Abstract: The injector has a hollow body carrying a nozzle; a metering valve; and an electromagnet for activating an armature controlling the valve. The electromagnet is housed inside a jacket integral with an end wall fitted with a discharge fitting of the valve. The armature is guided by a guide member having a thread engaging a corresponding thread of the hollow body, independently of fitment of the body of the valve. A spacer of modular size is provided between the electromagnet and the guide member to adjust the gap between the armature and the electromagnet. The jacket is fitted to the hollow body by screwing a cover onto the hollow body.