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Fettling of production steel rods – is it worth the risk?

The easiest route into motorsport is to use what already exists and then re-use it – for decades the use of a lightly modified production car or motorcycle has been a traditional way to start racing. In motorcycle road racing in particular there has been a shift away from bespoke racing machinery in recent years: where thoroughbred machines filled the paddocks of many race meetings in years past, now production-based racers are the norm.

The use of production machinery has not, however, curbed the competitive streak among racers and engineers. Production engines can benefit in terms of performance and reliability from the careful fettling of various parts of the engine – ports, for example, have long been a favourite among tuners of production engines at all levels.

Con rods are also often a target of the fettler’s tools. Steel rods are forged in huge quantities, and their surface finish and appearance can leave much to be desired. Moreover, some tuners like to ‘match’ the weight of each rod and the ‘end weights’ of the rods with a view to minimising vibrations.

In terms of appearance, forging ‘flash’ is more of an eyesore than of any real engineering concern, but it does stand out as making the rod look rough. It is common for people to want to remove this, sometimes because they labour under the impression that there are large frictional gains to be had from making the con rods smooth and shiny. The only real and quantifiable gain from improving the surface finish of the rod though is that the fatigue limit of a material improves with improving surface finish – provided that there is no residual stress in the surface of the rod. If a forged rod has also been shot-peened then any improvement in surface finish may be overcome by the reduction in fatigue limit due to the removal of the residual compressive stress due to peening.

Another point that can be missed when fettling parts is the importance of aligning any machining/filing marks with the stress field in the rod. The stresses in the beam of the rod are very much axially aligned, because the rod is loaded alternately in tension and compression. This is the direction that any remaining marks should go, if any remain after fettling. If the machining marks run perpendicular to the stress, they will act as a strong stress concentration and can lead to early failure of the rod – in fact there only needs to be one going in the wrong direction to cause a catastrophe. I have seen this exact failure on a fettled production rod.

One process that will always be worth considering after any fettling of rods is to have them shot-peened in order to put the surface into compression. The improvement in fatigue life makes the process worthwhile – anything that prevents an unwanted engine strip and rebuild following a con rod failure has to be worthwhile. Although the surface finish after shot-peening can be rougher than before the process (especially if the pre-peening finish was polished), the improvement in fatigue life from the peening more than overcomes any deterioration in surface roughness.

Rod failure can often wreck the whole engine, rendering almost all of the important parts of the engine scrap, so the decision not to peen after fettling needs to be considered carefully.

Written by Wayne Ward

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