plane 1- C/S rear plate found 2 ring cracks around bolt holes.
plane 2- found one rivet missing, two others loose, between rear plate doubler and mounting plate
plane 3- C/S rear plate found crack from center hole thru to outter edge, suprisingly the doubler plate hadn't cracked (yet)
These planes rear mounting plates were replaced and props balanced, problems haven't returned in over 5 years.
plane 4- C/S front plate found cracks around bolt holes, customer complained of low RPM shimmy
This one was different, couldn't get good balance for low RPMs and finally found gross imbalance in spinner itself (Chromed aluminum), changed out spinner and re-balanced, problem cause solved.
Maintain lots, upgraded - repaired - & modified more, rebuild a few, & built some too.