A study was initiated to determine how to increase the fatigue strength of gray iron cylinder blocks. The approach was to determine what combination of metallurgical properties and foundry processes would yield the highest fatigue strength while maintaining the castability necessary for these complex castings. This study, unlike much of the literature available on fatigue strength in gray iron, was conducted using hundreds of castings and thousands of test bars. A baseline data base was established. This information was used to determine which factors would be most likely to effect the fatigue strength. An experiment was designed to determine how the foundry processes and the metallurgical factors interacted with more than twenty-five response variables.