Welding cast iron: the exhaust manifold repair that actually held

Cast iron cracks because it cannot stretch. The weld shrinks as it cools and the base metal next to it cracks. So: preheat the whole part to 400 to 500°F (propane weed burner, or the kitchen oven if you can get away with it), V-groove the crack and drill a stop hole at each end, weld 1" stitches with 99% nickel rod (Ni-99) at low amps, peen each stitch with a hammer while it is still red to relieve stress, let it cool to touch-warm between stitches, then bury the whole thing in a bucket of sand or wrap it in a welding blanket to cool overnight.

Mine has been on a tractor for two years. The one I did quickly without preheat cracked next to the weld before I got it bolted on.