From what I have read it seems it was noted that turbines fitted with O rings ran quieter and also helps to stop the outer case of the bearing from rotating.

If not using a rear O ring then you would need to be certain the bearing is a good fit in the tunnel. My old Jetjoe tunnel was 16.01 at the front and why I fitted a new one when I rebuilt it, the new one I have for my wren is 15.99