I have a kitchenaid superba dishwasher. As far as I can tell for the lower washing arm--there are 4 parts The arm itself the bottom and top plastice bearing and the screw knob. My question is when screwing the knob down --it does not go flush with the top bearing. Should there be play(wobble)in the arm?

I have a Kitchenaid dishwasher (KUDS25SHAL1) and need to replace the lower wash arm upper bearing ring.... I got the part but it's all hard plastic and when I install it, the wash arm won't turn.... the old part is hard plastic on the inner ring, but is flexible/thinner plastic on the outer ring.... I have been told that I did get the right part, but how do I install the part so that the wash arm can turn?