It was reported that the pilot and passenger were conducting a stock survey at 200 ft above ground level when there was a loud noise and severe vibration from the area of the engine. The pilot immediately entered an autorotation descent and turned the helicopter into wind. A check indicated that no power was available from the engine. The terrain was rocky and covered in trees so the pilot attempted to cushion the landing as much as possible. On touchdown one skid collided with a large rock causing the helicopter to roll over onto its side. It caught fire immediately and the passenger, who was the first to exit the wreck, assisted the pilot to escape. The helicopter, including the emergency locator beacon, was destroyed by fire. The operator maintained its own search and rescue watch on its aircraft and a search was commenced as soon as the aircraft was reported as overdue. The pilot and passenger were found the following day by search aircraft attracted by a signal fire lit by them. Post accident inspection indicated that the engine problem was caused by the failure of a big-end bolt in the conrod of the no.1 cylinder which allowed the conrod to come loose and punch a hole in the crankcase. The engine had been overhauled by the manufacturer 300 hours (operating time in service) prior to the accident. The fire was probably started by oil that had leaked from the hole in the crankcase onto the exhaust system.