STAINLESS STEEL FORM C FLAT WASHERS BS 4320

Form C Flat Washers BS 4320 — Form C sits at the far end of the BS 4320 range from Form A, with the largest outside diameter and thickness combination available for a given bolt size in that standard - the option to reach for when a joint needs more bearing area than Form A or B provide but does not yet call for a dedicated large-diameter product like DIN 9021. That extra bearing area matters most where the clearance hole is oversized, the clamped material is relatively soft or thin, or the fastener is working close to its rated preload and needs the load spread as widely as practical to avoid crushing or embedding the surface underneath. It is a general engineering and fabrication washer rather than a structural one in the formal sense: where a bolted connection is designed and certified to structural steelwork standards, such as an HSFG (friction-grip) assembly, a washer specifically designated for that duty - DIN 7989 is the usual reference - should be used instead of a general BS 4320 form. Material selection follows the usual pattern: mild steel with a protective plating for indoor and general-purpose work, A2 (304-equivalent) stainless where the assembly is outdoors or subject to washdown, and A4 (316-equivalent) reserved for genuinely corrosive conditions - coastal exposure, marine equipment, or chemical contact - where the extra cost buys real protection against pitting rather than just a marginal upgrade. To choose correctly, confirm the bolt diameter and, critically, whether the drawing or specification explicitly calls out Form C rather than A or B; where no form is specified, Form C is worth considering specifically when the substrate or hole condition pushes toward maximum bearing area, with Form A or B being the more economical default otherwise.

FAQs
When does Form C make sense instead of Form A or B?
When clearance holes are oversized or the clamped material is soft; as a rule of thumb, if a metric fastener would otherwise need a penny or 3xd-style washer, Form C is usually the first thing to check within the BS 4320 system.

Can Form C washers be used in structural steelwork?
It is commonly used in general structural and fabrication work, but where a connection is formally designed to structural steelwork standards, such as HSFG assemblies, a washer specifically designated for that duty, like DIN 7989, may be required instead.

Are BS 4320 forms interchangeable with ISO large-series washers like ISO 7093-1?
No - each is governed by its own dimensional table, so bore, outside diameter, and thickness should be checked before treating them as equivalent.