Fuel tank cap seal Gen 1 AP8120894
Fitment - RSV 98-03, Tuono 02-05
OEM Aprilia fuel tank cap rubber seal, part number AP8120894, the bottom item in the diagram.
As this is a direct replacement it will fit easily in place and do it's intended job of preventing the ingress of any unwanted water or other fluids into the fuel tank. Fuel and water don't mix and so that will lead to running issues if it happens.
If you notice water in your fuel it shows that your rubber seal may be defective. If you notice it on the rim of your tank when you open the filler cap, it shows that your seal is working but it does highlight that you potentially have a blocked breather/drain hole that will need to be sorted.
Please click on the links above for a couple of short videos that we here at AP Workshops have produced explaining just that issue and how to remedy it.