How to Weld and Repair Cast Iron Parts



If you put ten people in a room who are familiar with auto restorations, whether they perform their own work, or are professionals, when discussing the challenges of restoration, the topic of repairing cast iron is likely to come up. Body rebuilding or reproducing ornamental pieces during a restoration are fairly common and usually can be achieved without too much difficulty. But mention a cracked or broken cast iron part and shivers are likely to resonate through your body. Cast iron, a ferrous metal containing approximately 95 percent iron with the remainder being alloys such as silicon and carbon is often used for many vintage auto parts. It casts well into a shape, is easily machined, is ideal for high heat situations (like exhaust manifolds) and even has good compression qualities. Cast iron however doesn't do well under tension and is brittle. A repaired crack in a cast iron part using conventional brazing techniques is likely to reappear when the part is put back into service, particularly if the part is usually under some sort of stress or load. Traditionally, dealing with a crack in a cast iron part called for preventing the spread of the crack. Closely spaced screws screwed into the root of the crack heads ground off could achieve this goal. Better is repairing cast iron to join the broken surfaces and prevent further cracking from occurring.

This article focuses on the repair a model L Lincoln cast iron water pump, but it also applies to repairing cast iron in general. While disassembling and separating the front and rear halves of the water pump, the flange surface of the rear half broke. This probably occurred because too much pressure was exerted on just on one side of the flange. In hindsight a better way would have been to first soak the shaft and bearing/packing areas and then gently press on the shaft and impeller to separate the halves. But it can't be undone and must be repaired.

Water pump cast iron part, after bead blasting, showing the pieces broken from the flange.

#72 for heat-affected cast iron, typically used on parts such as exhaust manifolds. (The heating of cast iron to high temperatures changes carbon content as well as the handling properties of the cast iron. Cast iron that has been used under these conditions is sometimes referred to as dirty cast iron because of the carbon content.) The #77 rod is used for "clean" cast iron which also can be machined. When repairing dirty cast iron, the #72 rod is applied first to fuse into that grade of cast iron and "topped" over with a few passes of the #77 rod. Only the #77 rod can be machined or ground and this can be useful when finishing an exhaust manifold which will be top coated or porcelain coated.

Before trying to weld the water pump, an old marine cast iron manifold was retrieved from the junk yard for practice welds. Several cuts were made using a die grinder and cut off wheel simulating crack areas and then lightly Vee'd out as is customary in performing deep welds. Using an electric arc welder at about 90 amps, negative polarity, and a back stitch method, the rod penetrated the scrap manifold well.

Diagram of performing a back stitch arc welding technique.

Scrap marine cast iron exhaust manifold used to practice. A thin gap was cut using a cut off wheel simulating a crack and slightly Vee'd out. Two passes over the gap proved the rod would penetrate and fill the gap.

Cross section view of the same practice manifold. A wider Vee would have permitted the weld to be more in the middle of the cast iron material.





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