Ford C4/C5 sun shell saver shim raises the sun gear slightly with a worn sun shell. When the sun gear splines "sink" into the shell, the bottom of the front planetary carrier can rub the inside of the shell causing significant transmission damage. The sun shell rub is not an aftermarket planetary problem; it can happen with an OEM 3 pinion front planetary and is something that needs to be checked by the builder.
 
The Shell Saver from Andrews Racing Transmissions allows a slightly worn shell to be used instead of being thrown away.
 
Worn/rubbed planetary and shell shown in pictures for reference.