On March 20, 2008, about 1026 Pacific daylight time, a Piper PA-31-350, N4098A, landed with the right main landing gear retracted at Portland International Airport, Portland, Oregon. Ameriflight LLC was operating the airplane, owned by UAS Transervices, Inc., under the provisions of 14 Code of Federal Regulations (CFR) Part 135. The certificated commercial pilot was not injured. The airplane sustained substantial damage to the aileron. The cross-country cargo flight departed Portland en route to Roseburg, Oregon. Visual meteorological conditions prevailed, and an instrument flight rules flight plan had been filed. The pilot was preparing to land at Roseburg when he noted that the right main landing gear failed to extend, which ground personnel confirmed during a flyby. The pilot diverted back to Portland. Ameriflight personnel alerted the tower at Portland that the airplane was returning, and would make an emergency landing with one main wheel retracted. Maintenance personnel discussed the problem with the pilot; he tried various remedial actions without success. A flyby confirmed that the wheel remained retracted. The pilot declared an emergency, and landed; there was no fire, and no injuries. The right propeller, right flap, right wing outboard of wing station 190, and right aileron sustained damage. During the recovery efforts, the airplane was jacked up in order to extend the right main landing gear, which locked in place. The airplane was then towed to a hangar. Examination revealed that the upper pivot bolt had not been installed through the pivot hole in the upper end of the actuating rod during recent maintenance. The mechanic who did the repair and the inspector who signed off the repair did not detect the discrepancy.