Compressor wheel assembly

A compressor wheel assembly comprises a compressor wheel (7) mounted to a rotating shaft (8) which extends through a bore provided along the rotational axis of the wheel. The bore has a first axial portion (21) corresponding in diameter to the diameter of the shaft, and a second axial portion (22) of enlarged diameter, such that the inner surface of the second portion of the bore (22) is radially spaced from the shaft (8).