An air atomising nozzle assembly for electrostatic spray guns is disclosed. The nozzle assembly is made of polyamide-imide («Torlon») plastic parts and includes an air cap having a central bore which surrounds a fluid nozzle tip. The fluid nozzle tip has a nut formed on its forward end to facilitate threading of the rearward end of the nozzle tip into a threaded bore of the gun body. Formed on the front part of the nozzle, either as a part of the nut or a cylindrical surface adjacent the nut, is an accurately machined surface engageable in press fit relationship with a mating accurately machined bore of the air cap. These mating press fit surfaces centre the bore of the air cap relative to the nozzle tip so as to leave an annular, evenly dimensioned air flow passage surrounding the nozzle tip and defined between the nozzle tip and the bore of the air cap. The air cap is retained in a one-piece resilient retaining ring by snapping the air cap into an annular lip of the retaining ring. The retaining ring is then threaded onto the gun body to secure the nozzle assembly onto the gun body.