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Fracture-splitting of joint faces

We have looked at and discussed several ways in which split con rods are machined on their joint faces. When they are split, there is a requirement for the surfaces to be accurately machined so that the faces are perfectly parallel, and that the plane forming the split line (or lines) coincides with the big-end axis. There is a further requirement for some location features so that the two parts can be accurately positioned relative to one another with repeatability. The normal ways to achieve this are either by providing dowel pins or hollow dowels around the bolts. Where studs are used, location diameters on the studs are common. A less common approach is to use joint face serrations.

However, in the roadcar industry, where costs are more important, it is very common to use a different method for steel rods - fracture-splitting. The idea is simple: a defect is deliberately introduced to the rod and, with the big end supported close to the split line, the rod is struck laterally. The high strain rate causes an otherwise suitably ductile material to behave in a brittle manner. The fracture produced shows no deformation, and the surface shows a random rough texture. This is very far from what we produce with the usual precision machining used in the production of racing rods.

The technique has significant merits compared to precision machining. The significant cost reduction is clearly attractive, especially during a period when everyone is looking to cut costs. However, lack of control over the contour of the surface does not in this case mean lack of precision. A fracture-split con rod has two parts that mate absolutely perfectly together. Also, in the same way that no two snowflakes or fingerprints are the same, there are no two identical fracture surfaces. For an experienced engine builder, it should be impossible to assemble a rod and cap that couldn't be assembled without it being obvious that something was wrong.

Given these desirable traits, why are fracture-split rods not in common use in motorsport? It isn't a method that can be applied to every material; many materials do not exhibit sufficient strain-rate sensitivity to allow them to fracture every time. One of the main 'crackable' steel materials in use for production engines is C70, a high-carbon wrought steel. The high-carbon composition reduces the content of ductile phases in the steel. This steel can be fractured using special machinery at room temperature.

Powder metallurgy materials have also been developed especially for fracture-split con rods. The powder metal materials developed for rods are favoured for larger, low-revving engines as produced for the US market, while European and Asian engines tend to be smaller and higher-revving, and these tend to use the wrought materials. However, the fatigue strengths of these materials are not as high as we would expect from high-quality wrought materials used for racing rods. For high-performance motorcycles, a method has been developed for fracture-splitting carburised con rods. Yamaha introduced the method for the 2003 R1, and found a 30% rod cost reduction compared to the previous machined split-face rod.

Written by Wayne Ward

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