Just spoke with Pat Gonsolan about this.  He is pretty sure the builder of my A/C did not properly align the spinner rotation axis before drilling the mounting holes, thus causing excessive loads to the back plate due to wobble.

He also mentioned a fix some have used involving a large gasket or o-ring to form a front "bulkhead".  He said this item came from a 5-gallon office water cooler but did not have access to the source anymore.  DOES ANYONE KNOW HOW TO LOCATE THIS PART?

This part fit tightly over the dome of the prop and must have been fairly thick because it would support the inner diameter of the spinner.  You simply put it on the dome near the front and pushed on the spinner.  On succeeding trials you would slide the o-ring farther onto the dome, then re-try the spinner until the spinner would just reach the proper location on the back plate.  When the fit was just right, the o-ring was secured in place by tie wraps fore and aft so it could no longer move.

Tom Thibault
N4XB