During climb, at about 2500 feet, a loud noise was heard from the engine coincident with a power loss and severe vibration. The choice of suitable forced landing areas was restricted by the strong wind and the terrain features. The aircraft touched down 188 metres from a fence and levee banks either side of a narrow irrigation channel. An attempt to avoid the fence and levee banks by overflying to an adjacent paddock was unsuccessful because of low airspeed. The aircraft collided with the levee, catapulted over the channel and came to rest 15 metres beyond. It was determined that the loss of power was due to a spark plug, complete with heli-coil, becoming dislodged from the Number 1 cylinder. The cylinder had been installed on the engine 7 flying hours previously, after being repaired by an approved organization. The most probable cause of the defect was overtorquing of the spark plug during installation, thereby damaging the tapped alloy of the cylinder into which the heli-coil had been screwed. With this thread damaged, the spark plug was able to vibrate to further damage the alloy thread. Exhaust gas escaping between the tapped thread and the heli-coil accelerated the damage process until eventually the spark plug was ejected by engine combustion pressures.