I have brazed, welded, and epoxied cracked blocks. Welding with nickel-alloy rod is the best, but also the most difficult fix. The crack must be stop-drilled at both ends, V-ground along the entire length of the crack, and then welded in beads of no more than an inch in length, after pre-heating with a torch. Don't get the block red, just hot enough to radiate a pretty good bit of heat to a hand held near the repair. Weld a short bead, then peen the weld for several minutes with the pointed end of a slag hammer to stress-relieve it. Heat again, weld another inch, peen- - - -repeat as necessary. Overlap each bead a little to minimize the possibility of creating multiple seep leaks at the bead starting and stopping points.
I strongly reccomend perfecting this technique on a junk block before doing the real job! Experiment with AC and DC current, ground polarity, and heat range to find the technique that gives the smoothest, least porous weld. Different block alloys require different approaches. A slag pocket can ruin your whole day! Once you're confident in the strength and continuity of your weld, the outside can be cleaned up by grinding, but not too deep. Don't try to hide the repair by grinding it completely smooth. Cool the block thoroughly before dressing up the bead.
Weld 10 seconds- - - -peen 10 minutes! Keep the block hot during the welding procedure, then bury it in sand or layers of some sort of insulation to cool it down slowly. If it goes "TINK!" during the cooling process, look for another crack, right alongside the weld bead. It happens! Good luck!
Jerry
