I know ZERO people who have purchased an original equipment unit from Mazda and had someone weld it. I've heard ZERO stories on the forums of people purchasing an original equipment unit from Mazda and having someone weld it. Where have you seen this? Why would you pay $500 for an OEM diff, then pay someone $50 to weld it and it be guaranteed to be out of balance, when you could purchase an MFactory unit for $630???
But, more importantly, 1,000,000:1 it's the differential.
