Sealing structure for a tank which is closed by a cap

A sealing structure is provided by an O-ring disposed on a tank, suitable for use in a sprayer, and closed by a cap adapted to carry a pump actuated by a handle which extends into the tank when the cap closes the tank. The O-ring is maintained in a notch between a rim around an open end of a neck of the tank and a shelf extending outwardly from the rim. The cap has a shoulder forming an exterior angle approximately the same as the interior angle between the rim and the shelf on the neck. The shoulder has a chamfered surface which truncates the exterior angle of the shoulder. When the cap is connected to the tank by external threads around the neck and internal threads below the shoulder, the O-ring is compressed in two directions against the rim and against the shelf on the neck of the tank thereby providing an effective compression seal between the cap and the tank.