On March 31, 1994, at 1715 hours central standard time, a Cessna 210, N732YK, operated by E. Beaver of McKinney, Texas, made an emergency landing with one main landing gear retracted at West Plains Municipal Airport, Missouri. The instrument rated commercial pilot and two passengers were not injured. Visual meteorological conditions existed at the time of the accident, winds were three knots, and no flight plan was filed. The flight operated under 14 CFR Part 91, and originated from Addison, Texas, at 1308 hours central standard time. Upon entry to the landing pattern at West Plains, the pilot observed unsafe landing gear indications. Visual inspection indicated that the right main wheel was not down and locked. The pilot orbited for two hours, obtained radio communications with a manufacturers representative, and attempted several methods to lock the wheels down, with no success. During troubleshooting, the hydraulic fluid reservoir was refilled with onboard engine oil, requiring 1 3/4 quarts. When landing gear extension was attempted, black fluid was observed venting overboard by ground personnel. The pilot landed with only the left and nose wheel locked down. On final approach, the engine was shut down. During landing the right wing contacted the ground and the aircraft departed the runway, damaging the propeller, right wing and right horizontal stabilizer. The last annual inspection was conducted September 1, 1993. No record of maintenance on the landing gear actuator system was noted since 1986. The airplane was recently purchased by the pilot/owner and had accumulated 7 hours under this ownership. The previous owner was Addison Aircraft Sales of Addison, Texas, who owned the airplane for a period of 3 months. Previous of Addison Aircraft Sales owning the airplane, it had a single owner since 1981. All maintenance was performed by the same individual. The last maintenance on the system was a left gear door replacement in 1990 due to structural door cracks. Post accident inspection revealed the Landing Gear Door Actuating Cylinder Piston (p/n 1281031-1) had seperated from the actuator cylinder (p/n 1281010-2F) during the landing gear extension cycle, allowing all hydraulic fluid to be pumped overboard. The snap ring (MS 16625-1068) which retains the piston within the cylinder was found attached but seperated from its retention groove. The assembly (p/n 1281028-1) was reassembled, and several retraction cycle tests were conducted. All cycles were successful with no leakage noted. Metallurgical examination of the actuator and snap ring revealed permanent deformation of the snap ring, and damage on the outboard lip of the snap ring groove. It could not be determined if these deformations were pre-existing or a result of the actuator seperation. The following machining dimensions (inches) were specified measured Inside Diameter (ID) of cylinder: .688-.690 .687-.688 Depth (radius)of snap ring groove: .020-.023 .020-.022 Diameter of snap ring groove: .730-.734 .727-.733 Width of snap ring groove: .039-.042 .035-.041 Distance from end of cylinder to edge of groove: .158-.161 .1566 Inside radius of groove closest to cylinder opening: .005R max .0032-.0039 ADDITIONAL INFORMATION The airplane was repaired and placed back into service. During the repairs, STC #SA5934SW for removal of the Landing Gear Doors and Actuator System was complied with.