Sequence of Events
The pilot reported that during cruise flight, the engine began "missing." In response, the pilot enriched the mixture, switched fuel tanks, and adjusted the propeller and throttle, but the engine continued to run rough. The windshield became 60 percent covered with oil, and smoke entered the cockpit. The pilot carried out a forced landing in a field. After landing, the aircraft was destroyed by fire.
Examination and Findings
A teardown inspection of the engine revealed that the #5 cylinder's intake and exhaust valve heads were missing. The exhaust valve stem and guide were sent to the NTSB Materials Laboratory for further analysis. The fracture surface of the exhaust valve stem (Part Number: SA643767P005 C FAA-PMA) exhibited features consistent with a brittle fracture mechanism, such as fatigue. The clearance between the exhaust valve guide and stem was measured at 0.019 inch, which exceeded the maximum serviceable limit of 0.006 inch specified in the 1989 Teledyne Continental Motors overhaul manual for the IO-520 engine.
Maintenance History
Maintenance records showed that an oil analysis taken on June 23, 2004, at 663 hours since time since overhaul (TSO) included the note: "Note Extreme Nickel Concentration. High Concentration of Nickel is Caused By Valve Guide Wear. Contact Engine Manufacturer's Service Rep. Resample in 25 Hours." The aircraft owner reported that no maintenance was performed in response to this warning. The engine failure occurred approximately 2.5 years later, after accumulating an additional 300 hours of operation.