Man that's weird. I had a older t4 I had since 2005 and never had to replace any gears. I ran mm7700 and 8.5t tekins no issues. Metal gear isn't going g to actually fix the issue. The only thing I can think of is that you are tighting your transmission case so tight that the binding is causing the older to lock up and fail. Your transmission your roll butter smooth. Just an idea.