Flat Washers are used to increase the bearing surface of a nut or fastener's head thus spreading the clamping force over a larger area. They can be useful when working with soft materials and oversized or irregular shaped holes. The washer also ensures a smooth base for the nut to press against. If you already have a washer and you are trying to match it measure the inside then look for a size slightly smaller that your measurement. For example the inside measurement of the washer you measured is approx 6.50mm you need to look for a M6 washer. If the washer measured 17.25mm internally then you need a M16 Washer. It is aesthetically pleasing to the eye and very practical because of its excellent corrosion protection in most environments. Most products can be identified by the following markings: A2 A2-70 304 or G304 but not all washers are stamped. All material is advisory only and its use by anyone is entirely voluntary. Reliance on it for any purpose by anyone is at the sole risk of that person and Bolt and Nut Australia is not responsible for any loss claim or damages arising therefrom.