The aircraft was cruising at an altitude of about 300 feet above ground level when the pilot heard a loud bang, then felt a severe vibration. The engine continued to operate for about two to three seconds before stopping. Because of the unsuitable terrain over which the aircraft was flying, the pilot was forced to land into trees and the aircraft rolled on its side. An inspection of the engine found that the number 3 connecting rod had failed. The failure was initiated by galling between the big end bearing and the connecting rod, inducing a fatigue failure of the connecting rod. Specialist opinion indicates that the engine was improperly rebuilt following a bulk strip some 200 hours earlier, in that excessive clearance was permitted between the crankshaft and big end bearing.