I've heard of this method from a very old timer
... Preheating is not necessary. Clean area to be welded, drill small hole at start and and end of where the weld will be (along the crack) I guess drilling the hole will stop the crack, because the crack will just get bigger when welded if you don't. Use a rod suitable for cast iron. And here's the tricky part... only run about a 1" bead at a time. Immediately after you put the torch down, use a hammer and tap on the weld until it is cooled down. I forget exactly what that does, but it's necessary. Inch by inch and tapping on it is what the old guy said to do. He also said that he repaired a block on a tractor one time- it was about a 8" crack and took all day!
