I am in the process of sorting out a spare tank for my 550 f2, and it has some slight differences from my other tank. the main issue is the thread on the fuel tap, on the old one its 16mm and the taps are easy to find on ebay. However the new tank has a 20mm thread and they seem to be harder to find. While i have been looking for one i checked David Silvers website and saw on the 400 four repro tank you have to use a later usa type tap and i was wondering if that is a 20mm? The outside spanner size is only 22mm, if anyone could confirm or deny that would be a great help.