New sending unit leaks at lock ring

I had to replace the sending unit on my daughter's 74 Dart. The new gasket didn't seal properly so I went to a real hardware store and bought some sheet stock fuel-resistant rubber that was a little thicker than the replacement gasket. I used the old gasket as a template and cut my own gasket. I used the original locking ring as it was and it was very tight when it hit the stops. There were no leaks.