Cylinder head cracking has been an engine development problem since the first high Performance diesels were designed and manufactured in the early 20th century. Valve bridge cracking is a common failure mode that is very dependent on engine application and operating conditions. Cracking failures cause increased engine maintenance and downtime, costly part replacement and in rare cases catastrophic engine failure. Cylinder head cracking continues to be problematic for modern diesel engines as peak firing pressures increase to meet exhaust emissions legislation and BMEPs increase for improved power density.The root cause of cylinder head cracking is often difficult to diagnose due to large number of design, manufacturing and engine operational variables involved. This paper summarizes the methods, results and conclusions of a study to determine the root cause of cracking in cylinder heads of large diesel engines used for standby power generation in nuclear plants. The root cause study included investigative, analytical and engine testing activities.The study revealed that head cracking was primarily affected by the seat insert counterbore design, fire deck thickness, engine load, and casting anomalies adjacent to the cracked locations. Recommendations to extend the service life of the cylinder head are discussed.