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When fitting a set of taps I was always taught (many moons ago) that the rubber washer went topside i.e the tap sat on the rubber washer and then the backnut was tightened up. I come across alot of sets of taps where the taps are not sat on anything and the washer is underneath, with the backnut tightened in to the washer. I have also read fitting instructions where it tells you to put the washer underneath. I cannot see the point of this as when the washer is underneath it prevents you from tightnening the backnut tight enough and can also split the washer.

So the question is when fitting taps should the washer go on the top or bottom??