This is a moulded seal using a hard Viton compound.

Note: Spring loaded tap designs (i.e. all Laverda/Ducati) will leak if any pressure is put on the lever other than to rotate it. To avoid damage to the seal it is recommended to only rotate the lever when there is the presence of petrol on the seal. The dimensions of these seals are, OD 18.5mm x 2mm thick. These seals are sold in pairs.