Close the crack using several turns of bailing wire if possible. Vee it just a little bit, just from the outside. If there were no other method, a wood fire on the ground wood work. Drill a small hole at the very end of the crack, between a 1/16 and an 1/8" diameter. To start, find the terminus of the crack. This leaves tig-brazing(equipment not available) and finaly torch brazing. You already eliminated Nickel stick repair, with your asumption that the metal is too thick. Just so you know, 7018 stick and 70 series mig wire will also weld cast iron and give good color match. The best repair would be square cast iron rods and torch welding, but also the most difficult and not needed since you are not worried about color match. Since you mentioned methods of repair which would not be good color matches, I asume you will paint it. The solution is pre-heat, post heat, slow cooling. Not that it can't be done, I just want you to understand the forces or problems you are dealing with. Heat it at one spot(weld, braze, whatever, the iron expands from the heat, only it is restrained by itself, and being brittle and non maleable, it just breaks somewhere else. Now take the same flat bar and form it into a circle. You heat it, weld it, whatever, and it expands whatever amount it needs to as a result of that heat. Imagine your piece was a flat bar, broken in half. What ever method you use, tig or torch brazing, stick or torch with cast iron rod, you will face the same problem. What makes it so difficult is mostly the circular shape.
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