An electromagnet housed in a skirt pressed on to a cap integral with the in conduit of the control valve. The skirt presents two shoulders separated by a distance greater than the height of the core of the electromagnet and on which the core and the cap rest. At least one Belleville washer between the core and the cap holds the core elastically on the respective shoulder and detached from the cap. A retaining ring coaxial with the washer is also provided for sealing the fuel flowing in an axial hole on the core and in the drain conduit.