Denny, I have repaired many blocks and heads in my time as a machinist, the plugs are made of the same alloy as cast iron, if the repair was done correctly it shouldn't change or leak, I use the bigger plugs because you can tighten them more, the smaller plugs seam to break off too soon for me, yes I have seen a lot of repaired heads and blocks that leak after they have been run and have gotten to hot, but the fact that it cracked the first time means there was over heating or it froze. I would find the reason it cracked the first time, fix that and you should not have a problem. also if this was a 250 or 350 head or block I wouldn't repair them , I would find another head or block that wasn't cracked, because they are cheap and everywhere but being a 1937 GMC 286ci that nobody has, I had to repair the head, it will work great. if its your only choice you have to do what you have to do to make it all better again. Dan
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