I did go to the spring supplier and had some interesting talk.

I just got 3 different discs of 8.2 x 15 in 0.6, 0.7 and 0.8 mm thickness. I grinded down the 0.6 discs to a diameter of 12.2 mm and they do fit.
It seems that only the nut has to be tighten so no real pe-tension is needed.
I do not come to 12 discs, it fits but it is very difficult to screw on the nut.