Hi John, Are your caps the type made by both Enots (Benton and Stone Ltd) and Best and Lloyd Ltd., with alloy ‘ears’ sticking up from the cap ? Originally the sealing expansion rings were made of leather, but these days you have to be VERY selective to choose a material that is resistant to oil and to the ethanol in petrol. In other words you need one made from Viton, for petrol, and one made from Nitrile, for oil, and to be very careful that you put them onto the correct side of the tank !! They are made by Simon Grigson, the vintage racing Norton guru. I bought some only last week, and the price was £12 for the Viton, and £6 for the Nitrile, plus £2 P&P. Simon’s phone number is 01256-381560, mobile 07909-784222. Do measure the internal diameter of the filler necks carefully as they are nominally 2.5″, but Velocette and Norton for instance, made theirs a good few thou’ bigger. Simon can advise you accordingly.