ENG FAILURE IN SMA BI-PLANE CREATES THE NEED FOR AN OFF ARPT LNDG.

Narrative

RETURN FLT FROM PROSSER; WA; TO RICHLAND; WA; AT 1700 FT MSL. ABOUT 10 MINS INTO FLT ENG RPM DROPPED TO APPROX 2000 RPM. CRUISE RPM WAS 2450. ENG PITCH CHANGED AND ENG STARTED TO RUN ROUGH. PERFORMED MAG CHKS; CARB HEAT; MIXTURE; ETC CHKS; NOTHING WORKED. AT THIS POINT IT BECAME OBVIOUS THAT I COULDN'T MAKE ARPT BECAUSE I COULDN'T MAINTAIN ALT AT 90 MPH (NORMAL LNDG APCH SPD) AND ENG CONDITIONS WERE DETERIORATING. BELOW WAS A SEMI- DESERTED MOBILE HOME PARK. THE 2 NORTHERN-MOST E/W STREETS WERE TOTALLY UNOCCUPIED. I PROCEEDED TO MAKE A 360 DEG APCH TO THE MOST NORTHERN STREET AND PERFORMED A ROUTINE LNDG. ACFT WAS UNDAMAGED. SUBSEQUENT ENG EXAMINATION REVEALED THAT #3 PISTON WAS GONE. ALSO; THE EXHAUST VALVE WAS GONE. PROBABLE SEQUENCE; SUPPORTED BY VISIBLE EVIDENCE; INDICATES THAT THE EXHAUST VALVE SEPARATED AT THE VALVE END AND PROCEEDED TO DESTROY THE #3 PISTON AND OTHER PARTS OF THE ENG. THERE IS VERY LITTLE SALVAGEABLE FROM THIS ENG. PLANE IS AN EXPERIMENTAL AND THE ENG IS A CONVERTED XYZ WITH APPROX 150 HRS SINCE OVERHAUL BY AN FAA CERTIFIED SHOP.