Abstract: A preassembled unit or inner cartridge for a multi-stage barrel type centrifugal pump including bearings, seals, balancing device, impellers and diffusers is aligned, balanced and pressed together into assembled position by a resilient means for stressing the parts against a pair of spaced handling clamps at opposite ends of the preassembled unit which permits the shaft to act as an elongated tie bolt to hold the preassembled unit or cartridge so that it can be installed or removed easily from the fixed outer casing of the multi-stage barrel type centrifugal pump.The unit or cartridge is preassembled and aligned in a special supporting frame which duplicates the engagement shoulder and end face on the pump casing so that all adjustments can be made easily and precisely at a point away from the pump.