Injection pump having pilot and main injection

A liquid pump comprising a cylinder, a piston reciprocable within the cylinder and generally sealing with the cylinder, the cylinder having a closed end including a liquid outlet, port means to introduce a liquid into said closed end of said cylinder, and said piston including, on its peripheral surface, helical groove means fluidly connected to the closed end of the cylinder said piston including further means to communicate said port with said closed end of said cylinder when the piston is in a position between a position in which the port directly communicates with the closed end of the cylinder and a position in which the port communicates with the helical groove means, said further means comprising a second groove in the peripheral surface of the piston between said helical groove means and the part of the piston adjacent the closed end of the cylinder, characterised in that said further means further comprises a cutaway portion in the wall of said cylinder to communicate with said second groove, said cutaway portion being coaxial with an axis of said port means.