Bolts are the most common line item in almost any fastener bill of materials, and they are also where small specification gaps cause the most rework. When you source bolts from China, the supplier will build to whatever drawing or standard you reference, so the quality of the result depends largely on how precisely you define the part before the order is placed. This guide covers the decisions that matter most for procurement engineers and mechanical designers: standard and designation, property class, thread and length conventions, material and finish, tolerance, and the inspection steps that catch problems before parts reach your line.
For a full overview of the range we handle, see our bolts category page. If you are still comparing fastener families, the how to choose topic hub walks through the trade-offs between bolts, screws, studs and anchors before you commit to a part number.
Start with the standard, not the drawing: specifying bolts correctly
A bolt specification should name a published standard wherever one exists. That gives the supplier a shared reference for dimensions, thread tolerances and mechanical properties, and it gives you a defensible basis for inspection. The most common references in Chinese production are DIN, ISO and GB, with ANSI/ASME appearing on North American projects.
| Standard | Typical scope | Notes for buyers |
|---|---|---|
| ISO 4014 / ISO 4017 | Hex head bolts, partially and fully threaded | Metric; widely produced and easy to cross-reference with DIN 931 / DIN 933 |
| DIN 931 / DIN 933 | Hex head bolts, partially and fully threaded | Common in European drawings; confirm whether the drawing means DIN or ISO dimensions |
| ISO 4762 | Hexagon socket head cap screws | Often called socket head bolts in catalogue listings |
| GB/T 5782 / GB/T 5783 | Chinese national hex bolt standards | Largely aligned with ISO; state the ISO equivalent if your drawing uses it |
| ANSI/ASME B18.2.1 | Inch-series hex head bolts | Inch threads and head dimensions differ from metric; do not mix |
Two details trip up buyers repeatedly. First, DIN and ISO hex bolts are not identical in every size, particularly across flats and head height, so a drawing that says "DIN 933" should not be switched to ISO 4017 without checking. Second, partially threaded and fully threaded versions are separate standards (DIN 931 versus DIN 933, ISO 4014 versus ISO 4017). State which one you need, and state the thread length if the default does not suit your joint.
Property class, material and finish: matching bolts to the joint
Property class defines the strength of a metric steel bolt. Classes 8.8 and 10.9 cover most structural and machine-building work; 4.6 and 4.8 appear in lighter duty and general assembly. Class 12.9 is available but is less forgiving of surface defects and hydrogen embrittlement risk, so it should be specified only when the load case requires it. For stainless parts, the designation shifts to grades such as A2-70 and A4-80 under ISO 3506 rather than property classes.
- Class 8.8: the default for general structural and mechanical joints; good balance of strength and toughness.
- Class 10.9: higher preload capacity for compact joints; confirm the mating material can take the clamp load.
- Class 12.9: high-strength applications; discuss coating choice carefully because of embrittlement risk.
- A2 / A4 stainless: corrosion resistance; A4 (316-type) for chloride exposure, A2 (304-type) for general indoor and outdoor use.
Finish selection follows the environment. Plain oiled finish is adequate indoors and in dry assemblies. Zinc plating is the common default for indoor and sheltered outdoor use. Hot-dip galvanizing suits heavier outdoor and structural work but changes thread dimensions, so nuts must be matched and overtapping confirmed. Zinc-nickel and zinc-flake coatings are options where a higher corrosion requirement exists, but the supplier should state the coating system and the standard it follows rather than a vague "anti-rust" description.
Material and finish interact with strength. A class 10.9 bolt that is electroplated requires a de-embrittlement step after plating, and that step should be confirmed in the process description. If the application is safety-related, ask how the supplier controls this rather than assuming it is handled.
Thread, length and head style: getting the details right
Thread specification should be explicit: nominal diameter, pitch and tolerance class. M10x1.5 is the coarse default; M10x1.25 is fine pitch and must be stated. For inch parts, give the thread callout in the ANSI/ASME form, for example 1/4-20 UNC. Tolerance classes such as 6g for external threads are worth naming when the joint has a controlled preload requirement.
Length measurement is defined by the standard, not by habit. For most hex bolts, length is measured from under the head to the end of the shank. For countersunk and other head styles where the head sits flush, length is measured from the top of the head. Mixing these conventions is a frequent source of short or long parts, so state the convention on the drawing or purchase order.
Head style follows the tooling and clearance available. Hex heads give high torque capacity and are easy to inspect. Socket heads suit recessed joints and tight envelopes. For lighter assemblies, pan head and countersunk Phillips screws are common, and the same sourcing discipline applies: name the standard, the thread, the length and the finish. Two examples in our range are the pan head Phillips screw and the pan head Phillips screw variant, while flush joints typically call for a countersunk head Phillips screw. These are screws rather than bolts in the strict sense, but they follow the same specification logic and are often ordered alongside bolts on the same purchase order.
Related resources: Cap nuts.
Related resources: Spring washers and Flange bolts.
What to check before and after the order ships
Most bolt quality problems are detectable with basic dimensional and documentation checks. Build these into the purchase order rather than treating them as optional extras.
- Drawing and standard review: confirm the supplier has quoted against your standard and revision, not a similar catalogue item.
- Material and class confirmation: ask for the mill certificate or material declaration for the heat used, and check that the property class on the certificate matches the order.
- Dimensional inspection: thread gauge, length, head dimensions and, for hex bolts, across-flats and head height. Sample size should be agreed in advance.
- Mechanical properties: tensile, proof load and hardness testing where the class matters. Agree whether this is done per lot and who witnesses it.
- Coating check: coating type, thickness and appearance; confirm de-embrittlement for high-strength electroplated parts.
- Marking and traceability: head marking should show the property class and the manufacturer's mark; packaging should carry the lot reference.
If a supplier cannot state which standard a bolt is made to, treat that as a specification gap rather than a detail to sort out later. It is much cheaper to resolve before production than after the container arrives.
For repeat orders, keep a controlled drawing and a signed first-article report on file. That gives you a baseline for comparing later lots and makes it easier to raise a clear non-conformance if something changes.
FAQ
Are DIN and ISO bolts interchangeable?
Not always. Many sizes are close or identical, but head dimensions and some tolerances differ between DIN and ISO standards. Specify one standard and keep it consistent across drawings and purchase orders rather than assuming a direct swap.
What does property class 8.8 mean on a bolt?
It is a metric strength designation: the first number relates to nominal tensile strength and the second to yield strength ratio. In practice, class 8.8 is the general-purpose structural and machine-building grade, and class 10.9 is used where higher preload is needed.
How is bolt length measured?
For most hex head bolts, from under the head to the end of the shank. For countersunk and flush-fitting heads, from the top of the head. State the convention on the drawing so the supplier and your inspection team measure the same way.
Ready to compare options? Start with the bolts category page, or use the how to choose hub to narrow the fastener family before you write the specification.