A fuel pump for a direct injection system having: a pumping chamber; a piston, which is mounted in a sliding manner on the inside of the pumping chamber; an intake channel, which originates from a wall of the pumping chamber and has a housing portion; an intake valve, which is inserted into the housing portion of the intake channel; and a locking ring nut with an annular shape, which is threaded and is screwed into the housing portion of the intake channel so as to strike against the intake valve; the housing portion has an annular abutment, which reduces the diameter of the housing portion and against which the intake valve strikes; the locking ring nut is screwed into the housing portion with a tightening force that is sufficient to determine a plastic deformation of the annular abutment against the intake valve.