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. A thin layer of zinc is applied by either electroplating or mechanically. It is intended to be used internally as it only offers an average level of protection against corrosion. Whilst some people use it externally it isnt recommended if corrosion is a concern. 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.