Why are copper washers required on banjo fittings?

Question

1. Why are copper washers required on banjo fittings?
2. Why do they need to be replaced each time you remove/loosen the banjo fitting?

I'm assuming that copper is a softer metal than bronze, brass or steel so they will squish when torqued to make a seal. However, I've seen copper crush washers that had an embossed ring in the center of the washer that would actually crush down when under pressure. Today I had to replace the washers for an oil feed line for a turbo but the manual calls for copper washers and not crush washers. Do the flat copper washers work in the same way?

Answer 1

You are correct that the copper washer is required to create a seal.

Even flat copper washers will deform during tightening to create a seal. They're made from annealed copper, which is very soft. The raised ridge just reduces the load required to initiate the seal.

Generally, these washers should be replaced rather than re-used, as the act of deforming an annealed copper washer effectively "hardens" the material, making it more difficult to crush next time, therefore you may not get a good seal when re-using a copper washer.