The crew of the McDonnell-Douglas DC-8 cargo airplane retracted the landing gear during its climb to cruise altitude. The crew advised air traffic control that the landing gear had not retracted properly. During an attempt to ensure the landing gear was in the down and locked position, the crew heard a loud bang. Coincidental with the aircrew's actions, a person residing in a single family home reported that a metal object had penetrated her home's roof and imbedded itself in the floor. The airplane returned to land without further incident. An on-scene investigation revealed the right main landing gear retract pin had separated from the landing gear leg. Portions of the gear leg retract pin boss area had separated and were found wedged alongside the gear assembly. Subsequent metallurgical investigation revealed the pin boss area had metal fracture features consistent with overload, one of which was consistent with a preexisting fracture area. The pin boss area had previously been over-bored and chrome plated prior to a return to service. The airplane manufacturer stated the pin boss bore should not be chrome plated.