What cause broken head stud?

[caliber60]

It seems like a common problem. Manufacturer defect? Owner abuse? natural wear?

[Jim Sims]

Engine is being pushed far beyond original power and torque design outputs and emissions standards. Parts (like the studs) were originally sized to achieve a design or safety factor (corrosion allowance, stress safety factor, etc.) based on original engine requirements with some plans for engine growth. As the engine output is raised with the same original layout or geometry parts begin to fail. The only easy solution is better/different materials; these are sometimes heavier, more expensive and more complicated. The stud material upgrade was complicated as they were also used to compensate for a thermal expansion mismatch that became worse as power output and operating temperatures rose. The requirements to achieve lower emissions signifcantly contributed to the temperature problems. There's more to the issue but that's a start. Jim