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Shaping up!

ringsNumbers, I would dare to suggest, are the language of engineering. A true and simple enough statement but when used in an engineering context it can only describe the approximate size of things. In the purer forms of science and mathematics, numbers are precise. For instance, the atomic number of iron is precisely 55, or in the world of Euclidean geometry, pi, the ratio of the circumference of a circle to its diameter is exactly 22/7.

In engineering however, we are rather more pragmatic, so things tend to be far less certain. Thus the position of a hole may be 100mm ± 0.5mm from a datum, or the diameter of a bearing journal could be somewhere between 44.980 and 45.000 mm. This lack of precision is called tolerance, and brings with it the simple admission that we can't make things as precise as we might wish.

A lack of precision in size is one thing but engineers as a rule tend to deal in more than one dimension, so the idea of shape - or as metrologists prefer, 'form' - comes to assist. Thus concepts like straightness, cylindricity, roundness and flatness have been defined to describe shapes more completely. And what better shape to describe than that of a piston ring and the groove in which it operates?

In trying to describe the attributes of a piston ring, therefore, we may have a tolerance of ±0.000050 in on the thickness or one of ±0.001 in on the as-installed bore diameter, while at the same time the surface finish may be described as being less than 4 microinches (< 0.000004 in). All very impressive and perhaps the very minimum I might expect in a piston ring, but to understand how a good ring relates to the piston and the groove in which it locates, I talked to a piston manufacturer who has an exemplary record and continues to supply pistons to all classes of motorsport. I must admit, I was unprepared for the response.

"There is a lot of rubbish written and talked about piston ring grooves," says the owner and managing director, who asked to remain anonymous. "Surface finish and flatness - absolutely ridiculous," were his exact words. He went on to explain that under running conditions the ring is a clearance fit inside the groove, and with a couple of 'thou' (0.002 in) or so axial and radial clearance the ring will be moving dynamically and unpredictably inside the piston groove. So while flatness of the grooves in the piston may be important - and by implication that of the ring also - it is not the overriding importance that some care to say.


Having described some of the research he had recently undertaken into logging piston temperatures in a running engine, it was clear that these results had produced an insight into piston design never previously fully appreciated. Using high-speed data logging of the piston temperature at up to eight critical points around the piston, and encoding the data against the crankshaft position, much was learned apparently. Furthermore, by feeding the data into a finite element model of the piston at the different times throughout the engine cycle, the analysis went on to give a dynamic picture of the piston groove and an insight into the resulting geometry, not only of the piston lands but the grooves in which the ring has to operate.

"Rather than the flatness of the groove," stated my contact, "under the very high temperature gradients induced, its shape - particularly that under the precise conditions of running - is the more important factor."
Taking care not to expand on this statement, and unable to divulge any engineering numbers, clearly the results were a revelation.

Fig. 1 - Shape, not flatness, is the key

Written by John Coxon

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