STAINLESS STEEL FORM B FLAT WASHERS BS 4320

Form B Flat Washers BS 4320 — BS 4320 exists because British engineering drawings needed their own flat washer series alongside the DIN/ISO metric system, and Form B is one size step within it - a specific outside-diameter-and-thickness combination for a given bolt size, sitting apart from Form A and Form C in the same standard's dimensional tables. It performs the same core job as any flat washer: spreading the clamping load from a bolt head or nut over a wider bearing area, protecting the clamped surface's finish, and correcting for an uneven or rough substrate. Where it earns its place is on legacy British-specification drawings, older plant, and repair or maintenance work where the original design called up BS 4320 rather than DIN 125 or ISO 7089/7090 - dimensions between the systems are close but not identical, so a BS 4320 Form B should not be swapped in for a metric DIN/ISO washer without checking bore, OD, and thickness against the drawing first. Material choice follows the same logic as any general washer: mild steel with bright zinc or similar plating covers indoor and short-term outdoor use economically, while A2 (304-equivalent) stainless is the default upgrade for anything exposed to weather, washdown, or general outdoor atmosphere. A4 (316-equivalent) is worth specifying only where the environment adds chlorides or process chemicals into the mix - coastal steelwork, marine fittings, or plant handling aggressive cleaning agents - since that is where A2 starts to pit and A4's molybdenum content earns its premium. Selecting the right washer starts with confirming the bolt diameter and grade specified on the drawing, then checking whether the form letter (A, B, or C) is explicitly called out; if it is not, Form B is a reasonable general-purpose default within the BS 4320 range, sitting between the tighter Form A and the larger-bearing Form C.

FAQs
Does the form letter (A/B/C) affect a washer's load rating, or just its footprint?
Mainly footprint - outside diameter and thickness change bearing area and stiffness, while hardness class and material govern maximum load capacity, so the form should be picked for fit and spec compliance rather than assumed to change strength.

What is the risk in substituting a DIN or ISO washer for a BS 4320 Form B on an old drawing?
A bore or outside-diameter mismatch that leaves too little bearing area or fouls adjacent fixings, so always check the drawing's dimensional callout, not just the bolt size, before substituting.

Is BS 4320 still specified on new designs, or mainly seen on older drawings?
It is mostly seen where a design deliberately follows British Standard for legacy consistency; new designs typically default to ISO 7089/7090 as the current international reference.