The breaking part is the butterfly plate screws, and it's the flutter in the plates themselves that fatigues and breaks them. I don't know if the plates are whanging against the sides of the intake or just fluttering in free air, but it's the worn bushings that allow the flutter. I'm not sure if the internal bushing wear is enough to allow the shafts to flutter, or if the end bushings have to be worn out for it to happen. I guess Nate will find out for us.

Not responsible for advice not taken...