There is provided an airless paint spray pump wherein the pump is a double acting piston pump having an inlet communicating with a source of paint, a motor for driving the pump, a pressure controller for controlling the pressure of the pressurized paint delivered by the pump, and a filter for filtering the paint delivered by the pump. The pump includes features which increase the accessibility of the components thereof, prevent the incorrect installation of the seal packings in the pump cylinder, and allow the assembly of the piston in the pump cylinder properly aligned with the cylinder so as not to damage the seal packings therein.