A fuel injector includes a valve member for selectively preventing and permitting flow of fuel through a valve seat by selectively engaging and disengaging the valve seat, an actuator for selectively applying a force to the valve member in order to unseat the valve member from the valve seat, and a return spring which biases the valve member into contact with the valve seat when the actuator is not applying the force to the valve member. A calibration tube is positioned within the fuel tube and holds the return spring in compression so as to set a force on the return spring to achieve a desired dynamic flow of fuel through the fuel injector. The calibration tube includes at least one protrusion extending outward therefrom such that the protrusion engages the fuel tube, thereby creating a spring fit between the calibration tube and the fuel tube.