Silver soldering large copper boilers, how do you get the heat required?

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Kaleb

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I've got a copper loco type boiler that I'm working on that's about 90mm (3 1/2") in diameter by around 380mm (15 3/4") long, but I'm having problems getting enough heat into it to perform silver soldering. Repeatedly heating it up to fill in leaks and put bushings in is leaving me short of gas very quickly. The obvious solution would be to use oxy-acetylene, but the cylinder hire and gas fees for even the small D size cylinders would cost me well over $500 a year, and that doesn't include the expense of the other equipment needed. It seems you can't buy the cylinders outright in Australia, so oxy-acetylene is out of the question as far as I'm concerned. What does everyone else use for this kind of work? I've currently got a Primus LPG torch and a MAPP gas torch, and I have been using a hearth.
 
I've seen an aluminum casting for a snowblower engine preheated on a gas BBQ grill. The brazing was then done on the grill. Crazy, but it might work for your boiler. My boiler is 3" in diameter and 12" long. It is a riveted boiler so it only needed a low percentage silver solder to make it water tight. This solder has a lower melting point. My total experience is one boiler so don't trust this to be gospel. There are many more experienced boiler builders here so I'm anxious to hear their thoughts.
 
What size burner are you using on your propane torch, you want something about 30-40mm dia to get a decent btu output and at least a 7kg bottle to supply it.

Also surround the boiler with proper firebricks to reflect the heat back into the work
 
Fire bricks all round enclose as much as you can just leave one side to work in , I use a 30mm torch with a flare of about 100mm long , to stop it blowing out from the blow back that has no oxygen left in it .
 
Oxy-acetylene is actually too hot. It's very easy to melt the copper and put a hole right through the boiler. A rosebud tip in the hands of an expert can be very effective, but for those of us in home shops it's probably not the best choice for silver brazing. A large propane torch (what a friend of mine calls "small dragon size") with firebrick stacked to create a heat-retaining chamber for the boiler is likely to be your best solution.

Just stacking up the firebrick around what you're working on makes a huge difference. Your present torch may turn out to be large enough.
 

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