I can think of a way to do this.  Not sure if is the best.  Group the report by Piece_Id.  Create three formulas.  The first one would define a shared StringVar and set it to "" (example: shared stringvar defect_cd := "";).  The second one (of course would have the shared stringvar, the same as the first formula) which would be in the detail section, would have a line something like this: defect_cd := defect_cd & ", " & {defect_code}.

You would not be able to print what you want in the details section, thus that is why I suggested putting it in the group footer (suppress group header and details).  To anyone viewing the report, they should not be able to tell it is not in the details section.

Ok thanks I actually got it working but I also had to just show the one defect code if the piece_ids did not match.  If not the code just appended all of the defect_codes together...