The place I've sourced my spring washers is
ASI Springs in Victoria. The fellows there have been so amazingly helpful. Called them out of the blue and they nearly fell over backwards when I told them where I was to use them. But, they were so patient and helpful. Bloody awesome stuff,
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What you need to do is determine the outside diameter of the main collimation bolt. You need to use an inside washer diameter that is snug, but not tight. Too loose and the springs won't stack neatly. Too tight and they'll bind on the bolt. How strong depends on your use and preference. I don't have anything more to help strength wise as I'm experimenting too.