A soldering iron will not get hot enough for silver solder. A oxy-acetylene torch or maybe a MAPP gas torch must be used. Silver solder takes a specific flux which must be purchased too...unless you find a flux covered rod.

I guess the part of this thread I am missing is why use silver solder at all. The benefit of the silver solder is its high strength. Most radiator applications do fine with the strength provided by regular soft solder. It's much more about sealing two pieces for a water tight pressure joint than it is about brute strength.

Stuart