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There are a number of fasteners that are described as critical in an engine, but if an engineer is asked what these are, he or she will generally give a list of thread fasteners such as con rod bolts or cylinder head studs. Certainly high on any list of critical fasteners though should be piston circlips. Although at times in the past it has been common to use an interference fit to limit the axial movement of the piston pin, modern practice allows the piston pin rotational and axial freedom in both the con rod and the piston.

I would hope that most of you have not had to survey the wreckage of an engine whose moment of glory has been blighted by a circlip problem, either through component breakage or by a failure to fit the circlip correctly during build. If you do lose or break a circlip, you might be lucky and find that it does little damage itself, but once the piston pin starts to move, time is generally short before serious damage ensues - especially in a modern, highly optimised engine where the piston pin is relatively short, sitting in two narrow pin bosses and passing through a narrow rod. If the pin becomes disengaged from one piston pin boss, then the piston tends to flap around, and can easily break. If not, it is still likely to damage the valves as clearance is reduced and the pin itself can badly damage a liner or the cylinder block.

Piston circlips are generally round wire items, and are made from high-strength spring wire which is carefully formed into the correct shape. The amount of deformation from the 'free state' to the fitted state dictates the circlip load, and for high loads a high-strength wire is required so that the circlip, which is acting as a radial spring, behaves elastically and doesn't lose its load. Drawn wire springs, when subjected to load and relatively modest temperatures, tend to lose load over time.

The shape of the circlip is most commonly a section of a circle, with a small gap in the installed state; variations on this have a sharp oblique cut at the ends of the circlip. Some people will often dress the sharp edges of this type of circlip, while others maintain the sharp edge so that it 'digs' into the groove, preventing circlip rotation. Circlip rotation does happen and, on an engine with extended periods between rebuilds, this can wear the circlip groove so that some of the installed load is lost.

There are circlip designs with anti-rotation features that don't rely on anything so uncontrolled as the circlip end digging into the piston. Many suppliers can make circlips with a tag or ear on one end of the circlip that can locate in a slot or hole, thus preventing rotation.

Besides loss of load owing to rotation and wear, circlips can deform and fail due to vibration. A particular forcing frequency can excite a resonant condition in a circlip which will cause a fatigue failure.

Written by Wayne Ward

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