Stainless Steel Hex Bolts & Nuts Set (A2-70 / A4-80)

A2 vs A4 Grade Selection and Material Traceability

A2 (304 stainless) is suitable for most indoor and sheltered outdoor applications. A4 (316 stainless) contains molybdenum, which gives it significantly better resistance to chloride corrosion — salt water, coastal environments, swimming pools, and chemical plants. Substituting A2 for A4 in marine applications leads to crevice corrosion within 6–18 months. The visual appearance is identical; the only way to distinguish grades is XRF analysis or a mill test report (MTR). When sourcing stainless fasteners, always require MTR documentation.

MTR traceability is the baseline quality requirement for industrial and construction buyers: the MTR documents the heat number, chemical composition (Cr, Ni, Mo percentages), mechanical properties, and certifying standards. Reputable Chinese fastener manufacturers (Zhejiang Haiyan, Jiaxing, Ningbo production clusters) can provide MTRs as standard. Factories that cannot provide MTRs are either producing low-grade material or buying from unknown sources — avoid them for any application requiring material certification. A factory audit should include MTR verification as a standard checkpoint.

Dimensional Inspection and Packaging for Resellers

ISO 4017 tolerance class 6g/6H is the standard commercial tolerance for hex bolts and nuts — this is what buyers expect unless specifying fine threads. Check thread engagement on a sample bolt/nut pair: the nut should run down freely by hand but with no wobble. Galling (thread seizure) during assembly is a common complaint with stainless fasteners — verify that the factory passivates the threads after machining (passivation forms a chromium oxide layer that reduces galling).

For reseller kit packaging, verify the sorted assortment accuracy. A 240-piece kit should have exactly the specified count per size — short packing (270-piece boxes with only 240 fasteners) generates returns and reputation damage. Specify quality inspection with 100% count verification on a sample of 3–5 kits per production run. Bulk packaging in 1 kg sealed bags with clear labeling (grade, standard, size) reduces handling errors downstream. For a systematic approach to evaluating fastener suppliers, see our factory audit checklist.