A gray iron cylinder head cracked after approximately 16,000 km of service. The head was cracked on the rocker arm pan rail next to the No. 3 intake port and extended into the water jacket on the rocker-arm side of the head. Microporosity was revealed in the crack in the sections taken from the water jacket next to the plug and the area next to the No. 3 intake port. A wave of microporosity travels midway between the inner and outer surfaces of the casting was observed and was concluded to have caused the cracking. The reasons and remedies for shrinkage porosity were discussed. Controlled pouring temperatures, improved design and use of chills were recommended to avoid the casting defects.