I worked in a radiator shop for a few years after I left school. I know the repairs look very simple, but they’re only simple for someone with the right gear and knowledge. (We made a lot of money from fixing badly done amateur repairs.) So, if there is a repair shop near you, it would be a good idea to take it there and talk through the problems—they might be able to fix it relatively cheaply.
I could not find a YouTube video that you could follow so I will try to explain what to do.
If you do decide to have a go, you need to get some bar solder. We used 18-inch lengths of a ¼ inch by 1/8 inch solder, not the stuff you use for electronics. For flux, we used a liquid flux, not the paste flux plumbers use. The liquid flux is important because it’s used to cool things down if anything gets too hot and to wash away dirt.
The torches we used were very controllable, from a very thin pencil flame for fine work like fixing tubes in the core, up to a wide flame to run the solder out of the joint between core and tanks. And they were not that hot.
The first repair I would do would be the pipe going into the header, as it’s the easiest. First, clean the area with a wire brush, be careful not to hit the tubes in the core.
The next tool you will need is a piece of stiff wire with a flattened end. You could use a bike spoke--cut off the threaded end and flatten the last 1/8 to ¼ inch with a hammer. This tool is used to help melting solder flow, cleaning around the area to be repaired when hot, holding fittings in place when you run solder around them and a multitude of other uses.
With the blow torch add heat, just enough to melt the solder you want to remove and nothing else. Slowly heat the area around where the pipe goes into the header tank, using the wire (bike spoke) to help the solder on its way and to clean out any small bits of paint, rust, etc. The cleaner the repair area, the better.
If anything starts to get too hot, remove the flame and douse with the liquid flux. Use a small paintbrush, but put the flux in an old coffee cup and keep it close by. I would hold the paintbrush and the torch in the same hand so I could hit the repair with the flux instantly if needed.
Once cleaned, tin around the hole in the tank and the outside of the pipe. To tin means to use the smallest amount of solder to make a very thin film over the areas to be joined.
Once the area is tinned, heat the area again, then add the flux and start adding new solder. Use the smallest amount of solder, add flux, then add more solder until you have made the joint. With soldering, you do not need a big lump around the joint. Cool using the flux.
Rinse with clean water when cold to remove the remaining flux as this can corrode the copper over time.
To fix the core tubes: first, be careful, because it’s really easy to mess this bit up! Warm the area of the leak, using a pencil flame and as little heat as possible. While heating with the torch, clean the area with wire, add flux, clean with wire again, add more flux. The flux will help wash away any dirt you missed with the wire. If things get out of hand remember to use the flux to cool things down.
Add the smallest amount possible of solder to tin the metal, add more flux, and allow the solder to run over the hole, then remove the heat. This all should take 5 – 6 seconds per hole. Cool, then rinse with fresh water.
Pressure test the radiator to just above the working pressure. So, if the cap is 15lbs pressure, test at 17lbs. We did this with a compressor with a regulator, some rubber bungs/stoppers, and a tank of water.
The other way to fix a hole in a core tube or when a tube is too rotten to repair is cut through the tube at the bottom and the top and solder up the holes where the tubes go into the top and bottom tanks. This is skilled work and I would not recommend doing it unless you were a pro as you can quickly destroy a radiator core if you get this wrong.
I would suggest you get a few copper sheets and practice before doing any of this. Drill small holes and try filling them with the least amount of solder possible, drill two holes close together, solder over one hole, then try filling the other hole without making the solder in the first one run. Try joining two pieces together, etc. Once you’re happy with your technique, start on the radiator. [I had six months of someone looking over my shoulder all day long before I was allowed to repair customer’s radiators on my own.]
Finally, the worst thing you can do is get it all too hot, as the solder will run from the nearby joints, making the job much bigger. Also, too much heat can start damaging the copper.