Mr. H,
I would recommend getting a new stop collar. There are a couple of examples below from US suppliers, but I'm sure that you should be able to find something similar in your neck of the woods.
http://pricecutter.com/stop-collar-kits/p/198-8043/ http://pricecutter.com/35-piece-rout...it/p/199-9641/
Washers aren't the most precision manufactured items in the world, and with their irregular surfaces, diameters, etc, they might induce a little vibration in your machine spinning at 20,000 rpm. A machined stop collar is probably a more proper choice.
Just my 2 cents,