I just did 4 motors with 4mm shafts from HK along with collets. They were straight with only 1 flat spot near the end and 64mm long. I pressed them in and ground a flat for the bell set screw and about 1/2inch stick out for the collets. After asembly with the bronze thrust bearing I made another flat spot for a drilled out wheel coller to the rear shaft portion. I allowed a slight movement in the setting. Excess shaft can be cut off. You have to search the length shaft you want. They were out of shafts with circlip grooves. I just worked around it.