At several of the antique tractor and engine shows in the area, there is a man in a small trailer who demonstrates the use of an aluminum soldering rod that he sells. Upon the recommendation of other model builders and engine restorers who have used the product, I watched and listened to his demonstration. Then he let me solder some aluminum, right there at his display. I then invested in some of the aluminum soldering rod that he sells and took it home to experiment. From all of this I learned that aluminum solder DOES WORK and can be very valuable in our hobby.
Let's start by describing some of the things that can be done with aluminum solder, then we will discuss the methods. A small gasoline engine crank case that had the side broken out, because the broken connecting rod had gone through the crank case, had been patched with aluminum solder. A motorcycle valve cover, that had a threaded boss broken off of it had the boss soldered back in place. A gear case cover that had a loose chain rub its way through the cover was repaired with the aluminum solder. The carburetor mounting threads on an engine intake manifold had been stripped out. There was no room for an insert, but the part was saved with the aluminum solder. Model airplane engines were repaired with the product. Trim parts for antique cars, made of die cast 'pot metal' were repaired and patched with the solder. An aluminum boat propeller (a real propeller, not a model) that had a chunk broken out was repaired with this aluminum solder.
This is an improved version of aluminum solder. The earlier solder worked just as well, but required more surface preparation, namely, brushing the joint with a stainless steel brush, (never use a steel brush) while hot. The new solder does not require this preparation, although the old literature is still being sent with the new solder.
The process is quite simple. First, clean the surface of the metal to be soldered. The solder does not stick to dirty or oxidized aluminum. Remember this fact, because it will be of use to us later. The metal can be cleaned with sandpaper, an emery wheel, a burr bit in a Dremel tool, etc. Never use emery cloth because some emery cloth contains oil, which would contaminate the surface. You will want to 'vee' out some cracks to make a place for the solder to go. Experience will lead you here. Second, heat the parent metal with a propane torch, above the 732 degree melting point of the solder. The parent metal has to be hot because the heat opens the pores of the metal. The molten solder will fill these pores, making it stick to the parent metal. The parent metal should be hot enough to melt the rod. Melt the solder by scratching it along the surface of the parent metal, while the torch is aimed at the point where the solder rod touches the parent metal. The solder will 'puddle' and flow wherever you want it. Let the metal cool and grind the joint, or patch if you wish. That is all there is to it.
The only way to learn the use of aluminum solder is to try it and practice. A good place to start is an aluminum soda can. This will also show the strength of the solder. Take an awl and pierce the bottom of the can. (PHOTO 1) Sand the surface lightly and fill the hole in with solder. The solder has a tendency to skin over and fill in holes, so all you have to do is 'puddle' a thin layer across the hole. (PHOTO 2) After the can has cooled, try to pierce a hole through the solder with the awl. You won't be able to. The solder is much stronger and harder than the parent metal. This is a method of filling small holes. Now take two aluminum cans and solder them together. (PHOTO 3)
For larger holes, such as the side of the gas engine crankcase that had the connecting rod break through it, there is a different method. (PHOTO 4) Remember that the solder will not stick to oxidized aluminum. First, clean around the break with sandpaper. Take a piece of aluminum that fits the opening behind the hole and set it in place. Do not clean this piece. Now, heat the parent metal and begin melting the aluminum on one edge, working the molten metal across the opening. The backing piece keeps the molten metal from running out. It doesn't have to be a perfect fit, either, because the metal solidifies quickly, casting a base for more moltenmetal. When done, let it cool and remove the backing piece. The backing piece can be flat or round, such as tubing, whatever will conform to the inside contour. If you wish, grind the patch down flush and no one will never know the crankcase was patched.
To repair the boss that had broken out of the Honda valve cover, the parent metal crack should be 'vee'd out,' by grinding. The parent metal is then heated and the solder melted in the vee groove until it fills the groove. Again, grind the joint down flush and no one will know. (PHOTO 5)
To repair the threads that were stripped out of the manifold, where the carburetor mounted, a really novel method was used. First, drill the hole larger than the major diameter of the thread. Set the bolt in place in the hole and somehow hold it in place. Now, heat the parent metal and melt solder down into the hole around the bolt. The solder will stick to the open pores in the aluminum, but will not stick to the uncleaned steel bolt. When cool, unscrew the bolt and you will have new threads. If this sounds too good to be true, I saw this done! (PHOTO 6) Note in the photo that a portion of the manifold has also been soldered back onto the manifold.
Repairs can be made to die cast parts the same way, as the solder sticks to die castings and 'pot metal' also. A repair was made to a toy 'pot metal' spoked wheel that had a missing spoke, by making a new spoke with the molten aluminum. The spoke was 'cast' in place and filed to final shape. Thus an antique toy was restored. The metal can also be melted and cast into a mold.
The aluminum boat propeller had a chunk broken out. The broken edge was wire-brushed with a stainless steel brush to remove any paint and oxide. An oxidized piece of copper was placed against the flat edge of the propeller. Aluminum solder was used to form a new section from the beveled edge of the propeller. Make the repair a little larger than the original metal. After cooling, the propeller was ground to the original contour. The new metal patch is free of porosity, so the propeller can now be balanced and painted. (PHOTO 7)
Now, how does this relate to our hobby? Aluminum parts that would have to be thrown away can be saved, if a replacement is not available. Worn surfaces can be built up and re-machined. Castings that have flaws can berepaired. You can fabricate just about any shape by soldering sections together. Use your imagination and you can come up with more uses for this product. When the solder rod gets too short, don't throw it away, fuse it to a new rod.