I have a 42 inch craftsman lawn tractor model no. 917.275640 and I ran over 2 large stones. It made a terrible noise and I stopped the blades. As I inspected them they were pretty chewed up, removed them though it appears that where the bolt screws into (shaft assembly mandrel) is bent. Looking into replacing this part seems to be difficult for this novice lawnmower guy. Can anyone help or advise me?