A fuel injection pump for a piston engine, the pump comprising a cylinder element having a pressure plenum provided with an outlet chamber for removing pressurized fuel from the pressure plenum, a piston arranged to reciprocate inside the pressure plenum, an inlet chamber arranged outside the pressure plenum and at least one inlet channel arranged between the pressure plenum and the inlet chamber. At least one fill channel provided with a non-return valve is arranged between the pressure plenum and the inlet chamber, the valve allowing fuel flow from the inlet chamber to the pressure plenum but preventing flow from the pressure plenum to the inlet chamber.