Fasteners from China: A Buyer's Guide to Materials, Property Classes, Coatings and Verification

Last updated: 16 September 2026

Quick answer. Fasteners are specified by material, property class, thread system, coating and dimensional standard — not by appearance. For metric steel bolts and screws, ISO 898-1 defines the mechanical properties and property classes; for inch-series alloy socket head cap screws, ASTM A574 applies. Always state the standard edition on the drawing.

Editorial position: China Custom Parts is an independent sourcing knowledge resource. It is not a factory, broker or marketplace, and it publishes no prices, no MOQs, no lead times and no supplier lists.

Why fastener sourcing goes wrong

Of all the hardware categories, fasteners look the simplest to buy and are the easiest to get wrong. A box of bolts that photographs correctly can still be the wrong product, because almost everything that matters about a fastener is invisible: the steel grade behind the plating, the property class behind the head marking, the thread tolerance behind a smooth-running nut, and the coating thickness behind a shiny finish.

The failure mode is rarely fraud. It is usually specification drift — a drawing that said "M8 bolt, zinc plated" and a supplier who interpreted that reasonably but differently from what the application needed. The fix is not a better supplier; it is a better specification. This guide covers what to put on the drawing, and what to verify when the parts arrive.

Materials and property classes: how to specify

For metric steel fasteners, the property class is the specification. ISO 898-1:2013 sets out the mechanical and physical properties of bolts, screws and studs made of carbon steel and alloy steel when tested in an ambient temperature range of 10 °C to 35 °C. That ambient qualifier matters: the standard notes that fasteners meeting its requirements may not retain those properties at elevated or reduced temperatures.

Two practical consequences follow:

Property class is not a synonym for material. The class number encodes a set of mechanical requirements. Two suppliers can meet the same class with different steels, which is legitimate — but it is why "8.8" on a drawing is more useful than "high tensile".
Some geometries fall outside the tensile requirements. ISO 898-1 notes that certain bolts and screws may not fulfil its tensile or torsional requirements because head geometry reduces the shear area in the head relative to the stress area in the thread — low and countersunk heads are the named examples. If your design uses those head styles, do not assume the normal property class logic applies.
Set screws are a different standard. ISO 898-1 explicitly does not apply to set screws and similar threaded fasteners not under tensile stress; that is ISO 898-5.

For inch-series alloy steel socket head cap screws, ASTM A574 is the specification. It covers quenched and tempered alloy steel hexagon socket head cap screws from 0.060 through 4 in. in diameter where high strength is required, requires the steel to be made to fine grain practice and to conform to a specified chemical composition, and sets test requirements covering tensile or wedge strength, proof load, yield strength, elongation, and Rockwell and Vickers hardness. Note that A574 states its values in inch-pound units as standard. If your drawing is metric, A574 is the wrong reference.

For nuts, washers and non-tensile fasteners, specify the matching standard for that component rather than borrowing the bolt standard. A nut's property class is specified separately, and specifying "8.8 nut" on a drawing is a category error that invites whatever the supplier has in stock.

Thread systems: metric versus inch

Thread mismatch is a quiet, expensive failure. A metric M8 and an inch-series 5/16″ fastener are close enough to feel similar and wrong enough to damage threads under load.

For metric work, specify the thread the way ISO 898-1 does: triangular ISO metric screw thread in accordance with ISO 68-1, coarse pitch from M1.6 to M39 and fine pitch from M8×1 to M39×3, with diameter/pitch combinations in accordance with ISO 261 and ISO 262, and thread tolerances in accordance with ISO 965-1, ISO 965-2 and ISO 965-4. Putting those references on the drawing removes the guesswork about which pitch and tolerance class the supplier should work to.

For inch work, state the thread series (UNC, UNF or UNEF) and the class of fit. Do not assume a supplier will default to the series your assembly was designed around.

Coatings and corrosion protection

Coating choice is driven by the environment, the base material, and whether the fastener will be in contact with a dissimilar metal. The main families, and what each is actually for:

Electroplated zinc — the default for indoor and mildly corrosive service on steel fasteners. Thin, inexpensive to apply, and the usual starting point for general assembly work.
Zinc flake coatings — used where a higher corrosion load must be carried by a coating that does not introduce hydrogen-embrittlement risk during application; common on higher-strength fasteners for that reason.
Hot-dip galvanising — for outdoor and structural exposure, where a thicker sacrificial layer is wanted.
Passivation — not a coating but a chemical treatment applied to stainless steel to restore the oxide layer after machining; relevant to stainless fasteners rather than plated ones.
Phosphate and black oxide — mild protection plus a low-friction or low-reflectivity finish, often used where the fastener is oiled or where appearance matters.
Plain stainless — the corrosion resistance comes from the alloy, not a coating, and is sensitive to grade choice and to the chloride and temperature conditions of the application.

Two specification notes that prevent most coating disputes: state the coating standard and the required thickness class rather than writing "plated", and state whether the requirement applies to the whole fastener including the thread (it usually does, and it constrains the coating thickness that is acceptable).

⚠️ On coating numbers: this page deliberately does not reproduce the coating thickness tables or salt-spray hour figures that appear inside the relevant coating standards. Those values are standard-specific and edition-specific, and we did not retrieve the official standard pages to verify them for this edition. Ask your supplier to state the standard and edition their coating claims to, and verify that claim against the standard itself. The sources used on this page are listed at the end.

Anti-loosening: what actually matters

Preload, not friction, is what keeps a joint tight. In order of how often they solve the real problem:

Correct preload — the right property class for the joint, tightened to a defined torque or, better, to a measured preload. Most "loosening" failures are under-preloaded joints, not missing locking features.
Prevailing-torque features — nylon-insert nuts, distorted-thread nuts and similar elements resist vibration loosening without relying on friction alone.
Chemical locking — threadlocker applied and cured under controlled conditions; not a substitute for preload.
Washers — hardened washers distribute load and protect the surface; spring washers are widely used but are not a reliable vibration solution on their own in high-load joints.
Mechanical locking — cotter pins, safety wire and tab washers where the consequence of loosening is severe.

⚠️ On torque values: this page does not publish torque figures. Published torque values depend on the property class, the friction condition (lubricated or dry), the coating and the joint stiffness, and ISO 898-1 explicitly does not specify torque/clamp force performance — that is a separate test standard, ISO 16047. Any torque number given without stating the friction assumption is unreliable. Get the torque specification from the joint designer or from the fastener's own technical data.

Incoming inspection: the short list

Check these on arrival, in this order. They catch the great majority of specification drift:

Head markings match the specified property class and the manufacturer's identification mark.
Thread gauging — check with the correct go/no-go gauge for the specified thread class, not by running a nut onto it by hand.
Length and diameter against the drawing, including the length convention (under-head versus full-length, and whether the thread is full or part).
Material certificate — request a certificate that can be traced to the delivered lot, not a generic certificate for the material type.
Coating — confirm the coating system and ask for the thickness class the supplier claims; independent laboratory verification is the reliable route.
Hardness — where the property class is load-critical, hardness testing is the fast field check for whether the specified grade was actually supplied.
Documentation — supplier inspection report, and where applicable a first-article inspection report tied to your drawing revision.

Specifying fasteners on the drawing

A specification that leaves nothing to interpretation looks like this, and it is worth the five minutes:

Component and standard — e.g. hexagon socket head cap screw, ASTM A574 (inch series) or the metric equivalent for your design.
Size and thread — nominal diameter, pitch, thread series, and class of fit, with the governing standard (ISO 68-1 / ISO 261 / ISO 262 / ISO 965 series for metric).
Length and length convention.
Material and property class, with the standard edition.
Coating — system, standard, edition and required thickness class.
Head style and drive.
Marking requirements on the head, where traceability matters.
Acceptance criteria — the inspection items above, and who bears the cost of a failed lot.

If you are still choosing a supplier, how to find a manufacturer in China covers how to tell a factory from a trading company, and quality control for Chinese manufacturing covers how to set up inspection so that "we checked it" means something.

All sources retrieved 16 September 2026.

Source-level note

ISO standard pages on `iso.org` returned normally for this retrieval (ISO 898-1, standard 60610). Earlier attempts to retrieve other ISO standard pages from this environment returned HTTP 403 (WAF interception), so no other ISO standard numbers are asserted on this page — the ISO references listed above (ISO 68-1, ISO 261, ISO 262, ISO 965-1/2/4, ISO 898-5, ISO 16047) are all cited as they appear in the ISO 898-1:2013 abstract and scope, which is the page actually retrieved. No claim is made here about the content of those other standards.

Frequently asked questions

What is the difference between property class 8.8 and 10.9?

Both are property classes defined for metric steel bolts, screws and studs under ISO 898-1, and the number encodes a set of mechanical requirements rather than a material. The higher class calls for higher strength and correspondingly different behaviour under load and in coating — which is why a higher class is not automatically the safer choice for a given joint. Specify the class the joint was designed for.

Does ISO 898-1 apply to stainless steel fasteners?

No. ISO 898-1 applies to bolts, screws and studs made of carbon steel and alloy steel. Stainless steel fasteners are specified under separate standards for stainless property classes. If your drawing says "stainless" and cites ISO 898-1, the specification is internally inconsistent.

Can I use ASTM A574 on a metric drawing?

No. ASTM A574 covers alloy steel hexagon socket head cap screws and states its values in inch-pound units as standard, with diameter range given in inches. If your design is metric, use the metric specification for the fastener type rather than converting dimensions and keeping the A574 reference.

Why does the standard say the properties are "at ambient temperature"?

ISO 898-1 specifies the mechanical and physical properties when tested in an ambient temperature range of 10 °C to 35 °C, and notes that fasteners meeting the standard might not retain those properties at elevated or lower temperatures. If your application runs hot or cold, the ambient specification alone is not sufficient.

Is this page a substitute for the standards themselves?

No. This page explains how the standards are used in purchasing and what to put on a drawing. The standards are the specification, they have editions, and the edition matters — always work from the current edition of the standard your drawing cites.

Sources

All sources retrieved 16 September 2026. This page is an independent reading of the sources listed; the standards and official pages themselves are the specification.

Fact used on this page	Source
ISO 898-1:2013 specifies mechanical and physical properties of bolts, screws and studs made of carbon steel and alloy steel when tested at an ambient temperature range of 10 °C to 35 °C	ISO 898-1:2013 — Mechanical properties of fasteners made of carbon steel and alloy steel, Part 1
Fasteners conforming to ISO 898-1:2013 are evaluated at that ambient range and might not retain the specified properties at elevated or lower temperatures	ISO 898-1:2013
Certain bolts and screws might not fulfil the tensile or torsional requirements because head geometry reduces the shear area in the head compared with the stress area in the thread; low and countersunk heads are named examples	ISO 898-1:2013
ISO 898-1 is applicable to bolts, screws and studs with triangular ISO metric screw thread in accordance with ISO 68-1, coarse pitch M1.6 to M39 and fine pitch M8×1 to M39×3, diameter/pitch combinations per ISO 261 and ISO 262, and thread tolerances per ISO 965-1, ISO 965-2 and ISO 965-4	ISO 898-1:2013
ISO 898-1 is not applicable to set screws and similar threaded fasteners not under tensile stress (ISO 898-5)	ISO 898-1:2013
ISO 898-1 does not specify requirements for weldability, corrosion resistance, resistance to shear stress, torque/clamp force performance (test method ISO 16047) or fatigue resistance	ISO 898-1:2013
ISO 898-1:2013 was last reviewed and confirmed in 2025 and therefore remains current; the standard's stage is recorded as 90.92, to be revised	ISO 898-1:2013 — lifecycle and general information
ASTM A574 covers quenched and tempered alloy steel hexagon socket head cap screws, 0.060 through 4 in. in diameter where high strength is required	ASTM A574-21 — Standard Specification for Alloy Steel Socket-Head Cap Screws
ASTM A574 requires the screws to be fabricated from steel made to fine grain practice and to be alloy steel conforming to a specified chemical composition	ASTM A574-21
ASTM A574 defines tests to determine tensile or wedge strength, proof load, yield strength, elongation, Rockwell hardness and Vickers hardness	ASTM A574-21
ASTM A574 states that the values in inch-pound units are to be regarded as standard, and includes no other units of measurement	ASTM A574-21
Verification route for supplier management-system certificates: validate the certificate in the issuing accreditation system's database rather than accepting a scanned document	IAF CertSearch
Verification route for Chinese company registration details (legal entity, registration status, unified social credit code)	National Enterprise Credit Information Publicity System (GSXT)

Independent resource. This page is published by an independent information resource. It is not a factory, broker or marketplace, and no prices, minimum order quantities, lead times or supplier lists are published. Where a figure or a requirement depends on a standard or an official rule, the standard or the official page is the specification — verify the current edition before you commit to a purchase decision. Sources used for this page are listed below and were retrieved on 16 September 2026.