Hardware & Fasteners

Hardware, Tools and Fastener Inspection in China

Published October 2026 · By SSTI Inspection Team

Fasteners, hand tools and other hardware fail in ways that a quick look will not reveal. A bolt with the wrong heat treatment looks the same as a correct one until it snaps or strips. A thin zinc coating looks bright when it is packed and starts to rust within months. Inspecting hardware means measuring: threads, hardness, coating thickness and markings. This guide covers what an inspection should include and what still needs a lab.

Property Class and Markings

For metric steel bolts and screws, ISO 898-1 defines property classes such as 4.8, 8.8, 10.9 and 12.9. Each class sets minimum strength and a hardness range. Hexagon bolts and screws from 5 mm diameter are normally marked on the head with the property class and the manufacturer's identification mark. Inch fasteners use SAE grades, marked with radial lines on the head, such as three lines for grade 5 and six for grade 8.

A head marking is a claim, not proof. A bolt marked 8.8 that was not heat-treated correctly can be much weaker than the marking says. The marking is checked on site; the claim behind it is checked by hardness readings, and when it matters, by laboratory testing.

Dimensions and Threads

• Thread gauges. Go and no-go thread gauges confirm that threads are within tolerance. A thread that is too tight or too loose causes assembly problems and weak joints.
• Dimensions. Length, diameter, head height and width across flats are measured with calipers against the drawing or standard.
• Visual defects. Cracks, burrs, damaged threads, incomplete heads and mixed parts in the same batch.

Hardness, Torque and Coating on Site

• Hardness. A portable hardness tester gives a reading that can be compared with the range for the specified class or material. On small parts, readings need care and suitable test methods. A result outside the range is a strong signal that heat treatment or material is wrong.
• Torque tests. For bolts, a destructive torque test on a sample shows whether parts strip or break below the expected value. For hand tools such as wrenches and sockets, a torque test shows whether the tool deforms or cracks under load. The method and target values should be in the checklist.
• Coating thickness. A coating thickness gauge measures zinc, nickel or paint thickness in microns on the sample, which can be compared with the specification. Thin plating is one of the most common cost savings and one of the easiest to measure.
• Coating appearance. Blistering, uneven colour, bare spots and white rust from poor storage.
• Packing and labelling. Correct counts per box, labels with size, class and batch, and moisture protection for steel parts.

For the general setup of tests, see our guide to on-site inspection tests.

What Needs a Lab

The steel grade itself is often the hidden variable. Our case study on grade 20 steel replacing grade 30 shows how a material change passed every visual check and was found through the raw material documents.

When to Inspect

Check material certificates and heat treatment records at an initial production check, before a whole batch is made from the wrong steel. Then a pre-shipment inspection covers dimensions, threads, hardness, coating and packing on finished goods, sampled under AQL. Thread gauge, hardness and coating thickness checks can be written into an SSTI inspection checklist. See our inspection services.

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