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Repairing Cast Iron
There are two threads going here currently about broken cast iron manifolds. I have suggested brazing them, and don't currently have the equipment to do it. So I thought I'd post how to do it.
I repair cast iron using common bronze brazing rods and the very common Brazo flux. I have tried special cast iron brazing fluxes but haven't personally gotten good results.
The first step for a broken casting is to clean the parts well with a wire wheel to remove all surface rust. Then V out the two sides of the joint so that you have about a 30 degree v groove between the parts and they only touch on the inside edge. As much as possible you'd like the brazing to penetrate all the way through.
Don't be in a hurry, you need lots of patience here. Carefully clamp all the pieces back together. Theere aren't any large foreces involved, but once you start they must not move. Heat up the whole casting to at least 500 F. I often do this over a plumbers propane lead melting burner, especially if the part is large. Touching it with some lead solder should melt the solder instantly. Once you have the whole part hot you can then start heating the area around the joint using an oxyacetylene or oxy-propane torch. Unless the part is very small and you use MAPP gas you will not be able to use a torch without oxygen. You MUST use dark brazing goggles, it will be very bright, and you need to see well. Wear your reading glasses!
Heat the area around the whole joint until the metal glows a dull red everywhere, and quite bright where the flame is. Make sure you use a neutral flame, cut back the acetelyene until the blue streamers just go away. You don't want to add more carbon to the brazing joint. This heating is very important, almost everyone hurries this, the metal is too cold and the brazing metal doesn't flow into the joint well.
Once it is good an hot around the repair area, heat the brazing rod with the torch and dip it in the flux. Then heat the start of the repair. Cut the flame back, you aren't in a hurry, you want very good control of the temperature. Do not melt the rod with the torch. It must melt when it comes in contact with the hot metal. Keep heating, and touching with the rod until it starts to flow into the joint. You'll need to keep refluxing the rod as you do this.
Once it starts to flow you need to keep the temperature of the metal just bleow the melting point of the rod. At this temperature the rod will melt in the heat of the flame, flow and solidify on the metal. The puddle should be bright and shiny the whole time. You can stir it with the rod and the rod will slowly melt into it as you move. If you get popping you have too much heat, cut the flame back or get a smaller tip. Note that to keep the temperature just right you will need to keep moving the torch away from the puddle and back again.
Controlling the temperature is the whole secret here. Note that small pieces heat much quicker than large pieces. This means the flame needs to spend most of it's time on the large piece. Small pieces tend to overheat. When the metal is too hot the brazing material just runs off and won't stick. If it too cold it also won't stick. Get the temperature right and it is a thing of beauty how the metal flows.
You'll find that if you have the temperature just right you can build up missing areas using just the bronze. For parts actually missing just get some mild steel and use a grinder to fit it into the casting. Then just braze it in the same way.
Don't be in a hurry. Let the whole thing cool slowly until you can pick it up with bare hands. The flux will be a glassy coating on the bronze that chips off with a hammer. If you have areas built up a little too much a grinder or hander file will easily take care of it.
I wish I had pictures of some of the parts I have fixed this way, You'd be amazed at how well this can work! I've done quite a few exhaust manifolds, one broken in at least 6 pieces. I've done fork lift wheels with two of the bolt lugs broken completely off, and a third one misssing altogether. I recently repaired my cast iron keel where the bolt holes were completely gone and two tabs were broken off and missing. Last year I repaired several pieces of a bandsaw gear shift mechanism that were broken.
Gary H. Lucas