A turbocharger comprises a compressor wheel (7) assembled to one end of a turbocharger shaft (8). A nut (17) is threaded on to the end of the shaft (8) to clamp the wheel (7) on the shaft to prevent both axial movement of the wheel (7) along the shaft (8) and rotational slippage. The torque capacity of the coupling between the compressor wheel (7) and nut (17) is increased by treating contacting surfaces of the coupling to increase their co-efficient of friction.