The Boeing 707 cargo aircraft was being prepared for a normal push-back from the freight apron position V2 at Sydney-Kingsford Smith Airport, Australia. A towing tug was connected to the nose gear and number two engine was running at idle power. The aircraft was cleared to be pushed back to taxiway G where the other engines would be started. The aircraft brakes were released but as the tug took the weight for the push back, there was a loud report and the nose of the aircraft collapsed onto the cabin roof of the tug. The nose wheels folded back under the fuselage and arrested its fall, saving the tug cabin from further collapse. The engine was immediately shut down and the crew departed the aircraft to assess the damage. Investigation revealed the nose landing gear oleo outer cylinder had failed at the upper end of the cylinder bore. The failure was initiated by a fatigue crack in the radius at the bore end forward quadrant. The radius in the vicinity of the crack was found to be .039 inches, marginally less than the manufacturer’s minimum dimension of .040 inches. Extension of the crack due to stress corrosion was extremely rapid and may have occurred whilst the aircraft was parked at Sydney. Significant Factors: The following factors were considered relevant to the development of the accident 1. A fatigue crack developed in the bore top radius of the nose landing gear cylinder. 2. The marginal dimension of the bore top radius may have contributed to the development of the initial fatigue crack. 3. Stress corrosion then developed the crack at a rapid rate to final failure.