Why Fully Machine a Rod?
The con rod is something that almost all piston engines have in common, and certainly all successful modern racing piston engines rely on these components to link the piston and crankshaft. There are some obvious visual differences between many racing con rods and their counterparts in a passenger car engine, although their roles are the same, as is the type of material used for their manufacture.
As with many components in a modern road engine, economics play just as important a role as the engineering. Engineering parts and assemblies down to a cost is a real skill, which goes far beyond getting a few quotes against a drawing. The economics of con rod manufacture dictates that the parts have as few machining steps as possible. Ideally, for a split (two-piece) con rod, a basic forging will have very little machining carried out, with the bores on the small-end and big-end axes machined to size, thrust faces skimmed to width, the big-end fracture split and the fastener holes finished (though not necessarily in that order).
Conversely, a racing rod is often machined all over. We can expect this to be the case with parts produced from billet. However, it is quite common for forged con rod blanks to be turned into racing rods without a single square millimetre of the original forged surface remaining untouched by a machining cutter. There are two main reasons for this.
The first reason is reliability. Parts with machined surface finishes perform better in terms of fatigue strength than those with cast or forged surfaces. Fatigue crack initiation takes place at lower levels of applied stress (including any residual stresses) on rougher surfaces. Many engineering textbooks which cover basic fatigue calculations will include surface finish factors. For two parts of the same material and undergoing identical stress cycles, a part with a forged finish will fail before one with a machined finish.
The second reason is consistency of manufacture and its effect on component mass. A batch of parts which have been accurately machined will have a much smaller spread of wall thickness than a part whose dimensions are controlled by one or two forged surfaces. We can relatively comfortably specify a rod width of ±0.05 mm for a machined part, safe in the knowledge that this is quite easy to achieve. If we need to rely on ±0.25 mm for a forged part, we need to leave a further factor of safety against fatigue, because our service load must be borne by a potentially smaller area. There is also an equal chance that, having calculated stresses based on the smallest possible area, the parts are on the large side. The amount by which a forged con rod must be over-engineered to cope with manufacturing tolerances is much greater than is the case for a machined rod.
For these reasons, it is likely that fully machined racing rods will remain common, and for reasons of economics we will continue to see rods with very little machining in road vehicles.
Written by Wayne Ward