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Frequently Asked Questions About BS 3692

General

1. What is BS 3692?

BS 3692 is the British Standard for precision ISO metric hexagon bolts, screws, and nuts. The current edition is BS 3692:2014.

BS 3692:2014, published by the British Standards Institution (BSI), specifies the general dimensions, tolerances, mechanical properties, and marking requirements for precision hexagon bolts, screws, and nuts with ISO metric threads. It covers nominal thread diameters from M1.6 to M68 inclusive, encompassing both coarse pitch and fine pitch threads. The standard applies to carbon steel and alloy steel fasteners tested at ambient temperature, but its dimensional requirements also apply to non-ferrous and stainless steel products. It is the standard most commonly referenced on British engineering drawings when precision metric hexagon fasteners are required. The standard is available to purchase from the BSI Knowledge Store.

2. When was BS 3692 first published, and has it changed over the years?

BS 3692 was first published in December 1963 and has been through four editions, the current one being BS 3692:2014.

The publication history is as follows. The first edition appeared in December 1963. A second edition was issued in August 1967, incorporating a technical amendment. The third edition arrived in February 2001 (BS 3692:2001), which embodied the text of two earlier amendments (AMD 1720 and AMD 8227) and a corrigendum (13183). The fourth and currently active edition is BS 3692:2014, which came into effect on 31 October 2014. Its most significant change was the introduction of the vertical bar marking notation ( , , ) for nuts, to distinguish BS 3692 nuts from those made to the more demanding BS EN ISO 898-2. All previous editions are withdrawn. Source: BS 3692:2014 foreword, BSI, October 2014.

3. Is BS 3692:2001 still a valid standard?

No. BS 3692:2001 was withdrawn on 31 October 2014 and is superseded by BS 3692:2014.

If you hold a drawing or contract that references BS 3692:2001, it is generally acceptable to supply fasteners to the current BS 3692:2014 unless the earlier edition contained specific requirements not carried forward. The dimensional profiles for precision hexagon fasteners have remained broadly consistent across editions. Where heritage or legacy maintenance work demands strict compliance with an earlier edition, Trojan Special Fasteners can manufacture to the dimensions of that edition from drawing or specification. Source: BSI Standards page for BS 3692 at landingpage.bsigroup.com.

4. What does "precision" mean in the context of BS 3692?

"Precision" refers to the tight dimensional tolerances to which the fasteners are made — product grade A or B under BS EN ISO 4759-1 — as distinct from the wider-tolerance "black" or commercial-grade fasteners covered by BS 4190.

Under BS EN ISO 4759-1 (Tolerances for Fasteners), three product grades exist. Product Grade A is the most precise, typically applied to fasteners up to M24 and lengths up to 10d or 150 mm. Product Grade B is intermediate, applied to larger sizes. Product Grade C (sometimes called "commercial" or "black") carries the widest tolerances and is the subject of BS 4190. BS 3692 is a precision specification — tolerances on the width across flats, nut thickness, head height, eccentricity, and squareness are significantly tighter than their commercial-grade equivalents. This matters in applications where close fit, accurate load distribution, or interchangeability is important. Source: BS EN ISO 4759-1:2001, Tolerances for Fasteners — Part 1; BS 3692:2014 Clauses 4 and 11.

5. What is the difference between BS 3692 and BS 4190?

BS 3692 covers precision metric hexagon fasteners (tight tolerances); BS 4190 covers commercial-grade (black) metric hexagon fasteners (wider tolerances).

BS 4190:2001 (ISO Metric Black Hexagon Bolts, Screws and Nuts) covers fasteners from M5 to M68 made to wider tolerances than BS 3692. The term "black" refers not necessarily to the colour but to the manufacturing method — black fasteners are typically hot or cold forged and are not finish-machined to the same precision. BS 3692 fasteners are precision products and are held to much tighter tolerances on all key dimensions. In practice, BS 4190 fasteners are more common in general construction and structural applications where precise fit is less critical, whereas BS 3692 fasteners appear in precision engineering, machinery, and applications where dimensional accuracy matters. Source: BS 4190:2001 Clause 1 (Scope); BS 3692:2014 Clause 1.

Scope and Applicability

6. What thread diameters does BS 3692 cover?

BS 3692:2014 covers metric hexagon fasteners from M1.6 to M68 inclusive.

Both coarse pitch (preferred) and fine pitch threads are covered. Sizes shown in brackets in the standard's dimension tables are non-preferred and should be avoided in new designs where possible. For sizes beyond M68, Annex E of the standard provides guidance on deriving tolerances. Trojan Special Fasteners manufactures nuts to BS 3692 dimensions across our standard range of M3 to M52. Source: BS 3692:2014 Clause 1 (Scope).

7. Does BS 3692 cover fine pitch threads?

Yes. BS 3692:2014 covers both coarse pitch and fine pitch metric threads.

All threads must conform to BS 3643-1 (ISO metric screw threads: principles and basic data) and BS 3643-2 (selected limits of size), to tolerance class 6H for nuts and 6g for bolts and screws. Trojan Special Fasteners can supply nuts in the full range of coarse, fine, extra-fine, and hard-to-find pitches across our M3–M52 size range, including non-preferred pitches and left-hand threads. Source: BS 3692:2014 Clauses 3 and 7.

8. Does BS 3692 cover non-ferrous or stainless steel fasteners?

The dimensional requirements of BS 3692 apply to non-ferrous and stainless steel fasteners. However, the mechanical property requirements (proof loads, hardness grades) apply only to carbon steel and alloy steel.

For stainless steel fasteners, mechanical properties are governed by BS EN ISO 3506 (Property classes A2-70, A4-80, etc.). For non-ferrous metals such as brass, aluminium, or bronze, there is no standard strength grading equivalent to the steel system — dimensions are taken from BS 3692 but mechanical properties are specified by agreement or to a separate material standard. Trojan Special Fasteners manufactures nuts in stainless steel A2 and A4, brass CZ121/CW614N, aluminium 2011T3, and other non-ferrous alloys, all to BS 3692 dimensional profiles where required. Source: BS 3692:2014 Clause 1, Note on dimensional requirements for non-ferrous and stainless steel.

9. What temperature range applies to BS 3692 fasteners?

BS 3692 fasteners are rated for use from -50°C to +150°C without special precautions. Use up to +300°C may be possible with metallurgical advice.

The mechanical property testing specified in the standard is conducted at ambient temperature (10°C to 35°C). Use above +150°C and up to +300°C may be possible but requires the advice of an experienced fastener metallurgist. The standard is not applicable to fasteners used outside the range -50°C to +300°C. Additionally, free-cutting steels — commonly used in machined fasteners — should not be used above 250°C, as the sulphide inclusions that improve machinability can cause brittleness at elevated temperatures. For high-temperature applications, consult our technical team. Source: BS 3692:2014 Clause 1, Notes 1 and 2; Clause 16.2.

12. Are drilled bolts (for split pins) covered by BS 3692?

Yes. Clause 18 of BS 3692:2014 covers drilled bolts with split pin holes.

Drilled bolts are frequently used with slotted or castle nuts (BS EN ISO 7042 / DIN 935 type) to provide a positive mechanical locking arrangement using a split pin (cotter pin) in accordance with BS EN ISO 1234. The standard specifies slot positions and dimensions for slotted nuts at specific sizes, and gives the requirements for split pin holes in bolts. Trojan Special Fasteners can manufacture slotted and castle nuts from M4 to M52 to be used with drilled bolts. See our Slotted and Castle Nuts page for further details.

Strength Grades and Mechanical Properties

13. What strength grades are available for nuts under BS 3692?

Grades 4, 5, 6, 8, 10, and 12 are available for steel nuts under BS 3692:2014.

The grade number represents approximately one-hundredth of the proof load stress in N/mm². Grade 4 carries a proof load stress of 400 N/mm²; Grade 12 carries 1,200 N/mm². Grades 4, 5, and 6 are for general and medium-duty applications. Grade 8 is the standard high-strength grade, paired with Grade 8.8 bolts. Grades 10 and 12 are for high-tensile applications with 10.9 and 12.9 bolt grades respectively. A nut of a higher strength grade may always be substituted for a nut of a lower grade. Source: BS 3692:2014 Clause 15, Table 10.

14. What do the bolt and screw grade numbers mean — for example, 8.8?

The two numbers separated by a dot describe tensile strength and yield ratio. For an 8.8 bolt, tensile strength is 800 MPa and yield stress is 640 MPa.

The first figure is one-hundredth of the nominal tensile strength in MPa. The second figure is ten times the ratio of nominal yield strength to nominal tensile strength. So for a Grade 8.8 bolt: tensile strength = 8 × 100 = 800 MPa; yield ratio = 8 ÷ 10 = 0.8; yield stress = 0.8 × 800 = 640 MPa. For Grade 10.9: tensile strength = 1,000 MPa, yield stress = 900 MPa. For Grade 12.9: tensile strength = 1,200 MPa, yield stress = 1,080 MPa. This system is defined in BS EN ISO 898-1, which BS 3692 references for bolt and screw mechanical properties. Source: BS 3692:2014 Annex A; BS EN ISO 898-1:2013.
15. Which nut grade should I use with which bolt grade?

Match the nut grade to the bolt grade: use a Grade 8 nut with an 8.8 bolt, a Grade 10 nut with a 10.9 bolt, and a Grade 12 nut with a 12.9 bolt.

The matching is not arbitrary — it is designed so that the nut has sufficient strength to prevent thread stripping when the bolt is loaded to its yield point. Using an understrength nut risks thread stripping rather than bolt fracture under overload, which is a more dangerous failure mode because it may be silent and progressive rather than sudden and visible. The pairing table in BS 3692:2014 (Table 4) gives the full recommendations. Note: nuts of a higher strength grade may always be substituted for those of a lower grade. Source: BS 3692:2014 Table 4 (recommended nut grade for each bolt/screw property class).

1. What is a proof load test, and how is it carried out for nuts?

A proof load test checks that a nut can withstand a specified axial tensile load without stripping its thread or fracturing.

The test method is defined in Annex C (normative) of BS 3692:2014. The nut is assembled onto a hardened and tempered mandrel (hardness not less than Rockwell C45, thread tolerance class 5h) and a specified axial load is applied. The nut must withstand the load without failure by stripping or rupture, and must be removable by hand after the load is released. If the mandrel threads are damaged during the test, the test is invalid and must be repeated. Hardness testing (Brinell, Rockwell, or Vickers) on the top, bottom, or side face of the nut may be used as an alternative acceptance method for nuts with a proof load exceeding 500 kN. Source: BS 3692:2014 Annex C (normative).

1. What maximum hardness limits apply to BS 3692 nuts?

The maximum hardness limits depend on grade. For Grades 4–8 the Brinell maximum is 302 HB; for Grade 10 it is 353 HB; for Grade 12 it is 375 HB.

Maximum hardness is specified because excessively hard nuts can be brittle and may be susceptible to hydrogen-assisted stress corrosion cracking, particularly in electroplated assemblies. The equivalent Rockwell and Vickers limits are: Grades 4–8: HRC 30 max / HV 310 max; Grade 10: HRC 36 max / HV 370 max; Grade 12: HRC 39 max / HV 395 max. There is no minimum hardness specified in the standard itself — compliance is verified by proof load testing. Source: BS 3692:2014 Table 10.

The Critical Difference: BS 3692 Nuts vs. BS EN ISO 898-2 Nuts

1. What is the difference between a BS 3692 nut and a BS EN ISO 898-2 nut of the same grade number?

They look identical and share the same dimensions, but BS EN ISO 898-2 nuts carry a higher proof load. The marking differs: BS 3692 nuts show , ISO 898-2 nuts show 8.

This is arguably the most important distinction on this page. BS EN ISO 898-2 is the internationally harmonised standard for metric nut mechanical properties. When it was revised, higher proof load values were introduced specifically to ensure that under overload, the bolt fractures rather than the nut thread strips. Thread stripping is a less ductile, harder-to-detect failure mode than bolt fracture. BS 3692:2014 retains the older, lower proof load values inherited from the 1960s versions of the standard, and is now maintained primarily for backwards compatibility with existing British drawings. The BSI foreword to BS 3692:2014 states explicitly: "It is essential that new designs of nuts for use with BS 3692 bolts and studs conforming to this standard conform to BS EN ISO 898-2." Source: BS 3692:2014 Foreword; BS EN ISO 898-2.

1. Why is bolt fracture preferred over nut thread stripping as a failure mode?

Bolt fracture is visible, sudden, and easy to detect. Thread stripping is silent, progressive, and may leave a joint that appears intact but is carrying no load.

Research into fastener failure modes confirms that thread stripping under tensile loading is a less ductile failure mode than bolt fracture. When threads strip progressively, an assembly can lose its clamping force gradually without any outward sign of failure — the nut may still be present on the bolt, giving a false impression of integrity. Bolt fracture, by contrast, is immediately obvious and cannot be missed. The design philosophy behind BS EN ISO 898-2 is therefore to ensure that in any overload scenario, the bolt breaks before the nut threads strip. This is achieved by specifying higher proof loads in the nut than are required by BS 3692. Source: BS 3692:2014 Foreword; Grimsmo et al., "Failure modes of bolt and nut assemblies under tensile loading," Journal of Constructional Steel Research, 2016, doi.org.

1. What does the marking on a nut mean?

The vertical bars () identify a nut made to BS 3692 rather than BS EN ISO 898-2. It distinguishes the lower proof load British standard from the higher proof load international standard.

When BS 3692:2014 was published, BSI added vertical bars around nut grade numbers to distinguish BS 3692 nuts from ISO 898-2 nuts. A nut marked * is a Grade 8 nut to BS 3692 with a proof load stress of 800 N/mm². A nut marked simply 8* is a Grade 8 nut to BS EN ISO 898-2 with a higher proof load stress. The marking must appear on all nuts of M6 and above manufactured to Grade 8 or higher; it is not mandatory on smaller sizes or lower grades. Source: BS 3692:2014 Clause 20.2, Table 13.

1. Should I specify BS 3692 nuts or BS EN ISO 898-2 nuts for new designs?

For new designs, always specify BS EN ISO 898-2 nuts. BS 3692 nut grades are retained only for backwards compatibility with older drawings.

The BSI is unambiguous on this point. The foreword of BS 3692:2014 states that for new designs using BS 3692 bolts, nuts should conform to BS EN ISO 898-2. The reason is the higher proof load requirement in the ISO standard, which ensures bolt fracture rather than thread stripping under overload. The only reason to specify BS 3692 nuts today is if an existing drawing calls for them by edition and grade, and you need to match that specification precisely. See our ISO 898-2 page for full details of the ISO standard.

Chemical Composition and Material

1. What is the chemical composition of steel used for BS 3692 nuts?

The composition depends on grade. Higher grades require tighter control of carbon, manganese, phosphorus, and sulphur content.

The limits are given in BS 3692:2014 Table 9. For Grades 4, 5, and 6: carbon max 0.50%, no minimum manganese specified, phosphorus max 0.110%, sulphur max 0.150%. For Grade 8: carbon max 0.58%, manganese min 0.30%, phosphorus max 0.060%, sulphur max 0.150%. For Grades 10 and 12: carbon max 0.58%, manganese min 0.45%, phosphorus max 0.048%, sulphur max 0.058%. Alloying elements such as chromium, molybdenum, or boron may be added at the manufacturer's discretion to achieve the required mechanical properties — particularly for Grades 10 and 12, which require heat treatment. Source: BS 3692:2014 Clause 16.2, Table 9.

1. Can free-cutting steel be used for BS 3692 nuts?

Yes, but only by agreement between purchaser and supplier, and not above 250°C. Maximum limits of P 0.12%, S 0.34%, and Pb 0.35% apply.

Free-cutting steels contain elevated levels of sulphur, phosphorus, or lead (or combinations thereof) to improve machinability — they produce shorter, more manageable swarf and allow faster cutting speeds. However, these additions reduce elevated-temperature ductility and can cause brittleness above 250°C. They may also affect weld quality if the fastener is to be welded. Where free-cutting steel is used, it must be declared and agreed with the customer. At Trojan Special Fasteners we use free-cutting bright bar grades as our standard material, which is appropriate for the temperatures encountered in most general engineering applications. If elevated temperature service is required, we can supply in alternative bar grades — please specify this when ordering. Source: BS 3692:2014 Clause 16.2.

1. Does the BS 3692 standard specify which steel grade to use?

No. The standard specifies chemical composition limits and required mechanical properties. The choice of specific steel is left to the manufacturer.

BS 3692:2014 Clause 16.2 states that the composition of steels is governed by Table 9, but the choice of steel grade meeting those limits is at the manufacturer's discretion. The overriding requirement is that the finished product must achieve the mechanical properties (proof load and hardness) specified for the relevant strength grade. For higher grades (10 and 12), heat treatment — typically quenching and tempering — is required. For lower grades (4, 5, 6), the inherent properties of the bar stock may be sufficient without heat treatment.

1. Do Grade 10 and Grade 12 nuts require heat treatment?

Yes. BS 3692:2014 requires nuts to be heat treated where necessary to achieve the specified mechanical properties — and at Grades 10 and 12, heat treatment is invariably necessary.

The standard states that nuts must be heat-treated where necessary to meet the proof load and hardness requirements of the relevant grade (Clause 16.3). In practice, Grade 10 and Grade 12 nuts are always quenched and tempered — the proof load stresses of 1,000 N/mm² and 1,200 N/mm² respectively cannot be achieved from carbon steel in the as-turned or as-forged condition. Grade 8 nuts may or may not require heat treatment depending on the steel specification used. Grades 4, 5, and 6 generally do not require heat treatment when produced from appropriate bar stock.

Dimensions and Tolerances

1. What thread tolerance class does BS 3692 specify for nuts?

Thread tolerance class 6H for nuts, and 6g for bolts and screws — the standard medium fit.

Thread tolerance class 6H/6g is the ISO and British standard medium class fit for general engineering. It provides a reasonable clearance for assembly while still maintaining good thread engagement and load transfer. Finer fit classes (5H/6h or 4H/5h) are available for high-precision applications but are not required by BS 3692. The thread form and pitch must conform to BS 3643-1 and BS 3643-2. If oversize tapping is required for hot-dip galvanising, class 6AZ applies (see the question on galvanising). Source: BS 3692:2014 Clause 7 and Table 2.

1. What dimensional tolerances govern the width across flats of nuts?

Tolerance fields h12, h13, or h14 from BS EN ISO 4759-1 apply, depending on the size of the nut.

The minimum width across corners (emin) is calculated as 1.13 × smin for all products, where s is the nominal width across flats. This 1.13 factor is geometrically derived from the inscribed circle of a regular hexagon (cos 30° = 0.866, so emin = smin ÷ cos 30° ≈ 1.155 × smin; the 1.13 factor used in BS EN ISO 4759-1 is a rounded practical value). The eccentricity of the hexagon flats relative to the thread diameter must not exceed the values given in the standard's dimensional tables. Source: BS 3692:2014 Annex E; BS EN ISO 4759-1:2001._

1. Is there a minimum nut height for mechanical properties to apply?

Yes. Nuts with an effective height of less than 0.6D are excluded from the mechanical property requirements, as are nuts with a width across flats of less than 1.45D.

The exclusions reflect the practical reality that very thin nuts and very narrow nuts cannot develop the full thread engagement area needed to generate the specified proof load without stripping. The 0.6D height threshold is a widely used rule of thumb in fastener engineering — it ensures a minimum number of thread turns in engagement. If your application requires a thin nut (jam nut, half nut, or low-profile nut), note that the proof load requirements do not apply; the nut is designed for locking or adjustment functions, not for full axial loading. Source: BS 3692:2014 Clause 1, Note 4.

1. What chamfering requirements apply to BS 3692 nuts?

Nuts must have a 30° chamfer on both faces and a 120° countersink at both ends of the thread.

The chamfer angle is approximately 30° on both bearing faces. The thread countersink must be at an included angle of 120° ± 10°. The countersink diameter (da) is calculated as follows: for d ≤ 5 mm, da max = 1.15d; for 5 mm < d ≤ 8 mm, da max = d + 0.75 mm; for d > 8 mm, da max = 1.08d. These requirements ensure that thread engagement is clean and that the nut seats squarely on the bearing surface without rocking. Source: BS 3692:2014 Clause 12.

1. What squareness requirements apply to BS 3692 nuts?

The squareness of the thread axis to the bearing face must be within limits specified in the standard. A gauge method for checking squareness is given in Annex B.

The squareness gauge described in Annex B (informative) consists of a tapered mandrel engaging the nut thread. The small end of the mandrel thread has an effective diameter approximately 0.02–0.12 mm below nominal; the large end is approximately 0.035–0.12 mm above nominal (values vary by size). The nut is placed over the mandrel on a flat surface plate, and a feeler gauge equal to the maximum permitted squareness tolerance is inserted beneath the nut face. The standard specifies maximum angular deviation values for nut faces. Source: BS 3692:2014 Annex B.

Marking, Identification, and Inspection

1. Is marking mandatory on all BS 3692 nuts?

No. Marking is mandatory only for steel nuts of M6 diameter and above, manufactured to Grade 8 or higher.

The marking and identification requirements of Clause 20 of BS 3692:2014 apply only when both conditions are met: the nominal diameter is M6 or larger, and the strength grade is 8 or above. Nuts of M5 and below, or Grades 4, 5, and 6 at any size, are not required to be marked. When marking is required, the strength grade symbol (with vertical bars, e.g., **) and the manufacturer's identification mark must both be present. Marking may be applied to the top face, bottom face, or side of the nut. Source: BS 3692:2014 Clause 20.2, Table 13.

1. Where should the grade marking appear on a BS 3692 nut?

The marking may be applied to the top face, bottom face, or side of the nut.

Clause 20.2 of BS 3692:2014 requires the strength grade designation and manufacturer's identification to be marked on all nuts of M6 and above to Grade 8 or higher, but does not prescribe a specific face. In practice, markings on the top or bearing face are common for larger nuts, while side-stamped markings are used where face space is limited. For nuts produced by Trojan Special Fasteners, the marking method and location can be discussed at the time of ordering.

1. How are BS 3692 bolts and screws marked?

Bolts and screws are marked with the property class designation and the manufacturer's identification mark. Marking follows BS EN ISO 898-1.

The marking requirements for bolts and screws follow BS EN ISO 898-1, since BS 3692 defers to that standard for bolt and screw mechanical properties. The property class (e.g., 8.8, 10.9) and the manufacturer's identification mark must appear on the head of all bolts and screws of M6 and above manufactured to Grade 8.8 or higher. For smaller sizes or lower grades, marking is at the manufacturer's discretion. Source: BS 3692:2014 Clause 20.1; BS EN ISO 898-1:2013 Clause 10.

1. What are the inspection and testing requirements under BS 3692?

Tests for mechanical properties follow BS EN ISO 898-1 for bolts and screws, and Annex C (normative) for nuts. The purchaser may request certification by agreement.

The standard states that the manufacturer shall take steps to ensure conformance, but if the purchaser requires certification or demonstration of conformance beyond the manufacturer's assurance, the details and cost of additional inspection must be agreed between the parties. For nuts, the normative testing method is the proof load test in Annex C, backed up by hardness testing. For bolts and screws, testing follows BS EN ISO 898-1. Trojan Special Fasteners can provide material certificates and test documentation by arrangement. Source: BS 3692:2014 Clause 21.

Finishes and Coatings

1. What surface finish options does BS 3692:2014 define?

The standard recognises as-machined (natural), heat-treated (black), bright-finished, electroplated, and hot-dip galvanised finishes.

Clause 19 of BS 3692:2014 covers finishes as follows. As-machined: no coating or treatment applied — this is the standard delivery condition for bar-turned fasteners from Trojan Special Fasteners. Heat-treated (black): fasteners heat-treated after manufacture are typically dull black; larger sizes may be finish-machined after heat treatment. Bright: fasteners finished bright on all surfaces, or with a bright-drawn hexagon; must be stated in the enquiry or order. Electroplated: must conform to BS EN ISO 4042. Hot-dip galvanised: must conform to BS 7371-6; requires oversize internal thread tapping (see below). Source: BS 3692:2014 Clause 19.

1. Can BS 3692 nuts be hot-dip galvanised?

Yes, but the internal thread must be tapped oversize before galvanising to allow for the zinc coating build-up on the mating bolt thread.

Hot-dip galvanising deposits a significant thickness of zinc on all surfaces, including threads. For a nut, this means the threaded bore becomes narrower after coating if not tapped oversize beforehand. BS EN ISO 4042 and BS 7371-6 cover this requirement. Nuts for galvanising must be tapped to tolerance class 6AZ in accordance with BS ISO 965-5, which specifies limits of size for internal threads intended to mate with hot-dip galvanised external threads. Nuts tapped oversize in this way must be marked with the letter Z immediately after the property class mark (e.g., Z). The bolt must also have its thread produced to tolerance position g or h before galvanising. Source: BS 3692:2014 Clause 19; BS ISO 965-5; BS 7371-6.

1. What is the risk of hydrogen embrittlement with electroplated BS 3692 fasteners?

High-grade fasteners (Grade 10 and above, or above HRC 32 / HV 320) are at risk of hydrogen embrittlement from electrolytic pickling and plating processes. Grade 12.9 bolts should not be electroplated.

Hydrogen embrittlement is a failure mechanism in which atomic hydrogen absorbed during electroplating or acid cleaning diffuses into the steel lattice and reduces ductility, potentially causing delayed brittle fracture under sustained load. The risk increases with hardness — fasteners above HV 320 (approximately HRC 32) are considered at risk. Baking after plating (typically 200°C for 8–24 hours) can drive out much of the absorbed hydrogen, but cannot guarantee complete elimination. For Grade 12.9 bolts and high-hardness nuts, mechanical cleaning (shot blasting) followed by a non-electrolytic coating such as zinc flake (Geomet, Dacromet) is strongly preferred. Our Hydrogen Embrittlement page covers this topic in detail.

Manufacturing Methods

1. How are BS 3692 nuts manufactured?

BS 3692:2014 permits cold forging, hot forging, or turning from bar. Trojan Special Fasteners manufactures exclusively by CNC bar turning.

The standard leaves the choice of manufacturing method to the manufacturer (Clause 16.1). Cold forging is the dominant process for high-volume production of standard fasteners and produces good grain flow and surface finish. Hot forging is used for larger sizes. Bar turning (CNC machining from solid bar) is used for precision, special dimensions, non-standard pitches, and lower volumes where tooling investment for forging is not justified. Bar turning also allows far greater flexibility in adding features — collars, castellations, cross-drilling, prevailing torque mechanisms — that cannot be achieved by forging alone. At Trojan Special Fasteners, CNC bar turning is our sole manufacturing process, giving precision and flexibility across the full M3–M52 range.

1. What manufacturing marks or codes might appear on high-tensile BS 3692 bolts?

High-tensile 8.8 bolts are sometimes marked "CW" (cold worked) on the head. This indicates a cold-working manufacturing process used to achieve the required mechanical properties.

CW marking on Grade 8.8 bolts indicates that the bolt has been cold worked (typically by thread rolling after head formation) rather than conventionally heat-treated. Cold-worked 8.8 bolts are considered equivalent to heat-treated 8.8 bolts in terms of mechanical properties, provided they meet the BS EN ISO 898-1 requirements. Thread rolling after head formation is a common and effective production method that also has the benefit of improving fatigue resistance due to compressive residual stresses in the thread roots. Bolts of Grades 10.9 and 12.9 must be heat-treated (quenched and tempered) — cold working alone is insufficient to achieve those property levels.

Related Standards and Cross-References

1. What is the BS EN ISO 898-1 standard, and how does it relate to BS 3692?

BS EN ISO 898-1 governs the mechanical properties of metric bolts, screws, and studs. BS 3692 defers to it for bolt and screw properties, using its own (lower) proof load table for nuts.

BS EN ISO 898-1:2013 (Mechanical properties of fasteners made of carbon steel and alloy steel — Part 1: Bolts, screws and studs with specified property classes) is the internationally harmonised standard for bolt and screw mechanical properties. BS 3692:2014 requires bolts and screws to meet the requirements of BS EN ISO 898-1, and refers to it for heat treatment requirements, decarburisation limits, and marking. The standard references the 2013 edition of BS EN ISO 898-1. Source: BS 3692:2014 Clause 13 and Clause 14; BS EN ISO 898-1:2013.

1. What is decarburisation, and why does BS 3692 control it?

Decarburisation is the loss of carbon from the surface of steel during heat treatment, which weakens the thread flanks. BS 3692 limits it for Grade 8.8, 10.9, and 12.9 bolts.

When steel is heated in the presence of oxygen during heat treatment, carbon can oxidise and be lost from the surface layer. The resulting surface is softer than the bulk material, reducing fatigue strength and wear resistance in the thread flanks. BS 3692:2014 Clause 13.4 requires that for Grades 8.8, 10.9, and 12.9, the depth of the non-decarburised zone at the thread flanks shall be not less than two-thirds of the depth of the basic thread profile. This ensures that the load-bearing surfaces of the thread retain their full hardness and strength. The measurement method follows BS EN ISO 898-1. Source: BS 3692:2014 Clause 13.4.

1. What is BS 3643, and why is it referenced in BS 3692?

BS 3643 defines the ISO metric screw thread form, dimensions, and tolerances. BS 3692 requires all threads to conform to it.

BS 3643 exists in two parts: Part 1 covers principles and basic data (thread form, pitch series, diameter/pitch combinations), and Part 2 specifies selected limits of size including tolerance classes. BS 3692 requires that all threads in bolts, screws, and nuts conform to BS 3643, specifically to tolerance class 6H (nuts) or 6g (bolts/screws). BS 3643 is itself aligned with the ISO 68-1 and ISO 965 series of international thread standards. Source: BS 3692:2014 Clause 2 (Normative References).

1. What is BS EN ISO 4042, and when does it apply?

BS EN ISO 4042 governs electroplated coatings on fasteners — it applies whenever a BS 3692 fastener is electroplated.

BS EN ISO 4042 (Fasteners — Electroplated Coatings) specifies requirements for electroplated coatings (zinc, nickel, chromium, tin, cadmium, etc.) on threaded fasteners. It covers coating thickness, adhesion, surface preparation, and the treatment of threads after plating. BS 3692 references it in Clause 19 as the standard to which electroplated coatings on BS 3692 fasteners must conform. For guidance on specific coating options available from Trojan Special Fasteners, see our Finishes page. Source: BS 3692:2014 Clause 19; BS EN ISO 4042.

1. Is there a direct ISO equivalent to the whole of BS 3692?

No single ISO standard replaces BS 3692 entirely. The equivalent functions are split across ISO 4014/4017 (bolt dimensions), ISO 4032/4033 (nut dimensions), and BS EN ISO 898-1/898-2 (mechanical properties).

BS 3692 is a comprehensive British standard that combines dimensional requirements, mechanical property requirements, and marking requirements into one document. Under the ISO system, these are handled by separate standards. For nuts: ISO 4032 covers style 1 hex nuts (product grades A and B), ISO 4033 covers style 2 high nuts, ISO 4034 covers product grade C nuts. For mechanical properties: BS EN ISO 898-2 covers nuts. For bolts: ISO 4014 (partial thread) and ISO 4017 (full thread) cover dimensions; BS EN ISO 898-1 covers mechanical properties. BS 3692 is maintained by BSI as a convenient single-document reference that harmonises with these ISO standards for dimensions but retains its own (legacy) mechanical property tables for nuts.

1. How does DIN 934 compare to BS 3692 hex nuts?

DIN 934 is the German standard for metric hex nuts and is dimensionally very similar to BS 3692 / ISO 4032 for most common sizes, but the two are not identical in all specifications and should not be assumed interchangeable without checking.

DIN 934 (Hexagon nuts — metric) was the dominant European standard for hexagon nuts prior to harmonisation with ISO standards. Many German and European engineering drawings still reference DIN 934. The dimensions are closely aligned with ISO 4032 for preferred sizes, but there are minor differences in widths across flats for some sizes (notably M10, where DIN 934 historically used 17 mm across flats versus the ISO 4032 preference of 16 mm). In European practice, DIN 934 is now largely superseded by the EN ISO 4032 series following CEN harmonisation, but it continues to be widely referenced. Trojan Special Fasteners can manufacture to DIN 934 dimensions as well as BS 3692 and ISO 4032 — please state the standard when ordering if dimensional compliance is critical.

Practical Application and Procurement

1. Can I order BS 3692 nuts in non-standard sizes or pitches from Trojan Special Fasteners?

Yes. We manufacture to drawing, sample, or specification and can produce any size or pitch within our M3–M52 range, including non-preferred and special pitches, left-hand threads, and oversize pitches.

Standard preferred sizes are those listed without brackets in the BS 3692 dimension tables. Non-preferred sizes (shown in brackets) and special or non-standard sizes are not available off the shelf from most suppliers — this is precisely where Trojan Special Fasteners adds value. Our CNC bar turning process allows us to produce virtually any thread form, pitch, or external dimension to your specification, with lead times measured in days rather than weeks for most sizes. Contact us with your drawing or specification.

1. Can BS 3692 nuts be supplied in stainless steel?

Yes. Dimensional requirements from BS 3692 apply to stainless steel nuts. Mechanical properties for stainless fasteners are governed by BS EN ISO 3506.

BS EN ISO 3506 defines property classes for stainless steel fasteners: the most common are A2-70 (18/8 austenitic, 700 MPa tensile strength) and A4-80 (316 marine-grade, 800 MPa tensile strength). These are not equivalent to carbon steel grades 8 or 10 in terms of proof load, though the tensile strengths overlap. We supply nuts in A2 (304) and A4 (316) stainless steel to BS 3692 dimensional profiles. For particularly corrosive environments — marine, chemical processing, or food industry applications — A4 (316) with its molybdenum addition provides significantly better pitting and crevice corrosion resistance.

1. Can I get a material certificate with BS 3692 nuts from Trojan Special Fasteners?

Yes. Material test certificates and certificates of conformance are available by arrangement.

We can provide a Certificate of Conformance (CoC) confirming that fasteners have been manufactured to the specified standard, grade, material, and finish. Where required, a Material Test Report (MTR) or Chemical Analysis Certificate from the bar stock supplier can also be provided. For safety-critical applications in aerospace, rail, nuclear, or pressure equipment, please discuss your documentation requirements with us at the time of ordering. Contact us to discuss your quality documentation requirements.

1. What should a full ordering designation for a BS 3692 nut look like?

A complete designation includes the product type, the standard, the nominal diameter and pitch, the property class, and any coating requirement.

Following BS 3692:2014 Clause 3, a full designation for an enquiry or order should include: the product form (e.g., hexagon nut), the standard (BS 3692:2014), the nominal thread diameter and pitch (e.g., M16 × 2), the property class (e.g., 8), and any finish or coating requirement (e.g., zinc plated to BS EN ISO 4042, or hot-dip galvanised to BS 7371-6). An example full designation would be: Hexagon Nut BS 3692:2014 — M16 × 2 — Grade — Bright Zinc Plated CR3. The more information you provide, the more accurately we can quote and manufacture to your requirements.

1. Does Trojan Special Fasteners hold stock of BS 3692 nuts?

We are a manufacturer, not a stockholder. We manufacture to order, which means we can produce to your exact specification rather than only in standard stocked sizes.

Our manufacturing model means that lead times for most sizes within our M3–M52 range are typically a few working days for straightforward specifications. Because we machine from bar rather than buying pre-made fasteners, we are not limited to stocked sizes, grades, or materials. If you need a size, pitch, or material combination that is difficult to source elsewhere, we are likely to be able to help. Request a quote or call us on +44 (0)121 789 8586 to discuss your requirement.