Fasteners for con rods
Bespoke race engines are festooned with purpose-designed fasteners, each of which carries out a specific task. We need specially designed fasteners for many reasons; every engineer has his preferences, so all engines are different and fastener designs reflect this. In general, bespoke fasteners are designed to higher standards than commercial parts. In minimising stress concentration and selecting high-quality materials, we seek to make parts that will prove more durable than a commercial-grade component. Commercial fasteners are, for their price, of excellent quality, but we often want something better than a general-purpose component that has been designed to a price.
However, for the most highly stressed fasteners in the engine - as in the con rod bolts or studs - most people wouldn't consider designing their own fasteners. Con rod fasteners are very highly engineered, are made from very high strength materials and there are a number of significant risks for those who design their own and have them manufactured. The three most serious problems that someone is likely to encounter in having a bespoke rod bolt are deficiencies in design, manufacturing problems and incorrect material specification and heat treatment.
Anyone tempted to design their own con rod bolts or studs should select a manufacturer who is well-versed in their manufacture. Producing consistently high-quality rolled threads on very high strength materials is not easy. There are also a range of other production processes used on con rod bolts that are not often used for other fasteners.
If we turn to specific design points, there are a number of things that are specific to con rod fasteners which we wouldn't find in a commercial bolt. There is often a location diameter close to the head of the fastener, to ensure proper alignment. Where a bolt is used, the head itself is normally a 12-point head rather than a socket head or hexagon head, as they have a greater torque capacity than a socket or hex head.
The head itself is carefully designed so that there is no sharp edge on the seating face that might dig into the con rod cap and cause a stress concentration. The shank diameter of the bolt is, for a very good reason, carefully controlled by design and manufacture in terms of diameter and length.
Most con rod fasteners have their pre-load controlled by measuring the length of the bolt or stud. By knowing the axial stiffness of the fastener and accurately measuring the stretch, we can calculate the load. As the axial stiffness of the shank section is directly proportional to the length of the shank and the square of the diameter of the shank, we need to control the shank length and diameter very carefully if loads are to be calculated with any accuracy. Also, in order to help the engine builder measure the stretch in the fastener, both ends have specific design features that allow accurate measurement with a specially adapted micrometer.
The manufacturing processes are critical to the success of the fastener. Among fastener manufacturers there is some disagreement on the need to cold-forge the head, but in producing a 12-point head, forging is necessary. Great attention must be paid to the underhead fillet, and these areas are often fillet-rolled to impart compressive residual stresses.
The surface finish of the fastener is also important. Naturally we want the surface to be free of any blemishes that might produce a fatigue crack initiation point. However, a mirror finish is not necessarily what we are looking for; we want a surface finish that produces a consistent level of friction. While we don't control con rod pre-load by torque, lower friction produces lower torsional shear stresses during tightening and operation.
Fig. 1 - If you want your rod to last, don't take risks with the fasteners (Courtesy of L Travers)
Written by Wayne Ward