The valve body (1) has a central bore which accommodates a valve rod (11) which has a number of grooves in the top part so as to make a star-shaped section. The bottom part (13) is cylindrical and of larger diameter so that it fits closely in the bore and acts as a piston. The fuel runs from the grooves in the star-section portion into passages (29) in the piston which run at a slight helical angle. There are pairs of small radial bores (31) running from the passages to the outer diameter of the piston near its downstream end. These bores are uncovered if the piston is moved downwards. The stop mechanism at the top of the valve rod limits its axial movement.
