if you over tighten the nut the oring will crush and squeeze out. the fitting isn't design to have a pocket for the o ring. that's why we make the bulkhead washers. this washer, has a taper to seat the oring on the fitting and another o ring on the reverse side that seals flat against the pan. the washer has the appropriate pockets to use O-rings, then you can torque the nuts to the proper value depending on size of nut.