Cast iron rods have various componets included in their make up and they are classified as machinable and non-machinable so you have to take a bit of care in your selection as they are also polarity and position sensitive. I have welded many an exhaust manifold and have also use brazing in head related repairs to great success. Preperation of the weldement is most important but a good repair is possible....my question is why is it cracking therein lies the root of a problem. Steve grin