Turbochargers for internal combustion engines

Turbocharger comprising a drive-shaft mounted in bearing means in a housing, the drive-shaft drivably connecting an exhaust gas driven turbine wheel to the impeller of a compressor, a gas flow control device positioned upstream of the turbine wheel and operable to adjust the operating performance of the turbocharger, and an electrically drivable actuator motor for regulating the operation of the gas flow control device via linkage means in response to an electrical signal dependent at least upon delivery pressure of the compressor, the linkage means comprising a male threaded lead screw having a multistart thread and a correspondingly female threaded screw member threadedly engaged therewith and one of said screw member and lead screw being arranged to move generally linearly, substantially parallel to an axis of the drive-shaft, on rotation of the other of said screw member and lead screw by the actuator motor, thereby converting said rotation into movement of the gas flow control device.