Fuel sending unit leaking

My 72 is leaking fuel at the sending unit at the bottom of the tank. It did this a few months back and I replace the rubber o ring gasket and it stopped but the other day it started again. The locking ring just doesn't seem to be tight enough. You can almost lock it in by hand. Any suggestions?

Try going up one diameter in thickness on the o-ring. 

Just received a new seal from Ecklers today... it's thicker than the old one. Wondering if maybe they sent me the wrong one before. We will see what happens.