Incident Overview
During taxi, runup, and takeoff, the pilot did not notice any anomalies with the engine, and the flight progressed without incident until about 90 miles short of the intended destination. At that point, the engine started to run rough for a few seconds before it stopped producing power. The windscreen became covered with oil, and the pilot executed an off-airport forced landing, substantially damaging the airplane.
Examination Findings
External examination of the engine revealed three holes in the top of the engine case, and a crankshaft counterweight pin resting on top of the engine. Internal examination of the engine revealed heavy mechanical damage on the No. 5 counterweight, and the engine case in the vicinity of the No. 3 and No. 4 cylinders. Examination of the No. 5 counterweight assembly revealed that one of the two aft snap rings, plates, and pins were missing. The pin was located on top of the engine during the external examination, but neither the snap ring nor plate could be located.
Maintenance History
The engine had been operated for approximately 16 hours since overhaul. The witness mark on the surface of the counterweight suggested that the snap ring, plate, and pin were installed; however, it could not be determined if the snap ring was properly seated. A review of engine overhaul records revealed that on February 5, 2005, the counterweights, along with the pins, plates, and snap rings were installed on the crankshaft.