DIRTY STUFF

SOMETIMES in this often complicated go-faster game, we end up involved in strange and interesting things – in this instance, soldering brass to aluminium. The method behind this particular job came to me several years back when I was given a very old textbook with the title Modern Motor Repair & Overhauling, Volume III. When I spotted a chapter in this book on welding, brazing and metal-cutting that also made mention of soldering aluminium, I got all interested and read on.

I didn’t know that this could be done. I always believed that aluminium was one of the non-ferrous metals that couldn’t be stuck with simple soft solder, no matter what version of Bakers flux you applied. But here were these guys saying that this work skill was simple with enough care and patience and the application of heat from an ordinary oxy/acetylene welding set.

I filed those simple instructions in the mind, and then a job came up involving the fabrication of a combined aluminium swirl pot and header tank for a special cooling system. The top tank of the radiator in this car was lower than the cylinder head outlet or thermostat housing spout. This meant I needed to design a small cylindrical tank out of 100mm-diameter alloy pipe of about 3.2mm wall thickness, with top and bottom lids cut out of 1.6mm alloy plate. The inlet and outlet pipes would also be alloy and deliver the coolant into the top of tank, and then it would be rushed back out to the radiator top inlet through the swirl tank pipe at the bottom. This would need the fixing of a standardtype filler-cap neck to the top of this thing, and if we could solder the brass neck to the alloy plate forming the top of this compact swirl pot, that sure would save lots of fiddling, riveting and crossed-fingers glue.

So we had to make up this new tank and try the old skills to solder the brass filler neck solidly on top. Would it work? Was this new bendable 1.6mm alloy plate not that different in the alloy mix to still be successfully wiped with solder? This was a step into the unknown.

So we drew up and cut out the pieces for this tank, asking the local engineering shop to Tig-weld the top and bottom plates, as we didn’t have that form of metal welding gear. That done, we cut and filed new holes to accept the in and out pipes and filler neck spigot. Then I needed to tack-weld these new alloy stub pipes to the tank with our oxy gear to get the angles precise to meet the existing rubber coolant feed hose. Once that was done, I sent the swirl pot back to the engineering shop. They didn’t like our tacks, saying we had contaminated where they needed to Tig-weld around the pipes, making life difficult, but they got there anyway. We now had a neat little tank, complete apart from the necessary filler neck. From here on in, it was all up to us.

The old engineers’ technique for soldering one to the other is simple. First, lay a film of 60/40 stick solder on the base of the filler-cap neck prior to sweating this to the top of the tank, and remove all the flux to leave clean solder. Dead basic, up to here. Now, using a file or a sharpedge scraper, cut through the layer of grey oxidised aluminium in a ring around the tank hole that is ready to accept the filler neck spigot, and heat the tank with a small flame from the oxy welding set until it’s almost hot enough to melt the stick solder if this is rubbed on the top tank plate.

Through a combination of carefully applied heat, rubbed-on solder and constant scraping of the spot where you want the 60/40 solder to stick, you can actually cause a thin layer of hot solder film to form as you work the aluminium and rub on the solder. A flat silver sheen then covers the scrape marks, and if you don’t go overboard with the heat, the layer will stay there and accept enough solder to form a round bead that can then be sweated direct to the filler cap base. No flux, no inert gas needed; just constant hard scraping to expose new aluminium before oxidisation and by this mechanical action make the solder bond with the alloy. Use just enough heat to do this, as too much will simply make the lead layer ball up and disappear.

You also want to use minimal heat to sweat on the filler neck, and using Bakers liquid soldering flux on a brush, you can finish with a neat bead between the brass lower edge of the neck and the perimeter of the solder bead that you created on the top alloy plate.

This may not be the prettiest soldering joint in the trade, as scraping and adding 60/40 metal does tend to get a tad untidy. Yet it works well, and also raises the possibility of solder repairs to alloy radiators, oil coolers and tanks out in the field where MIG and TIG welders don’t exist. This skill of the old blokes could save a sad situation!

IF WE COULD SOLDER THE BRASS NECK TO THE ALLOY PLATE, THAT SURE WOULD SAVE LOTS OF FIDDLING, RIVETING AND CROSSED-FINGERS GLUE