The petcock AND THE THREADS the petcock screws into just came out of my radiator when I was trying to drain it. I have no idea how I could fix this other than buying a new radiator since there is just a dime sized hole left with no threads in it.
Pull the radiator out, wire brush the area where the petcock and boss came out and do the same to the petcock boss. Take the petcock out of the threaded boss so that you don't damage it when you apply heat to the threaded boss. When both pieces are clean and shiny, you are on the right track.Then get a propane torch and some paste solder flux and plumbing solder. Lightly brush the flux on the radiator and boss mating surfaces and heat it up good with the propane torch. Then solder away...
If you are any good at sweat soldering copper pipe used in most houses, you'll have a solid, permanent repair.