After reading about different methods of welding cast iron, I thought you could use one more that worked for me and others. Most restorers of old-iron have the necessary tools to perform this operation.
My dear departed friend, Perry Mathrew of Helena, Montana, showed me this trick.
The crack we fixed was along the bottom of the water jacket on a 1926 Model 'T' Ford that I own. We performed it over 20 years ago and it hasn't leaked a drop.
Small 1/8 inch holes were drilled at the ends of the crack. This crack was over 10 inches long. Then using a rotary file, the crack was 'V'd out but not all the way through the crack.
Using an acetylene torch, with a small tip, he played the heat on the crack while at the same time he would rub the crack with a piece of small copper tubing. You could see the crack was getting a copper color to it. Very little heat was used, just enough so a copper tone was showing in the 'V'd sides of the crack.
Next he took some solder and used just enough heat along the crack to melt the solder. After testing the job for leaks, the solder was filed down to the contour of the block. We applied paint and the crack is hard to find.
I keep antifreeze in the 'T' year around with no loss of coolant.
I have no idea of the chemistry between the copper and cast iron. All I know is it works.