Specifications and information

The information below is intended for informative purposes only and should only be regarded as such. A copy of any specification referred to should be consulted.

Property Classes for Bolts & Screws (ISO 898-1)

Externally threaded parts marking
The number to the left of the dot (one or two digits) indicates 1/100 of the nominal tensile strength in megapascals.
The number to the right of the dot indicates 10 times the ratio between the nominal yield strength and the nominal tensile strength.
e.g. 8.8 will signify that the fastener has a nominal tensile strength of (8 x 100) 800 MPa and a yield strength ratio of (8 ÷ 10) 0.8. Multiplying 800 x 0.8 gives a nominal 640 MPa of stress at 0.2% non-proportional elongation, the same applies to G10.9 and G12.9.
There can be a 0 before the digit on the left of the dot, which indicates reduced loadability.

Property Classes for Nuts (ISO 898-2)

Internal threaded nuts designation and marking
There are three styles of nuts
style 0 - thin nut with a minimum height of 0.45 x (nominal thread diameter) and maximum height 0.80 x (nominal thread diameter)
style 1 - nut with a minimum height of 0.80 x (nominal thread diameter) and a maximum height of 0.89 x (nominal thread diameter)
style 2 - nut with a minimum height of 0.89 x (nominal thread diameter)
style 0 has a number designation with a 0 as the first digit, indicating reduced loadability and the second number indicating 1/100 of the nominal stress under proof load in megapascals.
style 1 and style 2 have a number designation that corresponds with the number to the left of the dot for parts in ISO 898-1, the number denotes 1/100 of the nominal tensile strength of the mating bolt/stud/screw in megapascals.
e.g. a G8 nut will be mated with G8.8 bolt/screw/stud (note: for style 1 galvanized tapped oversize nuts, BS 3692 recommends that bolts should be mated with a nut of a higher property class).
The parts of ISO 898, that our products fall in to, deal with normative references, designation and design, the steel chemical composition, mechanical properties, proof loads and marking criteria. There is a section devoted to tests that can be carried out to ascertain that products meet the criteria specified.

Stainless Steel Fasteners (ISO 3506)

ISO 3506-1 deals with externally threaded parts.
ISO 3506-2 deals with Internally threaded parts
Stainless fasteners use a letter-number system instead — A2-70, A4-80, and so on. The letter refers to the steel grade (A2 is the common 304-type, A4 is the more corrosion-resistant 316-type), and the number is the tensile strength in MPa ÷ 10.

DIN 267-4 and BS 3692

DIN 267-4 Technical delivery conditions Property classes for nuts
Although DIN 267-4 is an old specification, 1983 being the last update, DIN 934 nuts are still manufactured that use this standard. Proof loads are lower compared to ISO 898-2, therefore to differentiate between the two standards bars are used either side of the property class marking e.g. |8|
BS 3692[2014] ISO metric precision hexagon bolts, screws and nuts
As with DIN 267-4, nuts specified in BS 3692 have a lower proof load value than ISO 898-2 and bars are used either side of the property class, or grade as BS 3692 names it.

Dimensional Standards (DIN 934, ISO 4032, etc.)

DIN 934 Hexagon nuts with metric coarse and fine pitch thread - Product grades A and B
Dimensions and mass; M1 - M160
ISO 4032 [2023] Hexagon regular nuts (style 1) - Product grades A and B
Dimensions, marking and designation; M1.6 - M64
ISO 8673 [2023] Hexagon regular nuts (style 1) with metric fine pitch thread - Product grades A and B
Dimensions, marking and designation; M8 - M39
DIN 936 / DIN 439 M8 to M52 and M8 x 1 to M52 x 3 hexagon thin nuts - Product grades A and B
ISO 4035 [2023] Hexagon thin nuts chamfered (style 0) - Product grades A and B
Dimensions, marking and designation; M5 - M39
ISO 8675 [2023] Hexagon thin nuts (style 0), with fine pitch thread - Product grades A and B
Dimensions, marking and designation; M8 - M64

Allthread / Stud Bolts (DIN 976-1)

DIN 976-1 [2016] Fasteners - Stud bolts - Part 1: Metric thread
DIN 976-1 gives length and weight information, as well as normative references; M2 - M72. In Section 6 are the marking requirements. These specify that at sizes of M5 and greater, for steel property classes greater than G4.8 and austenitic stainless steel A2-70 or A4-70, be marked at one end with the respective symbols (Section 6.1) or using colour coding (Section 6.2), as an alternative to symbols.

8.8

10.9

12.9

Why We Hardness Test

If done properly a hardness test is useful, not only to confirm that the material being tested conforms to hardness specification for the appropriate standard, but also to give a good indication of tensile strength and other mechanical properties of the material. However it must not be relied upon as a given that if material hardness is within specification other mechanical properties will be within specification.
A tensile test, to proof load and then increasing until fracture occurs is a more reliable way to confirm tensile strength and other properties of a material.

3.1 Certificates — What Should Be On Them?

There is a lot of confusion as to which results should appear on a 3.1 certificate, or an F3.1 test report, to give it the ISO designation.
ISO 898 parts 1 and 2, and ISO 3506 parts 1 and 2 state that fasteners produced shall be capable of conforming to the applicable requirements, when using the applicable tests and methods in the appropriate specification. They state that if a purchaser requires test results and any additional or specific tests they are agreed on at the time of order. These specifications add that they do not mandate which of the tests the manufacturer shall perform.
As an example, for property class 8.8 (and above), if the tensile strength of the material is known, the yield strength (the point at which steel will no longer return to its original shape when a load is applied) can approximately be worked out using the formula; tensile strength x (yield strength ratio for steel used), yield strength ratio for steel used for fasteners of these property classes are usually between 0.8 and 0.9 (.8 or .9 in the designation). A bolt with a tensile strength of 900 MPa and a yield ratio of 0.8, the yield strength of the bolt would be in the region of 720 MPa, so the manufacturer, as ISO 898-1 does not state which of the tests are mandated, only that the product conforms to the applicable requirements, need not do a test and will have high confidence that the product, if subjected to a test for 0.2% elongation, will pass (≤M16 = 640 MPa min., > M16 = 660 MPa min.) .

EN 10204[2004] Metallic products - Types of inspection documents

The European standard for 2.1, 2.2, 3.1, and 3.2 certificates, EN 10204 sets out the types of certificate and a general overview of the meaning of the respective numbers. 2.1 and 2.2 are inspection documents based on non-specific inspection, and 3.1 and 3.2 are inspection documents based on specific inspection. EN 10204 does not specify any test results that need to be on a certificate.
This is generally the specification number that will be referenced on a 3.1 test report/certificate.

EN 10168[2004] Steel products - Inspection documents

List of information and description, again a European standard. EN 10168 sets out code numbers and designations for products included in a certificate. e.g. A 02 is the type of inspection document, B 02 is the code for steel designation.

ISO 16228[2018] Fasteners - Types of inspection documents

The International standard that gives comprehensive information regarding minimum requirements for what needs to be included on a certificate, and prefixes the type of document with the letter F.
Section 5.7, Table 2, Minimum requirements for reporting of test/inspection results. This section deals with methods of control, measurements shall be expressed as either min or max or min/max and give measured values, attributes shall be expressed as Go or No-Go or Go/No-Go and expressed as Conform, and inspection which will be expressed as Conform.
Section 5.8, Table 3, Minimum test/inspection results for fasteners. Results to be included in F3.1 test reports are:-
ISO 898-1 Chemical composition, Tensile strength, Hardness for quenched and tempered property classes, and Thread acceptance.
ISO 898-2 Chemical composition, Proof load, Hardness for quenched and tempered property classes. and Thread acceptance.
ISO 3506-1 Chemical composition, Tensile strength and Elongation, Hardness for fasteners made of martensitic and ferritic stainless steel, and Thread acceptance.
ISO 3506-2 Chemical composition, Proof load, Hardness for fasteners made of martensitic and ferritic stainless steel, and Thread acceptance.
Section 6 Required content for each type of fastener inspection document.
Section 6.1 General, states other information and/or test/inspection results may be included at the discretion of the supplier or if requested by the purchaser at the time of the order.
Section 6.4 Content of the fastener test report F3.1, Table 6, Required content for F3.1.
First section, General information related to the supplier.
Second section, General information related to the delivered fasteners.
Third section, Test/inspection results for material properties.
For fasteners made of steel or stainless steel
- the fastener reference standard (e.g. DIN 934, ISO 898-2) and date of publication;
- the material category, grade and/or designation (e.g. alloy steel for property class 10.9, A2 for stainless steel, etc.);
- the code identifying the test facility type where the chemical analysis has been performed: S (supplier), M (manufacturer), D (distributor), L (laboratory);
- the heat, cast, lot number of the material, or trace number to the material lot, the chemical analysis which shall include the chemical elements, the required chemical composition limit(s) and the test results. The elements specifically required by the reference standard shall be reported, however other elements may be included; the chemical analysis can also be a copy of the original material inspection document attached to the fastener inspection document.
Fourth section, Test/inspection results for mechanical, physical, dimensional, functional and/ finish-coating properties.
For each reported property
- the mechanical or physical property, its symbol, if any, the test method, the reference standard or technical specification and its date;
- the dimensional property and its symbol, if any;
- the code identifying the test facility type where the chemical analysis has been performed: S (supplier), M (manufacturer), D (distributor), L (laboratory);
- the quantity of tested parts, except when all test/inspection results are reported;
- the required limit(s), if any;
- the test/inspection results according to section 5.8, Table 3.
These items shall also be included in F3.1 when functional and/or finish-coating properties are specified in the relevant product standard (and/or technical specification) and/or by the purchaser.
Fifth section, validation.

Summary
From the contents of section 5.8, Table 3 in ISO 16228, it is clear which test results are required to satisfy the minimum requirements for a 3.1 test report/certificate, for the applicable material chemical and mechanical properties standard, and any other tests that may/may not be included.
It is wrongly assumed that test results are included because they appear in a table in the appropriate specification e.g. ISO 898-1 Tables 8-13, which are headed with Test series (FF 1-4, and MP 1 and 2). It is assumed that because the words test series appear that these tests should be carried out and results should therefore be included in the 3.1 certificate results. The test series tables only provide the feasibility of a test.
We are often asked for Boron(B) results, as, rightly, they are part of the ISO 898-1 chemical composition limit (8.8 and greater). However if the steel used is, for example, 42CrMo4, an alloy steel without any boron added, it is likely boron will not appear in the results of the original material inspection document (the chemical analysis can also be a copy of the original material inspection document), so may not be shown in the results of a 3.1 certificate.

Basic Thread Diameters (Metric)

The basic major, minor and pitch diameters are calculated using portions of the heights of theoretical triangles, from thread root to thread crest (In reality the root and crest points of the triangles are flattened), generated by the thread pitch, with a 60° thread angle, minor being 5/8 height and pitch 3/8 height. The following equations are simplified versions, but yield the same results:
(D [internal]/d [external]) major diameter = nominal diameter i.e. M16 = 16mm
(D₁/d₁) minor diameter = major diameter(D/d) - 2 x (0.54127 x thread pitch(P))
(D₂/d₂) pitch diameter = major diameter(D/d) - 2 x (0.32476 x thread pitch(P))

Thread Fit and Tolerance (Metric)

Internal thread fit is denoted by a capital letter e.g. 6H, external thread fit is denoted by a lowercase letter e.g. 6g.
The thread fit is split into two parts, the first, the number, denoting the tolerance grade, values are given in ISO 965-1, Table 3 for external thread (Td) and Table 4 for internal thread (TD).
Different thread pitches have a tolerance expressed in microns (µm) (there are 1000µm in a mm), e.g. 1mm thread pitch for tolerance grade 4 of external threads = 112µm, tolerance grade 6 = 180µm, and tolerance grade 8 = 280µm.
External thread tolerance grades are split into three: 4,6,8 and are used to give tolerance for the major and pitch diameters. Internal thread tolerance grades are split into five: 4,5,6,7,8 and are used to give tolerance for the minor and pitch diameters.
The second part of the thread fit, the letter, which denotes the tolerance position and gives the fundamental deviation, ISO 965-1 Table 1, again expressed in microns. Internal fundamental deviation is split in to two, G,H, external fundamental deviation is split into eight, a,b,c,d,e,f,g,h, and as with tolerance grade is thread pitch dependant.
To find the tolerance value of a specific thread the tolerance grade value is added to or subtracted from a basic value plus the fundamental deviation value. The tolerance values for internal threads increase the diameter, and external tolerance values decrease the diameter.
e.g. M16 x 2mm pitch 6H full nut
Basic minor diameter = 13.835mm
Basic pitch diameter = 14.701mm
ISO 965-1, Table 1, 2mm thread pitch, fundamental deviation H = 0µm
ISO 965-1, Table 2, 2mm thread pitch, minor diameter tolerance grade 6 = 375µm
ISO 965-1, Table 4, 2mm thread pitch, pitch diameter tolerance grade 6, basic major diameter above 11.2mm, less than or equal to 22.4mm = 212µm
Using this information the calculations are:
Minor diameter
minimum value = 0µm + 13.835mm = 13.835mm, maximum = 13.835mm + 375µm = 14.210mm
Pitch diameter
minimum value = 0µm + 14.701mm = 14.701mm, maximum = 14.701mm + 212µm = 14.913mm
In the case of an M16 x 2mm pitch 6G full nut
Fundamental deviation G = +38, therefore the calculations would be:
Minor diameter
minimum = 38µm + 13.835mm = 13.873mm, maximum = 13.873mm + 375µm = 14.248mm
Pitch diameter
minimum = 38µm + 14.701mm = 14.739mm, maximum = 14.739mm + 212µm = 14.951mm

The majority of internal threaded nuts produced are 6H and external threaded bolts, screws, allthread etc. are 6g, 8g is also used, but is less common and for galvanised parts 6az and 6AZ (or for even greater tolerance 6ax and 6AX).
To work out the minor and pitch dimensions for tapped oversize nuts, the fundamental deviation of AZ (or az, external) is worked out by adding 300 to 20 times the thread pitch (ax/AX is 220 times thread pitch minus 20) The standard which covers this is ISO 965, part 4 for external threads, and part 5 for internal threads .
Again we will use an M16 x 2mm thread pitch nut
AZ = 300 + (20 x 2 = 40) gives a fundamental deviation of 340µm and applied as above calculations will give:
Minor diameter
minimum = 340µm + 13.835mm = 14.175mm, maximum = 14.175mm + 375µm = 15.550mm
Pitch diameter
minimum = 340µm + 14.701mm = 15.041mm, maximum = 15.041mm + 212µm = 15.253mm

External threads are worked out in the same way, but instead of the minor diameter being the crucial value, the major diameter is, and the tolerances are subtracted. There may also be two tolerance designations e.g. 5g6g, the first tolerance is the pitch tolerance and the second the major diameter. In the case of 6az the diameter of the external thread would be further reduced, screwed undersize, to accommodate the galvanized coating thickness, crucially a 6H nut would be used, not 6AZ.
ISO 10684 also states, in Annex F, that assembly strength, for 6az, 6ax, 6AZ, and 6AX, will be reduced and provides methods to combat this. For 6az, the bolt or screw should not be manufactured to the minimum tensile strength as specified in ISO 898-1.
For 6AZ, to fit nuts with a higher mating property class (e.g. G8.8 bolts with G10 Nuts) or use style 2 nuts of the same mating property class, and 6AX it says that in some national standards nuts of two mating property classes higher are mandated.