I have noticed some of the large rings are a flat (square) ring, and some are round.
The round ones seem to seal easier, but you pretty much have to use what you get I guess.
Putting the tank back on I will sometimes use a vice just to gently squeeze it over the sealing ring.
This is where the thin film of RTV on the lip of the tank helps a lot.
Keeping the tank from popping back off is a bit of a 3 hander sometimes.:D
