As most race engines are now of the four-stroke multi-cylinder variety, the vast majority of con rods designed and manufactured for racing are of the split type. This means having a reliable means of holding the two parts of the rod together with sufficient pre-load, and this is taken care of by threaded fasteners. There are two options here. The first and most common method is to use a bolt. The second is to install a stud, usually into the rod, and to provide the load by tightening a nut.
The bolt is often preferred, owing to lower parts count and lower cost. The head of a bolt is often physically smaller in terms of diameter and depth, although using a bolt with a very shallow head height can bring its own problems in terms of increased stress concentration and lack of resistance to head-rounding during tightening or disassembly. Providing that the bolt is designed with sensible proportions, one might question why anyone might want to use the stud and nut combination.
However, there are valid reasons why some engineers might choose to use a stud and nut rather than a bolt. If the engine is expected to be rebuilt, and you need to re-use con rods, you may want to minimise damage to the internal threads in the rod by leaving the male fastener installed permanently. This is a consideration for titanium con rods, whose threads are more easily damaged than their steel counterparts. The stud-and-nut combination also offers the opportunity to dispense entirely with a female thread in the con rod, thereby eliminating any possible thread damage and also removing the stress concentration associated with it. In such cases, the stud has to be designed with an anti-rotation feature, and this is often achieved by using a D-shaped head.
A further reason for the use of a stud and nut is that the stress field in the fastener differs from that in a bolt. A nut and stud can be more flexible than a bolt, especially if a nut material of relatively low modulus is chosen. This can put less bending load into the fastener. The flexibility in terms of nut geometry and modulus means the stress concentration at the first thread within the nut can be minimised. Where a male fastener is used with a nut of lower modulus, the stress concentration is reduced as the female threads flex more, thus bringing more of the female threads into use and improving the load distribution over the length of the nut.
Studs can also incorporate location diameters, precluding the need to provide separate dowel pins or serrations. While the same features could be added to a bolt, this is often not the case as any variability in interference can add extra friction into the bolt-tightening procedure. While con rod fasteners are most commonly pre-loaded using stretch control, increased torque requirements to overcome the friction at the interface between a rotating location diameter and the static con rod will increase shear stress during tightening.
Written by Wayne Ward