A casement window assembly has a sash window pivotally connected to a window frame. A window operator has a housing connected to the frame, including an upper and lower piece connected together. The housing has an offset countersink hole receiving a fastener. A spindle extends into the housing and has a groove therein. An arm is operably connected to the spindle and to the sash window to open and close the sash window. A resilient seal member is received within the groove in the spindle. A first gasket is positioned between the upper piece and the lower piece of the housing. A second gasket is positioned between the housing and the window frame. When the fastener is driven into the countersink hole, the housing is forced in a generally horizontal direction to compress the second gasket between the housing and the window frame.