In spite of making the arbors shown in the reference in post 2 above, I had also collected a number of those motor shaft arbors that fit a 5/8" shaft but I found it a nuisance that the set screws were not Shopsmith hex wrench friendly, plus the socket wall is so thin, the set screws were falling out onto the floor and disappearing.
Recently I gathered all mine up and made collars for them on my metal lathe out of tubing. The collars are thick enough to retain the loosened set screw. The collars were made to be a press fit onto the socket ( I have a 2 ton press) though a couple were bored out slightly loose and had to be fastened on with Loctite red threadlocker before drilling the set screw hole with a letter F drill and being tapped 5/16NC18 for the setscrew. Here they are:
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I was in Lee Valley Tools locally a year ago and their bargain table had the arbor washers on sale so I am glad I bought a bunch back then.
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BTW, the drill chucks on these arbors do not run as concentrically as a genuine Shopsmith chuck.
Edit: Sorry, Artfuldodger, I did not answer your questions. Note that my single set screw arbors are used on the Shopsmith slanted face shaft. IMHO, on a motor where the arbor will not be constantly removed and replaced, I would go with 2 setscrews. I noticed that a lot of the smaller original screws I removed had pointed tips to dig into the motor shaft. I think 2 screws at 90 deg. are better than 2 in line, but all the factory made ones I have seen, have them in line! Given the 4 mo. time since your post, what did you do?