Next, the crack must be fully “V” notched and ground smooth before welding can begin. I use a stick welder with a high nickel electrode, preferably with 85% nickel. This makes the weld softer and less likely to crack and easier to machine.
When a high carbon electrode is used the high carbon content of the cast iron (usually 2-4%) is pulled into the weld pool and that along with the high carbon content of the electrode makes the weld become very hard and possibly crack unless precise pre-heat and post-heat treatment procedures are followed. This is not always practical in a small shop.
It is always a good idea to do a simple pre-heat (even when using high nickel electrodes) with an acetylene torch getting the iron to about 250°F -500° F. If repairing a deep crack or complete break, do it in several passes to prolong the cooling rate. A cast iron weld using this procedure can be easily machined or ground smooth.
Kovach & Assoc. Performance Engine Building