(57) A multi-stage centrifugal pump with an axially split casing (10) and a number of impellers (14) carried by a drive shaft (12) has inter-stage bushings (24) and rings (22) for sealing each side of each impeller. Each bushing (24) or ring (22) is conventionally prevented from rotating by a radial pin protruding from the lower half of the casing (10) into an axial slot in the periphery of the bushing or ring. The pin is necessarily small and is liable to shear, difficult to engage in the slot and, in cases where the bushing (24) or ring (22) is diametrically split, does not allow rotation of the lower half for its removal without lifting the shaft (12). To avoid all these disadvantages, the exposed end (37) of a tangential pin (32) fitted in the casing (10) is flush with the join face (28) of the casing and abuts against a shoulder (44) formed by a recess (38) in the periphery of the bushing (24) or ring (22). This pin (32) can be larger than the known radial pin and is subjected to compression, not shear.