Thanks Peter, I’m not Very mechanical inclined. That makes plenty of sense though. I will see what I can doJust a further note: shims are used to get the exact thickness, trying to use as few as possible. There isn't usually a particular magic to two or three or four... it will depend on the wear and tolerances for your particular hub/gear and may differ left / right on the same tractor and of course between tractors. I.e. don't read too much into the fact that the figure shows four.
All the best,
Peter