On October 31, 2004, about 1805 mountain standard time, a Cessna 172RG, N6140R, landed with retracted landing gear at the Kingman Airport, Kingman, Arizona. The touchdown was firm, and the airplane was substantially damaged as it slid to a stop on runway 35. The commercial pilot, who possessed a certified flight instructor (CFI) certificate, was providing dual flight instruction to his student, who also held a commercial pilot certificate and was in training to become a CFI. Neither of the commercial pilots was injured. The airplane was operated by Sheble Aviation, Bullhead City, Arizona, under the provisions of 14 CFR Part 91. Visual meteorological conditions prevailed, and no flight plan had been filed. The flight originated from Kingman about 1635. The CFI reported to the National Transportation Safety Board investigator that after performing a series of practice takeoffs and landings with his student at the Henderson Airport, Henderson, Nevada, he and his student experienced a mechanical malfunction with the landing gear extension system during their landing approach. The landing gear would not fully extend using either the normal or emergency extension procedures. All attempts to remedy the problem were unsuccessful, and they returned to Kingman and landed with retracted gear. Under the direction of the Safety Board investigator, the landing gear extension system was examined by an airframe and powerplant mechanic. The mechanic reported finding the roller piston and bearing (part numbers 1281019-1 and S1997C7-8) to have fallen out of their assembly, thereby allowing the actuator piston assembly to reindex itself on the main gear pivot assembly. The mechanic further indicated that this event prevented the left main gear from being locked down. The actuator assembly (part number 9882015-2) had a total time of 7,156.0 hours.