How to specify fasteners correctly: size, thread, length, material, coating, standard, strength

An incomplete fastener specification is not a minor administrative shortcoming. In oil and gas, offshore, mining and energy infrastructure, every missing parameter is a decision left to whoever fills the order — and that is where misspecification begins. A correct technical specification covers seven parameters that together determine whether the fastener performs as required under actual load, environmental and compliance conditions.

The seven parameters — a complete specification

Example of a complete specification

A complete specification for a typical structural connection may look like this:

M16 x 100 mm hexagon bolt, ISO 898-1 strength class 10.9, hot-dip galvanized to ISO 10684, supplied with a 3.1 certificate to EN 10204.

Every parameter is stated explicitly: diameter (M16), length (100 mm), thread designation (hexagon bolt — ISO 4014 is assumed), standard (ISO 898-1), strength class (10.9), coating (HDG to ISO 10684), and traceability (3.1 certificate). This specification is unambiguous and cannot be misinterpreted by the supplier.

What does size mean in a specification?

Size refers to the nominal diameter, but diameter alone is not a complete specification. Size must be stated together with the thread designation: M12 x 1.75 (metric), 1/2-13 UNC or 1/2-20 UNF (imperial). Diameter without a thread designation is a procurement risk.

Size also determines load capacity. The nominal diameter governs the stress area that carries the tensile load. A fastener that is one diameter too small in a structural connection carries significantly less load than specified.

Thread specification — pitch or TPI

The thread specification defines how the fastener engages with its counterpart and how much preload it can generate. For metric: M12 x 1.75 (coarse) or M12 x 1.25 (fine). For imperial: 1/2-13 UNC (coarse) or 1/2-20 UNF (fine). Coarse thread suits general structural applications; fine thread gives higher clamping force and is the correct choice for connections exposed to vibration.

Length and thread engagement depth

The length of the fastener determines the thread engagement depth and the grip length. Rules of thumb for minimum thread engagement:

A fastener that is too short gives insufficient engagement. One that is too long places the threads in the shear plane or bottoms out in blind holes. Both are specification errors with direct consequences for joint integrity.

Material selection — from carbon steel to Super Duplex

Carbon steel to ISO 898-1 covers most structural connections in industry and construction. Special materials are used where the environment or the load requires it:

Material selection must match the operating environment, the load requirement and the applicable standard. Specifying carbon steel in a chloride environment, or A2 stainless in a high-load structural connection, are specification errors with predictable consequences.

Surface treatment and coefficient of friction

Surface treatment protects against corrosion and affects the friction at the thread surface — which directly affects the relationship between applied torque and achieved preload. The main options:

Every technical specification must state the surface treatment, the applicable standard and whether the torque values are adjusted for the coefficient of friction of the coating. Hot-dip galvanized fasteners require oversized tapped holes if ISOFIT is not specified. Hot-dip galvanizing on high-strength bolts can lead to embrittlement.

Strength class and torque calculation

For metric bolts, ISO 898-1 defines the strength classes. For nuts, ISO 898-2 applies:

Indicative torque values for dry, uncoated connections (μ ≈ 0.14), with 75% of the yield strength as target preload:

The values are indicative and assume standard friction. The actual torque must always be calculated for the specific lubricant and surface treatment. For applications where preload is critical, strain measurement is recommended over torque control.

Checklist: seven parameters on every order line

• Nominal diameter (always with the thread designation)
• Thread designation with pitch or TPI
• Length including verified grip length and engagement depth
• Material class (carbon steel, A2/A4, L7, Super Duplex, etc.)
• Surface treatment and standard (HDG ISO 10684, EZN ISO 4042, Dacromet, etc.)
• Applicable standard reference (ISO, DIN, ASTM, ASME)
• Strength class according to the applicable standard

Frequently asked questions

What is the difference between an 8.8 and a 10.9 bolt?

Strength class 8.8 has a minimum tensile strength of 800 MPa and a yield strength of 640 MPa. Class 10.9 has 1000 MPa tensile strength and 900 MPa yield strength — giving approximately 25% higher load capacity. 10.9 is harder and requires careful handling to avoid hydrogen embrittlement during hot-dip galvanizing.

When must I use A4 instead of A2 stainless?

A4 (AISI 316) contains molybdenum, which gives significantly better resistance to chloride corrosion than A2 (AISI 304). For seawater-exposed, saline or industrial wastewater environments, A4 is required. A2 works in dry indoor environments or areas with a low presence of chlorides.

Which certificate do I need from the supplier?

For batch-level traceability, a 3.1 certificate to EN 10204 is used. For independently tested certification, 3.2 is used. General 2.1 or 2.2 declarations are not sufficient for safety-critical or offshore applications.

Can I mix strength classes in the same connection?

No. The bolt and nut must have compatible strength classes according to ISO 898-1/898-2. A 10.9 bolt with a class 8 nut will cause the nut to fail before the bolt reaches the specified preload. The rules are clear: the class of the nut must match or exceed that of the bolt.

Conclusion

A complete technical specification covers seven parameters: nominal diameter, thread designation with pitch, length with grip length, material class, coating system, applicable standard reference and strength class. Omitting any one of them transfers an engineering decision to the supply chain, where it is resolved by availability rather than by engineering requirements.

INDUSTRISALG is ISO 9001:2015 certified, registered in Magnet JQS and StartBANK (ID 131384), and supplies one of Norway's broadest inventories — including ISO 898, ISO 3506, NORSOK L7, Super Duplex and imperial UNC/UNF. Send us the specification and we will confirm that all seven parameters are covered and propose improvements where something is missing.

Related reading

• Metric vs Imperial: UNC/UNF and where it is still used in industry
• Thread types 101: machine threads vs thread-forming vs thread-cutting vs wood screws

Get in touch

Send us the drawing, the order line or the specification — we will confirm that all seven parameters are covered, propose improvements where something is missing, and confirm same-day delivery time.