Incident Overview
During cruise flight, the pilot observed that the engine began running roughly. He executed a precautionary landing at a nearby airport but was unable to identify the cause of the roughness. A subsequent ground run-up produced only 2,100 rpm at full throttle. The pilot then contacted the operator. After discussion, it was decided that if engine performance improved, the pilot would fly the aircraft back.
Attempted Return Flight
On the next run-up, the rpm increased to the 2,200–2,300 rpm range, which the pilot considered acceptable for takeoff. The takeoff roll felt slower than normal, prompting the pilot to plan a climb over the airport in case of an emergency. During the climb at 100 to 200 feet above ground level, the engine again began running roughly. The pilot initiated a shallow turn back toward the runway and subsequently lost control of the aircraft. The aircraft stalled, impacted the ground, and nosed over.
Engine Examination
An inspection of the engine revealed that the rocker arm boss on the No. 1 cylinder had fractured due to fatigue cracking. Laboratory analysis determined that the boss had been welded during a cylinder overhaul, a procedure not authorized by Continental.