Sugar Bear, after reviewing your post in regards to what caused the old one to break. If you are referring as to why I had the rear transmission case housing to replaced is because the old case was missing the transmission mount hole at the bottom of the rear transmission case housing. So I assumed you are asking what caused to break? It was because of the chain reaction of flex discs failures that caused a series of damaging parts that I had to replaced. I found this website that practically explains it all. It is Failure of the Flex Disc on a 12. Sugar Bear, I hope I answered your question correctly.