That isn't a great way of doing the cap seal Swedewheeler, nothing is really retaining the O ring in place, unlike the original cap where the O ring sits in a grove and it seals on the side of the inlet pipe. That will deform the O ring and it could fall into the tank again. A new rubber seal made from some fuel resistant rubber sheet such as 3mm neoprene and retained with a large penny washer, as in the photo will cause fewer problems.
