A nozzle assembly for generating fluid jets, such as self-spinning fluid jets. A nozzle body includes a cylindrical nozzle cavity, an inlet, an outlet, and a tubular nozzle rotor assembly positioned with respect to the nozzle cavity. The nozzle rotor assembly can have a ball end and a tubular end and a central fluid passage routed through the nozzle rotor assembly. A fluid swirl enhancer forms communication with the inlet. A replaceable nozzle seat forms communication with the ball end of the nozzle rotor assembly and with the outlet. The ball end of the nozzle rotor assembly pivots, oscillates and rotates against the nozzle seat when pressurized fluid flows through the nozzle assembly.