Form F washers are flat metric fasteners governed by British Specification BS 4320. They are explicitly defined as having a large outside diameter (OD) and a normal thickness profile, historically categorised as "black" or heavy-series washers meant for structural and outdoor engineering tasks.
Form F vs Form C
In terms of pure physical measurements, a Form F washer shares identical dimensions with a Form C washer. However, they are separated by their historical industry finish and modern material usage:
Form C: Typically manufactured from bright mild steel and finished with Bright Zinc Plating (BZP) for general interior or light-duty tasks.
Form F: Traditionally categorized as un-machined "black" steel but overwhelmingly supplied in modern commercial trade with a Hot-Dip Galvanised (HDG) finish to survive structural outdoor use.
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