Cast irons are used extensively in the production of large diesel engines for power generation or oceangoing ships. One current trend is to replace grey iron with spheroidal graphite (SG) iron for fatigue critical components. In order to take advantage of the higher fatigue strength of SG iron and ensure the reliability of a design; better understanding of the typical defects for these large castings and the relationship between these defects and the endurance limit fatigue strength have been studied; Testing included uniaxial constant amplitude, constant amplitude torsion, and simple variable amplitude loading sequences in the long-life regime. Defects in several thick wall castings have been statistically evaluated. A fracture mechanics based model involving matrix hardness and root area successfully related the defect size to-the experimentally observed fatigue limit. For ferritic-pearlitic SG iron, the hardness of the tougher pearlite phase was used in the model; The model also correlated the torsion and tension endurance limits, Endurance limit as a function of mean stress is presented in the form of Haigh diagrams.