68-04-02: Exhaust Stack Clamp Failure

68-04-02 BELL: Amdt. 39-555. Applies to Model 206A Helicopters, Serial Numbers 4 through 110, 112 through 116, 118, 119, 122, 124 through 126, 128, 129 and 133. Compliance required within the next 25 hours time in service after the effective date of this Airworthiness Directive, unless already accomplished in accordance with Bell Helicopter Service Letter, Model 206A No. 38, dated January 10, 1968. To prevent possible hazardous damage to the main rotor or the tail rotor due to failure of the exhaust stack clamps which attach the exhaust ducts to the engine exhaust flanges, accomplish the following modification: Replace exhaust stack clamp as follows: Remove clamps, P/N N3657, from the exhaust stacks. Install two new clamps, National Utilities Corporation, P/N 4656AA, on exhaust stacks with studs facing outboard. Tighten the two nuts on each clamp evenly until each nut is finger tight. Torque nuts to 30 inch pounds. Grasp top of stack and shake, at the same time tap clamp lightly with a plastic mallet and then recheck torque. Repeat this procedure until torque can be maintained at 30 inch pounds. Safety wire nuts to stud shanks with safety wire, P/N MS20995C32. This amendment becomes effective on March 15, 1968.