I'm not sure what is causing it except the new engine... Its a strange thing... they let go with no warning... just explode inside the diff.... what I find are 3 gears with the shaft snapped off floating in the greasy soup... always a fun chore to repair.. I got it down to under 30 minutes... .. I will look into the ffs027... unfortunately, I just purchased 4 more sets... Never have enough spares...