Forging ahead, or not?
When an engine builder picks up a con rod bolt, examines it and lubricates it before fitting, he perhaps doesn't consider the manufacturing processes involved. However, it is very likely that the head of the bolt will have been formed by forging.
Con rod bolts are generally equipped with bi-hexagon (12-point) heads. Conventional sockets tighten bolts through contact close to the corners of a hexagon because of large clearances between socket and fastener. More corners equals more torque capacity without danger of the bolt becoming 'rounded'. Also, a small socket can be used to impart a given torque, requiring a smaller diameter bolt head, and a smaller bolt head means less friction, and less friction means that a lower torque is required to produce a given pre-load. Twelve-point heads can't be formed by conventional machining processes, although there are material removal methods such as spark erosion that could produce the required shape. Bolt heads with internal tightening features - socket-head bolts - are also often produced by forging.
It is commonly understood that forging offers other advantages too. Forging is a process that produces a shape by material displacement rather than by material removal. It is widely used in the production automotive engine industry for many components, most notably crankshafts, con rods and pistons. In racing, pistons are still mainly machined forgings, but forged crankshafts are more of a rarity owing to the cost of producing the tooling. Con rods occupy a middle-ground; where sufficient parts of one design are produced, forgings are a common starting point, and manufacturers will use a common forging for several similar designs. The grain elongation and flow in the forged material is reckoned to produce worthwhile improvements in fatigue strength.
For bolts, the case is less clear-cut than for some larger components, and there is certainly not universal agreement between manufacturers of forged-head bolts as to whether there is a real improvement in component strength and durability due to forging. When interviewing bolt manufacturers, I hadn't expected to find such diverse opinion. It can't be argued that the grain flow doesn't take place, or that the grain flow wouldn't be beneficial.
However, the underhead of the bolt is generally machined, and for very highly stressed bolts it is common to find that the underhead has been slightly undercut too. This serves to remove some of the forged material where it would present the greatest advantage; the machining operation also interrupts the grain flow.
Additionally, for a number of high-strength materials used only for very highly stressed bolts, the material has to be heated in order to forge the head detail. If incorrectly done, this can leave the material in the highly stressed transition between head and shank in a lower-strength condition than we would ideally like to find. As hardness and strength are generally related, finding a critical area of a bolt soft will not instil confidence.
However, for many designs of critical bolts, we can cope with a slightly softened transition between head and shank. If we take the example of a con rod bolt, it is often the case that the most highly stressed part of the fastener is the waisted shank; the underhead transition is usually larger than the shank and is therefore less highly stressed.
Written by Wayne Ward