@andre-ruiters It looks like someone has been trying to get a good seal by adding some material to the brass ferrule. If you have the length to spare, I would cut the tube as close as possible to the ferrule and start with a new ferrule and nut. There is only one size nut that goes with the tube size. Your local hardware or auto parts store should have what you need. Good luck.

Mike

@35mike Thanks for the tip. That's exactly what I did. Unfortunately, the fuel line got a kink in the process. It is tight, but I would still like to replace it. As I live in Germany, 5/16 pipes are rarely used here. Hence my question about the appropriate compression nut

Andre

@andre-ruiters I can't help you with the exact size but before giving up and replacing the fitting I would try one additional step first. Clean off all of the stuff others have put on the ferrule and put a liberal amount of gas and oil resistant pipe dope (sometimes called Pipe Joint Compound) around it, then tighten the nut. Plumbers here have been using it for well over 150 years successfully to plug off small imperfections. I suspect similar has been done on your side of the pond as well although it may have a different name there. Chemical resistant pipe dope is cheap and readily available.

@tiny Thanks for the tip, but as I said, the fitting is tight. However, the pipe has got a bend and there is a risk that it will leak or have too little flow.

the compression nut should be 1/2"-24 and the fitting into the tank most likely is a pipe thread and not soldered. I suggest you replace the entire fitting . 

I agree with Steve and believe the fitting in the tank can be unscrewed resulting in either 1/8 inch or 1/4 inch national pipe thread. I’d give it a try, and if successful, replace the existing fitting with a barbed fitting that you can push a short section of fuel line hose on.  Connect the other end to the existing fuel line past the kink.  A couple hose clamps should make it leak tight. Remember that the one piece fuel line acts as the gas tank ground (earth) so it will be necessary to separately ground the tank for the gas gauge to work correctly.  In later years the last few inches of fuel line between the frame and fuel pump was always fuel line hose.

I had tried to unscrew the fitting in the tank. But it didn't work and I didn't want to break anything off. In my case, the last few centimeters of the fuel line between the frame and the fuel pump are a fuel line hose.

@Steve-d - Thanks for the thread size