On December 6, 1999, at 1655 mountain standard time, a Boeing 767-232, N111DN, operated by Delta Air Lines, Inc., as flight 990, scheduled domestic passenger service from Salt Lake City, Utah, to Dallas-Fort Worth, Texas, sustained minor damage when the main landing gear retraction arm failed during initial climb immediately following takeoff from Salt Lake City International Airport, Salt Lake City, Utah. There were no injuries to the airline transport certificated captain and first officer, a first officer riding in the jump seat, 6 flight attendants, and 76 passengers. Visual meteorological conditions prevailed, and an IFR flight plan had been filed for the regularly scheduled domestic passenger flight being conducted under Title 14 CFR Part 121. The flight was originating at the time of the incident. The following is based on written statements submitted by the captain, check captain, and a deadheading first officer. The captain had just received his Boeing 767 type rating, and this was his first initial operating experience (IOE) flight. The crew heard a loud noise and felt a vibration when the landing gear was retracted after takeoff. An emergency was declared, and the flight returned to Salt Lake City. When the landing gear was lowered, the nose and right main landing gear DOWN AND LOCKED lights illuminated, but a GEAR UNSAFE light illuminated for the left main landing gear. Following a low pass over the airport, maintenance personnel advised the landing gear appeared to be down and locked. An uneventful landing was made at 1705, and the airplane was stopped on the runway. Maintenance personnel informed the crew that the landing gear actuator had sheared and could not be pinned for towing. Passengers were deplaned via mobile stairs and bused to the terminal. It was discovered the main landing gear retract actuator (p/n 273T1104-3) had failed. The part was shipped to the Boeing Airplane Company's metallurgical laboratory where, under FAA direction, it was examined. According to the Boeing report, stress corrosion cracking had occurred through 75% of the piston, followed by ductile separation. The corrosion had formed when a faulty fillet seal around the base of the rod end allowed moisture to enter the joint. The retract actuator was an original part, but it was determined that it had previously been reworked.