I am looking at soldering a cast brass finial to 38 mm brass handrail tube because the fit is too loose for using grubs-crews. Can anyone tell me if a decorators-style propane lamp be hot enough for this, and will the heat discolor the metal permanently? I am used to soldering copper tube joints as big as this, but not heavy gauge and not brass.
I have silver soldered this size in the past for a large commercial ice maker. I did use an expander to stretch the copper to be a snug fit in the fitting (German ice maker US made compressor) after silver soldering I cleaned up the pipe and manifold with a light (400 grit) sandpaper then sprayed with a clear sealer since this was used with food production and inspectors look for anything ugly. Even though this is just a menachical joint I would use a swage tool to expand the copper for a snug fit or your solder connection will be the same as a grub screw.
Solder doesn't really fill hole or gaps very well. In regards to discoloration, brass is usually a little better than copper but if it's a sheet type of brass I find can discolored quite a bit. Without expanding or shrinking or filling something as Ed Beal mentioned in his answer you will find it very difficult to fill with solder. Brass also seems to have a fairly high heat capacity, for example soldering copper into a brass fitting versus a copper fitting you need more heat for the brass. If you are able to contain the solder then you should be able to get a fairly good joint also using a solder with higher lead content may prove to be easier but also toxic. Lead solder may not be available anymore at hardware stores but you may find it at HVAC supply stores. I don't use it but some guys still use it on boiler systems. It's messy but melts at a lower temperature.