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Fastener purchase contracts and drawing tolerances: getting the quality requirements right the first time

2026-10-22

Compiled by JIALI Industrial (164.hk) — a Hong Kong fastener supplier specialising in screws, bolts, nuts and selection know-how.

Why 'M8x25' alone is not enough - hidden assumptions on a fastener drawing

Fasteners are mass-produced parts governed by international standards, so when a drawing says 'M8x25 hex bolt', the supplier defaults to the current standard - yet 'per standard' still leaves wide latitude. The same 8.8 grade may be hot-formed and quenched-tempered or cold-headed to property; zinc plating may be 5 um electroplate or mechanical/Dacromet; thread tolerance may be 6g before or after plating. These differences directly affect corrosion life, assembly and preload behaviour, but none of them appear in the bare part number. GB/T 3103.1 (equivalent to ISO 4759-1) divides dimensional and geometric tolerances for bolts, screws and nuts into product grades A, B and C: grade A is tightest (limits on bearing-face runout, shank straightness and more), grade C loosest. If the drawing does not state a grade, a low-cost supplier is entitled to ship grade C - the root of 'same name, different quality' disputes. The first job of a fastener purchase contract is to convert these hidden degrees of freedom into explicit clauses.

How to call out thread, geometric and material tolerances

Thread tolerance is the core: metric threads per GB/T 197 (ISO 965-1) are called out as tolerance classes, typically 6g for external and 6H for internal threads at commodity level. A 5-8 um plating consumes roughly 2-3x its thickness of thread clearance, so allow for it before plating (e.g. pre-plate 6g, verify 6g after) or specify 6e - otherwise the GO gauge will not pass. Strength grades per GB/T 3098.1 (ISO 898-1) are simply '8.8' or '10.9'; stainless per GB/T 3098.6 as 'A2-70' or 'A4-80'. For non-standard parts, geometric tolerances follow GB/T 1182 (GD&T): perpendicularity of the bearing face to the thread axis and flange runout directly decide whether preload is centred - specify perpendicularity within 0.25 mm on critical joints; unreferenced linear dimensions can invoke GB/T 1804-m (medium class, about +/-0.1-0.3 mm) to remove ambiguity. The drawing should also state inspection datum and sampling: GB/T 2828.1 general inspection level II with AQL 1.0-2.5, keeping critical items (thread, strength) judged separately from general items (appearance, dimensions).

Writing the acceptance clauses: standards, certificates and responsibility in one pass

A workable fastener purchase contract should carry four technical blocks. First, a referenced-standards list - dimensions and tolerances (GB/T 3103.1 / ISO 4759-1), threads (GB/T 197 / ISO 965), mechanical properties (GB/T 3098 series / ISO 898), coatings (GB/T 5267.1 / ISO 4042) and inspection (GB/T 2828.1) - each with its year edition. Second, certificates: routine batches with EN 10204 2.1/2.2, critical load-bearing or export items with a 3.1 material certificate, heat/batch traceability stated. Third, acceptance and re-inspection: incoming sampling level, AQL, replacement and line-stop liability on rejection, disputed parts sent to a neutral lab against sealed samples. Fourth, special requirements: coating type and thickness, salt-spray hours per ISO 9227, hydrogen-relief baking for embrittlement-risk parts, friction coefficient ranges. Add one line on the drawing - 'unspecified tolerances per standard XXX' - and the four readings of 'shipped per standard' collapse into one. Need fasteners supplied to drawing tolerances and contract requirements, with batch 3.1 certificates? Contact Garly Industry (164.hk) with your drawings or specification list for a matched solution and quotation.

Related Reading

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• JIALI launches Stainless Steel zone with full 304 / 316 range

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