Filtration Tank Retainer Ring

I recently replaced the motor recommended by you guys and it’s working awesome!  Since then, I’ve developed a small leak where the top and bottom of the jandy tanks come together.  I didn’t put the retaining ring back as it broke due to corrosion.  Is the metal retaining ring needed or could I just replace the O-ring?  Another dumb question…does the retaining ring go on the inside or outside of the filter tanks?