Abstract: A positive displacement type pump used for applications such as pressurizing beverage dispensing containers, filling fire extinguishers, and pressuring welding tanks. CO.sub.2 or similar fluids are drawn into and discharged through the pump by a reciprocating piston driven by a rotating eccentric main bearing mounted on a bearing frame. The bearing frame comprises an open, unsealed pair of structural plates connected by tie rods, the main bearing itself being sealed by one of two seal arrangements to make it easily convertible between oil-lubricated and oil-free operation. For oil-lubricated operation, the main bearing is supplied with a surrounding sealed bearing sleeve compressed between the plates of the bearing frame. For oil-free operation, the main bearing is internally sealed. The main bearing is driven by a shaft supported on bearings in the plates, the bearings also being changeable between sealed and unsealed bearings for oil-free and oil-lubricated operations, respectively.