- Modern Reproduction of the classic 3" butterfly filler cap as fitted to Norton International and pre-featherbed Manx Norton petrol tanks. Note: These filler caps as supplied to me do not have the cork sealing washer - which I think is pretty poor - so I have had a press tool made for making these from the correct gasket cork, I then fit them before sending out (if buying from elsewhere you may want to check they come with these washers - I suspect they will not!).