When the GDL/CDL multi-stage pump is assembled, all threads of bolts and nuts must be coated with a layer of lead powder oil. After assembling the final stage impeller, the axial distance between the hub and the two ends of the balance disc hub must be measured, and the axial dimension of the retaining sleeve in between should be determined according to this axial distance. The sum of the axial gap between the retaining sleeve and the impeller hub, the retaining sleeve and the balance disc hub is 0.30~0.50mm. Because in the initial stage of GDL/CDL multi-stage pumps, the parts on the impeller and other shafts are first affected by the medium with higher temperature and temperature, and the shaft is affected by heat. After that, their expansion has a time difference. The axial gap of 0.3~0.5mm is left to prevent the impeller, balance disk, etc. from expanding first and collapsing each other, resulting in greater tensile stress on the pump shaft.