Ok so I have been a Certified H1 technician since 2002. They dont crack due to a blown headgasket. The crack in the number 8 cylinder due to casting of the walls to be to thin. After time and milage goes by the begin to leak coolant into the cylinder. Thats a fact from a AM General field service rep. The years affected were 97-99, Customer complains low coolant light and no leaks. Remove glow plug install glow plug tester removed valve and apply shop while cylinder is at TDC, in about an hour or so the overflow will be blowing fluid out the tube. I have done about 10 engine for this concern. The biggest crack i have seen was 1 inch long and about the width of a pen point.