?? its the internal part of the differential that contains the spider gears. It also hold the ring gear.I assume your wanting to do this for drifting or drag racing.
BTW make sure your diff is not a limited slip its worth too much to weld...
Dave
__________________ Dave - PDX 1995 318ti - Active Black and Tan. 2005 330xi - Mtech 1 - 6spd - Orient Blue/Black