This piqued my curiosity, so I've just had a furtle in the shed with my set of BOC nozzles. I have a set of the swaged copper nozzles to suit a BOC Sapphire 3 torch, and by dint of poking bits of wire and small drills up the orifice I've come up with the following:

#1 nozzle is 0.6 mm dia, #3 = 0.8 mm, #5 = 1.0 mm

#7 = 1.3 mm, #10 = 1.4 mm, #13 = 1.6 mm

#16 = 1.8 – 1.9mm (1.8 is loose, 1.9 won't go), #25 = 2.0 mm, 35 = 2.4 mm

It's hardly a precision measurement but it'll give a rough guide.

If anyone has got one of the sets of wire reamers BOC sold for nozzle cleaners these would give the sizes more accurately. I've always assumed that the numbering system, though peculiar to BOC, was the same for all their torches, but I haven't checked. Am I right in thinking the numbering system is related to the fuel gas consumption in some way?

As Fizzy says above, for silver soldering something as large and as heat-dissipative as a copper boiler you'll want a bloody big flame and I would think quite possibly a second torch – probably propane – to help with getting the whole thing pretty hot, then go in with the oxy-acetylene to trim the local temp' and get the solder to flow. Getting a structurally sound joint is a matter of getting on with the job, getting a structurally sound and pretty result is another stage of magic again. My Dad was a silversmith, and together we hard soldered some pretty crazy things (the broken antler on a brass statue of a deer was one) but you don't have time to hang around before the flux stops absorbing oxygen and you're knackered. That was with JM EasyFlo flux; I gather there are better fluxes around now, so good luck!

Do let us know how it goes. Rgds Simon

Oops! My apologies, I think I've just committed the sin of trying to teach my grandmother to suck eggs, though in this case it's my grandfather to cook them!

But I'd love to see the results?

Anyone tried tig brazing a boiler?

John, I still dont get it? Just put a decent size tip in and turn it up or down as desired. Your not using the torch to weld so anything quoted for welding will be irrelevant (am I missing something?). A boiler the size of yours will need a lot of heat and quite rightly you are also using propane.If youve not used oxy for silver soldering before let me give you some advice (If you have then great). You want a big flame/nozzle size and you want it running oxy weak. The cone needs to be at least an inch long so as to protect your boiler. If you use a fierce flame as you would for welding you will either end up burning the copper or melting a tube, or both. Oxy looks like the ideal solution to heating but it requires a much greater degree of skill and caution than propane. Dont try to get it up to temoerature too quickly, and dont point the flame directly at the joint. If you do then the silder melts and is blown everywhere except where you want it to be. I tell you these things from experience, not from the armchair. Hope all goes well.