I recently bought a very used old brass S.P. tender on eBay from a chap in Australia. Turns out that he'd neglected to mention that both the trucks had been broken at some point and then soldered back together by someone with more ambition than soldering skill. Either both had rebroken at the badly soldered joints during shipment, or they went into the box that way...........no way to tell which.
So anyway, I break out the resistance soldering rig and put everything back together, only to discover that my normal "Tix" solder -that works great for replacing those loose bits that are constantly coming adrift from Korean brass locomotives- isn't strong enough to withstand the stress of bending the trucks back into precise alignment, and lets go all over again.
After trying this twice each on each truck with the same result each time -I'm apparently a slow learner- I dug through my soldering drawer and came upon a tube of Kester "Lead-Free flux cored silver solder" (98% tin, 2% silver) and gave it a try.
Results?
The good news is that both trucks are now correctly aligned and *much* stronger than they had been with either the original Japanese solder or my normal "Tix" stuff.
The bad news is that silver solder melts at a much higher temperature than lead-based solders, so using it on a loco boiler -or anywhere else that has a plethora of other solder joints that were made at lower temperatures- would be asking for the other joints to come loose before the silver solder even melted. But on the other hand it works just *dandy* in situations where you need strength and don't have to worry about melting previously soldered joints.
"So what kept the solder joints on the opposite sides of the trucks from melting while you were silver-soldering" you ask?
I just clipped several small alligator clips onto the truck's cross-member to act as heat-sinks, and proceeded as normal with no problems.
Pete