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The design of con rod fasteners, part 1

For many companies supplying a race engine, the choice of con rod can be a very straightforward affair. Often there are multiple suppliers of uprated engine internals, and there are a number of very competent and successful con rod manufacturers. The engineer has only to be satisfied of the advantages of choosing a rod from a certain manufacturer, who will have chosen to use some very specialised bolts (or studs and nuts) to hold together the two pieces of the rod (assuming a split rod design). Compared to a typical fastener, the material will be much stronger and more durable, and the design of the fastener will be much more detailed.

The con rod fastener is by some margin the most highly stressed fastener in the engine, and is almost certainly exceeded in absolute stress levels only by the valve springs. In restraining the mass of the rod and piston assembly at top dead centre on the exhaust stroke, the inertia forces are responsible for the peaks on the cyclic load curve for the con rod bolted joint. How this load is translated into a stress which the rod fastener sees is a function of the design of the rod itself, the design of the bolt and its initial tightening conditions.

The worst scenario is when the initial tightening of the bolt is insufficient to keep the con rod joint loaded during maximum inertia conditions. If the two parts of the rod become separated, leaving only the fasteners to maintain them as an assembly, then the fasteners take all of the inertia load, and they are not designed to do this. Engine failure is guaranteed in this case – it is simply a matter of time.

When the fastener is correctly tightened, the joint will remain loaded, and the extra load experienced by the fastener is much smaller than the earlier scenario. The proportion of the total load to which the fastener is subjected is defined by the internal load coefficient of the joint, and this is a function of the stiffness of the fastener and that of the clamped members.

It is of paramount importance that the fastener is correctly tightened and, in the case of con rod fasteners, that this is very accurately checked by measuring the length of the fastener before and after tightening. The elongation measured in tightening is proportional to the load in the bolt, and this is a very much more accurate way to confirm the load in the bolt compared to other methods such as torque tightening or torque and angle.

For this reason there are features on the end of the rod bolts that make measuring the elongation easier when using a specially adapted micrometer, which is provided with ball-shaped ends instead of the traditional flat measuring tips. These engage with centre drillings in each end of the rod bolt, providing a far more repeatable assessment of length than simply measuring over the ends of the fastener with a standard micrometer. The improvement is something you can assess for yourself if you have both types of micrometer available.

Written by Wayne Ward

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