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shakeproof washers
2 copper crush washers one under the head ...one after the bolt passes thru the line block.
Flat steel washers, galvanised washers, ripple washers, spring washers, fibre locking washers.
Yes, washers can rust. I have seen many rusty washers.
Flat washers. Spring washers. Rubber gasket. Felt washers. Asbestos gaskets.