On October 13, 1995, at 2051 central daylight time (cdt), a McDonnell-Douglas DC-9-31, N945AX, operated as Flight 135 by Airborne Express, Incorporated, of Wilmington, Ohio, and crewed by an airline transport rated flight crew, was substantially damaged during cruise flight. The left engine's top thrust reverser door became partially detached and struck the engine's thrust reverser door repeatedly. Visual meteorological conditions existed at the time of the accident. The 14 CFR Part 121 cargo flight was operating on an IFR flight plan. The captain and first officer reported no injuries. The flight departed Des Moines, Iowa, at 2043 cdt. The flight crew reported hearing a loud bang and felt the airplane shuddering shortly after leveling off from a climb. The flight crew declared an emergency after observing no problems on the airplane. During the landing the thrust reversers were deployed. The 2 yellow thrust reverser unlock lights illuminated and then the right engine's thrust reverser's blue light illuminated. The airplane decelerated and was able to taxi to the parking area. An on-scene investigation was conducted by a Federal Aviation Administration (FAA) Principal Maintenance Inspector (PMI). The PMI reported the left engine's thrust reverser door had one of its driver linkages broken. Damage from the thrust reverser doors striking each other broke the left engine mount and pylon spar strap. The thrust reverser doors and exhaust nozzle were wrinkled. The exhaust nozzle end had about 25 percent of its end crushed upward. The outboard driver linkage arm (arm) was broken near its pivot point. The arm was bent upward 90 degrees about 24 inches forward of the break. The bolt and associated hardware were not found. One overcenter link was bent about 45 degrees at its midpoint. Examination of the thrust reverser assembly revealed that a pivot bolt had worked its way out of the driver linkage arm assembly. Excerpts from the McDonnell-Douglas DC-9 maintenance manual are appended to this report. These excerpts illustrate the bolt's location and relationship to the thrust reverser assembly. Due to the missing bolt, washers, nut and the cotterpin the reason for the bolt separation could not be determined. Evidence of a broken bolt was not found. The company examined other airplanes in its fleet. One airplane was found that had a missing cotterpin in a castellated nut on the same bolt as was missing on N945AX. A second airplane was found with a loose lower thrust reverser door. Examination of this airplane revealed the washers were overly thick. The overly thick washers would not allow the bolt threads to protrude beyond the end of the castellated nut. This situation would not allow the cotterpin to be installed because the hole in the bolt was not adequately exposed. The manufacturer's illustrated parts catalog and maintenance manual show different combinations of washers being used on the pivot point bolt. The maintenance manual text calls for a countersunk washer to be installed under the bolt head and one plain washer under the nut. The manual's illustration of this area shows the washer under the bolt head and two washers under the nut. The illustrated parts catalog illustration of this area illustrates what the maintenance manual text called for.