posted by Capri,
There is a reason they do this instead of welding the steel tubes to the cast iron differential housing.... it's because welding on cast is a time consuming and expensive process to do correctly.
Here are two links explaining the proper way to weld cast iron.
http://www.lincolnelectric.com/knowl...ronpreheat.asp
Skip to page 5 for the good stuff in this link.
http://www.esabna.com/EUWeb/OXY_handbook/589oxy15_1.htm
Many people choose to weld their tubes, but it is almost always done incorrectly and results in a weld that looks good but has actually weakened the area instead of strengthening it.
So unless you completely disassemble the rear down to just the tubes and the carrier housing, heat it up slowly to just under 1000 degrees, weld the tubes with a nickle rod and then bury the whole piece in sand to allow it to cool very slowly, you are only giving yourself a false sense of security that the piece is stronger than stock.
If someone just grabs a mig welder and welds the tubes up with a common steel wire, when the stock factory plugs that hold the axle in place fail, so will the welds. They'll just tear away from the cast iron when the tube spins because of microscopic cracks that have formed from welding incorrectly on the cast iron.
My advice is NOT to weld the tubes unless you closely follow the directions in the link above or take it to shop that specializes in welding on cast iron.
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Found this to be informative, and since I am not a good welder, and its hard to find people you trust to do the proper work on your car i opted out of having them welded.