You need to order and install the transaxle mounting bracket kit PN 125255. They beefed up the metal thickness to stop the flexing and breaking the brackets. The main center bracket the attaches at the axles is now a lot thicker, is three bolt to frame design along a backer plate.
Also the Red Dog version of this mower also has the same problem. The only difference is the Red Dog is red and the Hustler is yellow painted.
I want to make a correction here. I keep calling the Big Dog mower a Red Dog. Just how I how I remember them. Sorry calling by the wrong name.
I just install the third kit this year on customer mowers.
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In the bottom center of the image you seen that third mounting hole. Installation instruction calls for drilling a 7/16 hole when it need to 3/8 to match the 3/8 screws.
No point making the hole bigger than necessary. Also this can be installed without removing if you are resourceful enough which is what I did on the last mower but still needed the jack stands and hydraulic jack. Also you can't simply to the jack stands at the rear as the engine guard is attached using the same screws as the rear transaxle brackets; hence, you raise the motor on the jack and replace the rear brackets first then you can use the jack stands for clearance on the rest of the job as the wheels has to be removed.