Motors come in different sizes and have different clearances. This gap is important. It is about 0.5mm for a motor of about 4 kilowatts. As the air gap increases, the magnetic resistance increases, the motor generates heat and the efficiency decreases. To prevent the rotor from getting stuck, the two bearing caps must be installed concentrically, which is mainly ensured by the accuracy of machining. We call it "Spike", which is a stop, or a positioning hole. Generally, the control accuracy of small factories is not good enough. During assembly, you can only use a soft hammer to lightly tap the edge to adjust. The stator can be rotated easily without rubbing, and then the screws are tightened.