You are absolutely correct...the thread on the spindle WAS 20mm X 1.0...but any more. The factory thread was somehow cut out of parallel (?) with the axis of the spindle and would not allow a threaded piece to sit flat on my poly pulley. I dialed the spindle in on the lathe and cut a new thread...3/4" X 16 and now everything is square, plumb and proper.
How those factory threads were cut out of whack is beyond me. I machined a disc and single pointed internal 20mmX1.0 threads in it, screwed it onto the spindle...it wobbled like a drunken sailor.
Stuart