The cathode collector bar end portion extending through a window in a sidewall of an electrolytic cell for refining aluminum is snugly received in a central opening of a seal assembly. Such seal assembly maintains a hermetic seal preventing ingress of air through the sidewall window while permitting longitudinal (horizontal) movement of the collector bar and also movement in a vertical plane (side to side, or up and down, or diagonally) which can be caused by changing heat conditions inside the cell.