Hattie, it's not a question of "good head" or "bad head". A "good head" cannot go "bad". If the casting has flaws internal to itself, these flaws, will, over time, generate small areas where localized cracking of the aluminum will occur. If the area is in a critical part of the head, then the life of the head is limited. Now we can discuss all the limiting factors until we are blue in the face. The variables are many and the strength of each variable is different for each specific vehicle. Overheating, maximum power operation, EGR use, trap-ox, engine cycles, are all variables. Nobody really knows the strength of each variable and whether one or more than one variable will effect the failure. However, I can tell you that no one variable can cause the head to crack unless it is predisposed to crack due to a casting flaw.

If the head is strengthened, it is less likely that any localized cracking will propogate and cause failure of the head. But, how much to strengthen it, and where to strengthen it, is a bit of black art.