I have serviced or restored over 40 Buren cal 11 through 15s in the past 10 years and I have found that the majority of the bearings in the micro rotors have excessive wear and are too loose. This can cause insufficient auto winding and in the worst cases, the rotors rub and wear on the inner movement plates. This is due to a design that was flawed from the beginning (steel on steel and a contact surface that is way too small) and certainly was never intended to last 40 years.
As far as I know, the bearing are not available separately and the last NOS rotor unit I could find cost me $175.00! So a couple of weeks ago I starting looking into a replacement bearing. I had hoped that I could find a ball bearing unit that small (1.58mm OD) but it was something most bearing makers did not even want to consider and if they did, the cost was too high. Additionally, the base (and/or the support shaft) would have to be modified making it all to expensive. I am now looking into having a ruby sapphire jewel bearing made as a direct replacement. This is still not going to be cheap. So, I would like some input from all of you collectors of chronos with these movements, especially those who have had the same rotor problems. Any ideas or suggestions will be appreciated. Thanks, Craig