A fluid injector, comprising: a fluid inlet disposed at a first end and configured to receive a fluid, and a fluid outlet disposed at a second end of the fluid injector for discharging the fluid, the fluid injector defining a fluid path for the fluid from the fluid inlet to the fluid outlet; a tube member having an end disposed at or near the fluid inlet of the fluid injector, the tube member configured to pass fluid along the fluid path; a calibration tube disposed in the tube member, and includes a bore defined in an axial direction through the calibration tube, the bore defining at least a portion of the fluid path through the fluid injector; an actuator unit disposed within the fluid injector downstream of the calibration filter tube, relative to a direction of flow of fluid along the fluid path from the fluid inlet to the fluid outlet of the fluid injector, the actuator unit engaging an end of the calibration tube; a valve assembly operatively coupled to the actuator unit, wherein a position of the calibration tube within the tube member sets an opposing opening force of the valve assembly; and a spacer member (180,180') disposed between a downstream end portion of the actuator unit and a portion of the valve assembly; wherein the valve assembly comprises a valve body portion and a pin member at least partly disposed therein, the valve body portion including a shoulder portion disposed orthogonal to a longitudinal axis of the fluid injector; and wherein the spacer member (180,180') is disposed downstream of a downstream end portion of the armature and disposed upstream of the shoulder portion of the valve body portion, and characterized in that an inner surface of the valve body portion gradually protrudes radially inwardly and then gradually widens downstream of the radial protrusion to at least partly define a space in which the spacer member is disposed, a diameter of the inner surface of the valve body portion defining the space being greater than a diameter of the inner surface of the valve body portion elsewhere. a fluid inlet disposed at a first end and configured to receive a fluid, and a fluid outlet disposed at a second end of the fluid injector for discharging the fluid, the fluid injector defining a fluid path for the fluid from the fluid inlet to the fluid outlet; a tube member having an end disposed at or near the fluid inlet of the fluid injector, the tube member configured to pass fluid along the fluid path; a calibration tube disposed in the tube member, and includes a bore defined in an axial direction through the calibration tube, the bore defining at least a portion of the fluid path through the fluid injector; an actuator unit disposed within the fluid injector downstream of the calibration filter tube, relative to a direction of flow of fluid along the fluid path from the fluid inlet to the fluid outlet of the fluid injector, the actuator unit engaging an end of the calibration tube; a valve assembly operatively coupled to the actuator unit, wherein a position of the calibration tube within the tube member sets an opposing opening force of the valve assembly; and a spacer member (180,180') disposed between a downstream end portion of the actuator unit and a portion of the valve assembly; wherein the valve assembly comprises a valve body portion and a pin member at least partly disposed therein, the valve body portion including a shoulder portion disposed orthogonal to a longitudinal axis of the fluid injector; and wherein the spacer member (180,180') is disposed downstream of a downstream end portion of the armature and disposed upstream of the shoulder portion of the valve body portion, and characterized in that an inner surface of the valve body portion gradually protrudes radially inwardly and then gradually widens downstream of the radial protrusion to at least partly define a space in which the spacer member is disposed, a diameter of the inner surface of the valve body portion defining the space being greater than a diameter of the inner surface of the valve body portion elsewhere.