The pilot stated that while in the cruise at 3500 feet, approximately ten minutes after take off, the engine began to lose power and run roughly. Remembering that he had flown over a cleared area a short time before, the pilot elected to attempt to return to this clearing. During the descent he noticed that even with the throttle closed a rough vibration was being transmitted through the airframe. A decision was then taken to make a power-off landing. Approximately ten metres into the landing roll the right main wheel assembly separated from the strut when it impacted a small gully. The aircraft then came to halt without further incident. The investigation revealed that the piston assembly in the number five cylinder had failed due to prolonged and severe detonation. The exact cause of the detonation was not determined.