Incident Overview
During the first flight following a 100-hour inspection, the aircraft experienced a partial loss of power while en route to its destination. The pilot initiated a precautionary landing at a nearby airport but soon realized the runway was not reachable. The pilot subsequently performed a forced landing on a football field.
Maintenance History
The 100-hour inspection included the replacement of the number 3 cylinder. The flight during which the power loss occurred was the first flight after this maintenance was completed.
Post-Incident Findings
An examination of the engine by a Federal Aviation Administration (FAA) inspector revealed that the number 3 cylinder exhaust rocker arm set screw had backed out, and the jam nut was found lying in the rocker cover. Additionally, the '1102' plug was missing from the intake port of the number 3 cylinder. The tappet clearance between the rocker foot and the valve stem tip was measured at 0.300 inch. According to published Lycoming Service Instructions, the correct tappet clearance should have been between 0.006 and 0.012 inch.