I just replaced my electric priming pump on my SM #335 Onan Genset. I did not have access at the time to a proper FLARE NUT WRENCH when I did this. I ended up distorting the "B-nut" on the small copper fuel line that goes from the fuel pump to the fuel filter adapter and could NOT get a proper seal upon replacing this line (a continuous small seep of fuel that was really annoying and messy). I had to order a replacement line (not that expensive) and buy a proper set of flare nut wrenches to adequately get a seal. I wish I could remember the size of the flare nut but it was SAE (not metric) sized. Make sure the fuel tank is low on fuel (think 250 liter will work) otherwise a continuous stream of fuel will flow by syphon from the fuel tank fuel return line while you make this plumbing change. I managed to turn this otherwise simple replacement into a major project based on A) not using the proper tools to loosen and re-install the B-nut of the fuel line, B) not have lowered the fuel in the tank (mine was full at the time of my work).
Good luck. The pump was about $65 as I recall (not that expensive in the scheme of things) and the fuel line was about $8
Gary S. Silver
Amel SM #335 Puerto Rico