Abstract: Two axially bored tappet valves 11, 12 are rotatably mounted in a mixing head housing 10. Primary fluid components A and B are supplied to the respective bores through radial passages 21, together with fluid additives C and D supplied through longitudinally displaced radial passages 22, whereby these components A, C and B, D undergo preliminary mixing in collection chambers 19, 20 defined by the bores. The pre-mixed components are then supplied to a final mixing chamber bore 13 defined in the housing, through nozzles 23, 24, from which they may be expelled by an axially reciprocable push rod 14. Prior to expulsion the valves are rotated in unison, which blocks the inlet passages 21, 22 and communicates the pressurized fluid components with respective tangential return passages 25, 26 via partial circumferential grooves 27, 28 in the valves. Such rotation also aligns recesses 29 in the valve bodies with the chamber 13 to allow the push rod to travel freely therethrough.