In these expensive times, you naturally do not want any of the fuel tank to empty your tank. So then becomes a fuel cap with a lock? Then it is important to be careful about what kind of fuel cap you are going to 'assemble'. You can easily get a problem with it. Your well-cared-for classic suddenly stops (...), but just starts again and then drives normally again. The joy is of short duration, because a little later the engine stops again. The reason, your original fuel cap had a vent and the new fuel cap did not. If the fuel is sucked out of the tank to feed the engine, air must also enter the tank and that does not happen. The tank draws vacuum and therefore the 'flow' of fuel stops. The same can also happen with a hydraulic brake or clutch, when the small hole in the cap is hidden. The vehicle then continues to brake or the clutch keeps decoupling. Because that small hole brings air in when you depress the pedal and disappears again when you take your foot off the pedal ... Something to keep an eye on, to remember ...