Repairing cast iron radiators?

If it is one that has the bolts at the top and bottome, then you can remove the nuts and separate the sections at the cracked one and take it out. If you are lucky, the push nipples will come out so you do not have to shift them between sections. If you do have to move them, use a piece of pipe to drive them out of the cracked section. If the ones you do not need stay in the good sections, then you have to use a chisel and sort of rock them back and forth to remove them. Put pipe joint compound on the nipples before pushing the good sections back together. Then cut a new set of shorter bolts, or cut off and rethread the old ones. You will then have to modify the piping to accomodate the shorter radiator. The success rate is 99%