A bolt or screw is identified by six attributes: head type, drive, diameter and thread pitch, length, property class, and finish. Quotes are comparable only when all six are fixed in writing, and property class and finish carry most of the price spread.
How a fastener is actually identified
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Head and drive
Head type is the external shape: hex, hex flange, socket cap, button, countersunk (flat), pan. Drive is how torque is applied: external hex, internal hex socket, hexalobular (Torx-type, ISO 10664), cross recess type H (Phillips) or type Z (Pozidriv) per ISO 4757, slotted, square. The two are independent choices: “countersunk screw” without a drive is not a specification.
Diameter, pitch, length
Metric threads are written diameter × pitch: M8×1.25 is coarse, M8×1.0 is fine. Coarse is the default in the ISO general purpose metric series (ISO 261), so “M8” alone means M8×1.25. Fine pitch gives a larger tensile stress area at the same nominal diameter, but it is a different part. Length is measured from under the head to the tip on protruding-head fasteners and over the whole part on countersunk heads. Tolerance class also needs stating: 6g external and 6H internal in metric, 2A/2B in inch.
Standards families a Chinese factory will recognise
- ISO metric: ISO 4014 (partially threaded hex bolt), ISO 4017 (fully threaded hex screw), ISO 4762 (socket cap), ISO 10642 (socket countersunk), ISO 4032 (hex nut style 1), ISO 7089 (plain washer).
- DIN legacy: DIN 931 and DIN 933 (hex), DIN 912 (socket cap), DIN 7991 (socket countersunk), DIN 934 (hex nut), DIN 125 (plain washer), DIN 127 (spring lock washer) — still the everyday trade language in China. DIN and ISO hex parts differ in width across flats at some sizes, M10, M12 and M14 being the usual traps.
- Chinese GB: GB/T 5782 (equivalent to ISO 4014), GB/T 5783 (ISO 4017), GB/T 70.1 (ISO 4762), GB/T 6170 (ISO 4032), GB/T 97.1 and GB/T 93 for washers. GB/T 5780 and GB/T 5781 are the product grade C hex bolts: looser tolerances, cheaper, not a substitute unless you say so.
- Inch/US: ASME B18.2.1 (square and hex bolts and screws), ASME B18.3 (socket cap screws), ASME B1.1 (unified inch threads), with strength as SAE J429 grade 2, 5 or 8, ASTM A307, or ASTM F3125 for structural bolting.
A factory quoted on a DIN number will often build to the GB equivalent; ask which document they will run to, and get it onto the order acknowledgement.
Property class is the line that matters most
ISO 898-1 property classes for carbon and alloy steel bolts and screws are arithmetic, not a catalogue code: the first number × 100 is the nominal tensile strength in MPa, and the two numbers multiplied × 10 give the nominal yield strength. Class 8.8 is nominally 800 and 640 MPa, 10.9 is 1000 and 900, and 4.8 is 400 and 320 — cold-formed low-carbon rather than quenched and tempered.
Nuts are classed separately under ISO 898-2, in classes 5, 6, 8, 10 and 12; an under-matched nut can strip before the bolt reaches its proof load. Stainless follows ISO 3506-1, with designations such as A2-70 and A4-80: A4 is the molybdenum-bearing (316-type) grade with better chloride resistance, and the second number is minimum tensile strength in units of 10 MPa, so A4-80 is 800 MPa minimum. Austenitic threads also gall readily when run dry.
ISO 898-1 requires the property class mark and a manufacturer’s identification mark on hexagon head bolts and screws and socket head cap screws from M5 upward. The inch convention is head marking by grade: SAE J429 grade 5 carries three radial lines, grade 8 six.
Finish: corrosion life and price
- Electroplated zinc (ISO 4042; ASTM B633 service condition classes run from a few microns to roughly 25 µm): cheap, thin, and the default. Passivation is now normally trivalent, hexavalent chromium being restricted under the EU RoHS and End-of-Life Vehicles directives.
- Hot-dip galvanizing (ISO 10684 for fasteners; ASTM A153 Class C and D): thicker and longer-lived outdoors, but the coating does not fit inside a standard thread tolerance, so nuts are tapped oversize after galvanizing and supplied matched to the bolt. ASTM F3125 Grade A490 bolts must not be galvanized.
- Zinc flake systems (ISO 10683), dip-spun and cured rather than electroplated: no hydrogen charging from a plating bath, thin enough for standard threads, and specifiable with a controlled coefficient of friction.
- Black oxide and phosphate give little standalone corrosion protection and rely on supplementary oil.
Corrosion performance is verified by neutral salt spray to ISO 9227 or ASTM B117. Specify hours to white rust (the coating corroding) and to red rust (the steel underneath) separately, and treat them as batch comparison, not a prediction of service life.
Electroplating high-strength fasteners carries hydrogen embrittlement risk, conventionally taken seriously from property class 10.9 upward; those parts need baking promptly after plating, or a non-electrolytic coating.
What actually changes the cost
Property class is a real process step: 4.8 is cold-formed and not heat-treated, while 8.8, 10.9 and 12.9 are quenched and tempered, on medium-carbon, boron-treated or alloy steels. Stainless costs more than plated carbon steel, and A4 more than A2. Cold heading needs dies for each head shape and diameter, so a non-catalogue part becomes a production run with setup cost and a higher minimum. Fasteners are usually quoted per 1,000 pieces or per kilogram, and certification or third-party inspection adds cost and lead time.
What a Chinese factory needs from you
- Standard number and full designation, e.g. ISO 4017 M10×40, and whether the GB equivalent is acceptable.
- Thread detail: diameter, pitch, hand, and tolerance class (6g/6H, or 2A/2B for inch).
- Property class or grade for bolts, nuts and washers separately, plus material where the class does not imply it.
- Finish: coating type and standard, thickness or service condition class, passivation and chromium type, and RoHS/REACH compliance for the EU.
- Corrosion requirement: salt spray hours to white rust and to red rust, with the test standard named, plus marking: property class, manufacturer mark, any customer logo.
- Quantity, annual volume, delivery schedule, and packaging: pieces per box, box weight, labelling, pallet spec.
- Documentation: EN 10204 certificate type, test reports, a sampling plan such as ISO 3269, third-party inspection if required, and a drawing or sample for anything non-standard.
- Destination market and its compliance regime, such as CE marking under EN 15048 or EN 14399 for structural bolting.
- What the part fastens and at what torque or preload, if the joint is safety-related.