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Part 1 - Airworthiness Articles

AAC 1-39

Faulty Cylinder Attachment Nuts

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Investigation into an in-flight shut-down of an engine in a Cessna 42l as a result of vibration and declining manifold pressure esablished the cause to be failure of the attachment studs of one cylinder. Further examination at the CAA Materials Testing Facility has since established anomalies with the cylinder attach nuts which may have reduced bolt preload inducing fatigue.

The nuts were found to have a defective thread form, possibly from a fault during manufacture. The CAA is presently trying to establish the extent of the problem with Teledyne Continental, but as yet does not have sufficient information upon which to act.

CAA tests suggest that the nuts do not provide sufficient preload or that the irregular form is not capable of maintaining preload in service. The shape of the thread profile is such that there is a protrusion which could tend to cut a groove or embed itself in the bolt material causing the nut to effectively loosen.

Suspect nuts examined can be identified by the form of the base, but high magnification is required to establish the thread form irregularity. The solution to the problem appears at present to be a visual inspection of installed and spare nuts, but more detail is required before instructions can be issued. This is advance information while investigation continues.