Abstract: The present invention is directed to open nozzle unit fuel injectors for injecting a metered quantity of fuel into the cylinder of an internal combustion engine, as synchronously controlled by a drive train, wherein the unit fuel injector comprises a body with an axial bore and a plunger assembly movable therein between a retracted position and an advanced position. The plunger includes a major diameter section, slidably movable in the axial bore to open and close a fuel supply orifice and a minor diameter section which extends in a bore of a cup portion of the injector body. The cup portion has an internal surface including plural diameter portions connected by an annular step.