Establishments primarily engaged in manufacturing aircraft engines and engine parts. This industry also includes establishments owned by aircraft engine manufacturers and primarily engaged in research and development on aircraft engines and engine parts whether from enterprise funds or on a contract or fee basis. Also included are establishments engaged in repairing and rebuilding aircraft engines on a factory basis. Establishments primarily engaged in manufacturing guided missile and space vehicle propulsion units and parts are classified in Industry 3764; those manufacturing aircraft intake and exhaust valves and pistons are classified in Industry 3592; and those manufacturing aircraft internal combustion engine filters are classified in Industry 3714. Establishments primarily engaged in the repair of aircraft engines, except on a factory basis, are classified in Transportation, Industry 4581; and research and development on aircraft engines on a contract or fee basis by establishments not owned by aircraft engine manufacturers are classified in Services, Industry 8731.