During mustering activities the aircraft was operating between 50 and 80 feet above the ground, when the engine suddenly stopped. The wind at the time was a quartering tailwind, and during the attempted autorotation the aircraft struck the ground in a tail-low attitude. The tail boom was severed, the aircraft bounced, spun to the right and came to rest with the landing skids collapsed. An inspection of the engine revealed that the magneto idler shaft had sheared due to overload caused by foreign objects fouling the magneto drives. The foreign objects were identified as crankshaft flange bolts which had been incorrectly torqued at overhaul.