Fuel injector assembly

A fuel injector assembly (1,100,110,120) incorporating a fuel pump (5) of the type comprising an injector body (2) and a two-stage snap-action lift stepped piston (7) mounted in a bore (8) in the injector body (2) having a gas passage (9) at one end for communicating with an engine combustion chamber, the piston (7) having a wider part (10) slidable within the bore (8) and a narrowed part extending through the gas passage and urged by a timing spring (16) into engagement with a first internal sealing land (19) for the gas passage (9) to form a gas-tight seal until the pressure of combustion chamber gases acting on an outer portion of the narrowed part is sufficient to overcome spring pressure and move the piston (7) for inlet of combustion chamber gases to act on the wider part (10) of the piston (7) whereby the piston (7) snaps inwardly under the increased force acting against the spring (16) due to the increase in exposed area acted on by the gases, valve means (20,21) for sealing the gas passage (9) when the piston (7) is moved inwardly, a fuel pump (5) mounted on the piston (7), the pump (5) comprising a plunger (14) for reciprocal pumping movement within a complementary fuel pump cylinder (15) communicating with the bore (8) to deliver a measured quantity of fuel to a nozzle assembly (6) mounted on the outer end the piston (7) in the gas passage (9) in the nozzle assembly (6) forming part of the narrowed part of the piston, through fuel passage (66) in the piston (7) between the fuel pump cylinder (5) and the nozzle assembly (6), characterised in that the valve means (20,21) is formed by a second external sealing land and (20) for the gas passage (9) an the nozzle assembly (6) which is engageable against the second external sealing land (20) upon inward snapping of the piston (7) to form a gas tight seal for retention of purging air.