I replace the stock 5/16" sending unit on my 70' cuda with a aftermarket 3/8's with the additional 5/16th port for a return line. I used the new lock ring and gasket that came with the sending unit, but now the tank will not seal properly.

I wanted to put the stock lock ring back on but of course when I check the box with the old pickup the lock ring is missing.

Has anyone else had problems with this? Could the rubber gasket be too thin?

Would love to more than 6 gallons of gas in the tank at a time.