This paper describes the root cause investigation of cracking in cylinder liners installed in ALCO 251 diesel engines in nuclear standby service. The results of the investigation identify conditions which contribute to cracking of the upper cylinder liner flange. It is demonstrated that the cracks are initiated via high cycle fatigue, but that most cracks will arrest. A cylinder liner with an arrested crack of this type would be expected to continue to provide reliable service. For worst case conditions, low-cycle thermal fatigue may cause crack growth to continue beyond the normal arrest point, ultimately causing catastrophic failure of the cylinder liner flange. Several corrective actions are identified to preclude the cracking of replacement cylinder liners. These corrective actions are applicable to all operators of ALCO 251 diesel engines. The root cause investigation included finite element stress analyses and fracture mechanics analyses of the cylinder liner and a detailed review of industry experience. Results in each of these areas are presented.