Narrative:

I prepared the cessna 152 for a test flight and break-in procedure for the new O-235 power plant. The preflight, starting, and run-up went smoothly. The takeoff was normal in all respects. I backed off the throttle to 75% for the break in procedure. After a shallow cruise climb, I leveled off at 2500 ft AGL and kept the engine at 75% power. After 15 mins of cruise flight and normal operating temperatures, the engine gradually lost most of its power in the span of about 30 seconds. Using the emergency procedures for restart and rough running engine did not solve the problem. I circled to land on a ranch which was below me at the time of the incident. I touched down with full flaps and slowest speed possible. After touchdown, the plane came to a stop in about 300 ft, without any damage. The problem turned out to be the locknut on the rocker arm for the #3 cylinder. The repair was done the next morning by the ia and with permission from the ranch owner, the aircraft was flown back to ZZZ without incident.

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Original NASA ASRS Text

Title: A CESSNA 152 AT 2500 FT EXPERIENCED A LOSS OF ENG PWR. MADE AN OFF FIELD LNDG. CAUSED BY #3 CYLINDER ROCKER ARM LOCKNUT FAILURE.

Narrative: I PREPARED THE CESSNA 152 FOR A TEST FLT AND BREAK-IN PROC FOR THE NEW O-235 PWR PLANT. THE PREFLT, STARTING, AND RUN-UP WENT SMOOTHLY. THE TKOF WAS NORMAL IN ALL RESPECTS. I BACKED OFF THE THROTTLE TO 75% FOR THE BREAK IN PROC. AFTER A SHALLOW CRUISE CLB, I LEVELED OFF AT 2500 FT AGL AND KEPT THE ENG AT 75% PWR. AFTER 15 MINS OF CRUISE FLT AND NORMAL OPERATING TEMPS, THE ENG GRADUALLY LOST MOST OF ITS PWR IN THE SPAN OF ABOUT 30 SECONDS. USING THE EMER PROCS FOR RESTART AND ROUGH RUNNING ENG DID NOT SOLVE THE PROB. I CIRCLED TO LAND ON A RANCH WHICH WAS BELOW ME AT THE TIME OF THE INCIDENT. I TOUCHED DOWN WITH FULL FLAPS AND SLOWEST SPD POSSIBLE. AFTER TOUCHDOWN, THE PLANE CAME TO A STOP IN ABOUT 300 FT, WITHOUT ANY DAMAGE. THE PROB TURNED OUT TO BE THE LOCKNUT ON THE ROCKER ARM FOR THE #3 CYLINDER. THE REPAIR WAS DONE THE NEXT MORNING BY THE IA AND WITH PERMISSION FROM THE RANCH OWNER, THE ACFT WAS FLOWN BACK TO ZZZ WITHOUT INCIDENT.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.