Incident Summary
The pilot reported that while in cruise flight, the airplane shuddered and began to lose power. She changed the fuel selector from the right to left tank, checked the fuel mixture, exhaust gas temperature, and instrument panel. Despite these actions, engine performance continued to decay, and she initiated a forced landing to a pasture.
Examination Findings
Post-landing examination of the engine revealed that the number two cylinder exhaust rocker arm retaining studs had separated. The studs were examined by a metallurgist, who determined that over torquing of the attachment nuts during installation caused plastic deformation of the studs and probable cracking in the threads. This cracking led to a fatigue crack in at least one of the studs, which subsequently led to the failure of the exhaust rocker arm retaining studs.
Conclusion
The engine failure was traced to the failure of the exhaust rocker arm retaining studs on cylinder number two, caused by over torquing during installation.