BS 4320 is a British Standard that outlines the specifications for metal washers that are used with fasteners. The standard covers various types of washers, including plain washers, which are flat and used to distribute the load of a threaded fastener, and spherical and conical washers, which are used in specific applications to provide a self-aligning bearing surface. BS 4320 also provides information on the materials and dimensions of these washers, ensuring that they conform to the necessary quality and performance standards. This standard is crucial in maintaining the safety and reliability of fastened connections in various industrial and commercial applications.