 supersedes EN 4048:2004. The foreword lists these technical changes:

• EN 3497 was added to Clause 2.
• Heat treatment temperature tolerances and duration were modified.
• Thread and thread deformation requirements were modified.
• Thickness measurement methods were added.
• Axial load requirements at ambient temperature and after 425 °C baking were modified.
• The five-cycle self-locking torque method at ambient temperature was modified.
• The hardness requirement was modified.
• Table 5 was modified for wrench feature torque, push out load, and torque out test.
• Table 7 seating torque values and the related footnote were modified.

Which standards are used with kSIST FprEN 4048:2026?

kSIST FprEN 4048:2026 relies on and points to these related documents:

• EN 2491 - coating methods for molybdenum disulphide dry lubricants, and the reference for coating appearance and adhesion.
• ISO 1463 and ISO 3497 - methods for measuring coating thickness.
• ISO 5855-2 - MJ thread limit dimensions for bolts and nuts, used for fit and thread checking.
• ISO 8642:2008 - test methods for self-locking nuts with maximum operating temperature greater than 425 °C.
• ASTM E 112-96 - reference for grain size assessment.
• EN 9133 - qualification procedure for aerospace standard products.
• ISO 2859-1:1999 - attribute sampling plans used for lot-by-lot inspection.
• ISO 3452-1 - penetrant testing, used for surface discontinuity inspection.
• ISO 21920-3 - surface texture specification, used for surface roughness.

What does the kSIST FprEN 4048:2026 document contain?

The document contains technical requirement tables and test allocation tables that link each characteristic to a method and sample size. Table 1 is the main requirements table, Table 2 allocates qualification samples, Table 3 classifies defects, and Table 4 gives sampling plans for visual and dimensional inspection.

It also contains load and torque values by MJ thread size in Table 5, mechanical and metallurgical sampling plans in Table 6, torque limits in Table 7, and permissible surface discontinuities in Table 8. Those tables are used for design checking, qualification, routine acceptance, and final release of aerospace self-locking nuts.

SLOVENSKI STANDARD
oSIST prEN 4048:2025
01-januar-2025
Aeronavtika - Matice, samozapiralne, navoj MJ, iz toplotnoodporne zlitine na
nikljevi osnovi NI-PH2601 (Inconel 718), mazane z MoS2 - Klasifikacija: 1 550 MPa
(pri temperaturi okolice)/425 °C - Tehnična specifikacija
Aerospace series - Nuts, self-locking, MJ threads, in heat-resisting nickel base alloy NI-
PH2601 (Inconel 718), MoS2 coated - Classification: 1 550 MPa (at ambient
temperature)/425 °C - Technical specification
Luft- und Raumfahrt - Muttern, selbstsichernd, MJ-Gewinde, aus hochwarmfester
Nickelbasislegierung NI-PH2601 (Inconel 718), MoS2-beschichtet - Klasse: 1 550 MPa
(bei Raumtemperatur)/425 °C - Technische Lieferbedingungen
Série aérospatiale - Écrous, à freinage interne, à filetage MJ, en alliage résistant à chaud
à base de nickel NI-PH2601 (Inconel 718), revêtus MoS2 - Classification : 1 550 MPa (à
température ambiante)/425 °C - Spécification technique
Ta slovenski standard je istoveten z: prEN 4048
ICS:
49.030.30 Matice Nuts
oSIST prEN 4048:2025 en,fr,de
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

oSIST prEN 4048:2025
oSIST prEN 4048:2025
DRAFT
EUROPEAN STANDARD
prEN 4048
NORME EUROPÉENNE
EUROPÄISCHE NORM
November 2024
ICS 49.030.30 Will supersede EN 4048:2004
English Version
Aerospace series - Nuts, self-locking, MJ threads, in heat-
resisting nickel base alloy NI-PH2601 (Inconel 718), MoS2
coated - Classification: 1 550 MPa (at ambient
temperature)/425 °C - Technical specification
Série aérospatiale - Écrous, à freinage interne, à filetage Luft- und Raumfahrt - Muttern, selbstsichernd, MJ-
MJ, en alliage résistant à chaud à base de nickel NI- Gewinde, aus hochwarmfester Nickelbasislegierung NI-
PH2601 (Inconel 718), revêtus MoS2 - Classification : 1 PH2601 (Inconel 718), MoS2-beschichtet - Klasse: 1
550 MPa (à température ambiante)/425 °C - 550 MPa (bei Raumtemperatur)/425 °C - Technische
Spécification technique Lieferbedingungen
This draft European Standard is submitted to CEN members for enquiry. It has been drawn up by the Technical Committee ASD-
STAN.
If this draft becomes a European Standard, CEN members are bound to comply with the CEN/CENELEC Internal Regulations
which stipulate the conditions for giving this European Standard the status of a national standard without any alteration.

This draft European Standard was established by CEN in three official versions (English, French, German). A version in any other
language made by translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC
Management Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,
Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye and
United Kingdom.
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 supporting documentation.

Warning : This document is not a European Standard. It is distributed for review and comments. It is subject to change without
notice and shall not be referred to as a European Standard.

EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2024 CEN All rights of exploitation in any form and by any means reserved Ref. No. prEN 4048:2024 E
worldwide for CEN national Members.

oSIST prEN 4048:2025
prEN 4048:2024 (E)
Contents Page
European foreword . 3
1 Scope . 4
2 Normative references . 4
3 Terms and definitions . 4
3.3 Surface discontinuities . 5
4 Quality assurance . 7
4.1 Qualification . 7
4.2 Acceptance. 7
4.2.1 Purpose . 7
4.2.2 Responsibility . 7
4.2.3 Inspection and test report . 7
5 Requirements . 7
Bibliography . 21

oSIST prEN 4048:2025
prEN 4048:2024 (E)
European foreword
This document (prEN 4048:2024) has been prepared by the Aerospace and Defence Industries
Association of Europe — Standardization (ASD-STAN).
After enquiries and votes carried out in accordance with the rules of this Association, this document has
received the approval of the National Associations and the Official Services of the member countries of
ASD-STAN, prior to its presentation to CEN.
This document is currently submitted to the CEN Enquiry.
This document will supersede EN 4048:2004.
This document includes the following significant technical changes with respect to EN 4048:2004:
— Clause 2: EN 3497 added;
— Table 1, Clause 5.3.2: heat treating temperature tolerances and duration modified;
— Table 1, Clause 5.3.4: thread and thread deformation requirements modified;
— Table 1, Clause 5.3.6.2: thickness measurement methods added;
— Table 1, Clause 5.4.4.1: Axial load requirement at ambient temperature modified;
— Table 1, Clause 5.4.4.2: Axial load requirement after 425°C baking modified;
— Table 1, Clause 5.4.5.1: Self-locking torque at ambient temperature (five cycles) method modified;
— Table 1, Clause 5.4.9: Hardness requirement modified;
— Table 5: requirement modified (wrench feature torque, push out load and torque out test);
— Table 7: seating torque values and footnote modified.
oSIST prEN 4048:2025
prEN 4048:2024 (E)
1 Scope
This document specifies the characteristics, qualification and acceptance requirements for self-locking
nuts with MJ threads in NI-PH2601, MoS coated, for aerospace applications.
1 2
Classification: 1 550 MPa /425 °C .
It is applicable whenever referenced.
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.
EN 2491, Aerospace series — Molybdenum disulphide dry lubricants — Coating methods
ISO 1463, Metallic and oxide coatings — Measurement of coating thickness — Microscopical method
ISO 3497, Metallic coatings — Measurement of coating thickness — X-ray spectrometric methods
ISO 5855-2, Aerospace — MJ threads — Part 2: Limit dimensions for bolts and nuts
ISO 8642:2008, Aerospace — Self-locking nuts with maximum operating temperature greater than
425 °C — Test methods
ASTM E 112-96, Standard Test Methods for Determining Average Grain Size
3 Terms and definitions
For the purposes of this document, the following terms and definitions 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
batch
quantity of finished parts, of the same type and same diameter, produced from the same material
obtained from the same melt, manufactured in the course of the same production cycle, following the
same manufacturing route and having undergone all the relevant heat treatments and surface
treatments
3.2
inspection lot
quantity of parts from a single production batch with the same part number which completely defines
the part
Correspond to the minimum tensile stress which the nut is able to withstand at ambient temperature without
breaking or cracking when tested with a bolt of a higher strength class.
Maximum test temperature of the parts.
Published by: ISO International Organization for Standardization http://www.iso.org/.
Published by: ASTM International (US) https://www.astm.org/.
oSIST prEN 4048:2025
prEN 4048:2024 (E)
3.3 Surface discontinuities
3.3.1
crack
rupture in the material which may extend in any direction and which may be intercrystalline or
transcrystalline in character
3.3.2
seam
open surface defect
3.3.3
lap
surface defect caused by folding over metal fins or sharp corners and then rolling or forging them into
the surface
3.3.4
inclusions
non-metallic particles originating from the material manufacturing process. These particles may be
isolated or arranged in strings
3.4
test temperature
ambient temperature unless otherwise specified
3.5
simple random sampling
taking of n items from a population of N items in such a way that all possible combinations of n items
have the same probability of being chosen
3.6
critical defect
defect that, according to judgement and experience, is likely to result in hazardous or unsafe conditions
for individuals using, maintaining, or depending upon the considered product, or that is likely to
prevent performance of the function of a major end item
3.7
major defect
defect other than critical, that is likely to result in a failure or to reduce materially the usability of the
considered product for its intended purpose
3.8
minor defect
defect that is not likely to reduce materially the usability of the considered product for its intended
purpose, or that is a departure from established specification having little bearing on the effective use
or operation of this product
3.9
sampling plan
plan according to which one or more samples are taken in order to obtain information and possibly to
reach a decision
oSIST prEN 4048:2025
prEN 4048:2024 (E)
3.10
limiting quality
LQ
quality limit in a sampling plan which corresponds to a specified 10 % probability of acceptance
3.11
acceptance quality limit
AQL
quality limit which in a sampling plan corresponds to a specified but relatively high probability of
acceptance
Note 1 to entry: It is the maximum per cent defective (or the maximum number of defects per hundred units)
that, for purposes of sampling inspection can be considered satisfactory as a process average.
3.12
finished nut
nut ready for use, inclusive of any possible treatments and/or surface coatings, as specified in the
product standard or definition document
3.13
definition document
document specifying all the requirements for finished nuts
3.14
self-locking torque
torque to be applied to the nut or bolt to maintain movement of rotation in relation to the associated
part, the assembly being under no axial load and the nut locking system being completely engaged with
the bolt (minimum protrusion of two pitches including end chamfer)
3.15
seating torque
tightening torque to be applied to the nut or bolt to introduce or to increase the axial load in the
assembly
3.16
unseating torque
untightening torque to be applied to the nut or bolt to reduce or remove the axial load in the assembly
3.17
breakaway torque
torque required to start unscrewing the nut or bolt with respect to the associated part, with the nut
locking device still fully engaged on the bolt, but after the axial load in the assembly has been removed
by unscrewing half a turn followed by a halt in rotational movement
3.18
wrench feature torque
torque to be applied to the wrenching feature of the nut
oSIST prEN 4048:2025
prEN 4048:2024 (E)
4 Quality assurance
4.1 Qualification
The qualification procedure for aerospace standard products (e.g. according to EN 9133 or an
equivalent aerospace accepted and established qualification procedure) shall be used and documented
according to the specified tests if not otherwise agreed between customer and supplier.
Qualification inspections and tests (requirements, methods, numbers of nuts) are specified in Table 1.
They shall be carried out on:
— each type and diameter of nut;
— 46 nuts selected from a single inspection lot by simple random sampling.
The test programme may possibly be reduced, or the qualification of a nut be granted without
inspection or testing. Any such decision shall be based on the results obtained on similar types and
diameters of nuts provided that the design and manufacturing conditions are identical.
Table 2 indicates the allocation of nut samples for the inspections and tests.
4.2 Acceptance
4.2.1 Purpose
Acceptance inspections and tests (requirements, methods, numbers of nuts) are specified in Table 1;
they shall be carried out on each production batch or inspection lot. Nuts from the batch or lot to be
tested shall be selected by simple random sampling.
Each nut may be submitted to several inspections or tests.
If a more stringent inspection is deemed necessary, all or part of the qualification inspections and tests
may be performed during the acceptance inspection and testing. In this case, the number of nuts
submitted to these inspections and tests is the same as that submitted for qualification inspection and
tests.
4.2.2 Responsibility
Acceptance inspections and tests shall be carried out by the manufacturer, or under his responsibility.
4.2.3 Inspection and test report
A test report showing actual numerical values shall be provided if specified in the purchase order.
5 Requirements
Requirements shall be according to Table 1.
The same test sample may be utilized for more than one test provided that none of the characteristics of
the samples are altered during the examination procedure (see Table 2).
oSIST prEN 4048:2025
prEN 4048:2024 (E)
Table 1 — Technical requirements and tests methods
Sample
a
Clause Characteristic Requirement Inspection and test method Q/A
size
5.1   Material In accordance with the product Chemical analysis or certificate Q
standard or definition of conformity issued by the
A
document. manufacturer of the semi-
finished product.
5.2   Dimensions, In accordance with the product Standard gauging. Q 43
tolerances and standard or definition
A Tables
tolerances of form document.
3 and 4
and position
5.3   Manufacturing
5.3.1   Forming Nuts shall be formed by a hot or The method of forming shall be Q
cold forming process. indicated.
If hot formed, the forming
temperature shall not exceed
1 150 °C and they shall be air
cooled or faster.
The equipment shall ensure a
uniform temperature
throughout the production
batch.
5.3.2   Heat treatment The heat treatment medium or Calibration of the heat Q
atmosphere shall not cause any treatment equipment shall be

surface contamination except as confirmed.
permitted by 5.5.4.
Any scale which will not be Visual examination.
removed by subsequent
machining shall be removed by
abrasive blasting with an
appropriate equipment.
Solution treat (if applicable) at a Examination of the heat
temperature of 940 °C to treatment specification.
1010 °C, hold at the selected
temperature within ± 15 °C for
between 1 h min. and 2 h max.,
quench in oil, or alternative
medium or faster.
Precipitation treatment at
(720 ± 5) °C, held at
temperature for at least 8 h,
furnace cooled at (55 ± 5) °C
per hour to (620 ± 5) °C, held at
620 °C for at least 8 h , followed
by air cooling or faster.
Instead of the 55 °C per hour
cooling rate to 620 °C, parts
may be furnace cooled at any
rate provided the time at 620 °C
is adjusted to give a total
precipitation time of 18 h min.

oSIST prEN 4048:2025
prEN 4048:2024 (E)
Sample
a
Clause Characteristic Requirement Inspection and test method Q/A
size
5.3.3   Bearing surface In accordance with the product ISO 8642. Q 43
perpendicularity standard or definition
A Tables
document.
3 and 4
For non-floating plate nuts
having a bearing surface
exceeding × 1,5 the thread
nominal diameter, the
perpendicularity requirement
shall, unless otherwise specified
by the product standard or
definition document, apply only
to that portion of the bearing
surface of the part contained
within a diameter equal to × 1,5
the thread nominal diameter.
5.3.4   Thread and thread T
...