Pilot Report
The pilot reported that shortly after takeoff, the propeller RPM became erratic and uncontrollable, prompting a decision to return to the airport. During descent, the engine developed a rough running condition and eventually lost power, resulting in an off-airport landing.
Engine Examination
An external examination of the engine revealed a large hole in the crankcase adjacent to the number four cylinder assembly. The number four piston rod was fractured, and extensive heat distress, deformation, and discoloration were noted on both the number three and number four piston assemblies. A representative from Textron Lycoming stated that this type of damage is indicative of oil deprivation.
Further examination of the engine assembly showed that the hydraulic pump adapter pad cover gasket was protruding from between the accessory mounting pad and its associated cover. A section of the gasket had separated and was displaced from its installed position.
Investigation Findings
The FAA Airworthiness Inspector overseeing the examination reported that the installed gasket was intended to be used in conjunction with a hydraulic pump, if applicable. However, this particular engine configuration does not utilize a hydraulic pump, and therefore a hydraulic pump adapter pad gasket should have been installed instead.
Personnel at the Challis airport reported that a large puddle of oil was found in the run-up area following the airplane's pre-takeoff run-up and departure, consistent with the oil loss that led to the engine failure.