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Bolt Hole Details

Many of us will be aware that the con rod bolt is very often the most highly stressed fastener in any race engine. Not only is it highly stressed but if a rod bolt fails then the consequences can be catastrophic for the whole engine. I have seen engines where there has been a rod bolt failure, and very little of any real value could be used again. Thankfully though, by using careful tightening procedures and the correct bolt for the application, rod bolt failures are relatively rare.

The bolt hole itself is very important, not only for the reliability of the rod; it can also have an effect on that of the bolt. If we begin at the part of the hole at the end close to the head of the fastener, we obviously need to leave sufficient clearance for the bolt head. However, we need to consider carefully both the surface finish and the corner radius here, and also whether we simply plunge in with a cylindrical end mill with a corner radius or do something more ‘fancy’ in order to reduce stress concentration. Having access to finite element stress analysis is useful in such circumstances.

From the bolt head recess, we need to be sure that the chamfer provided at the top of the bolt hole is sufficient to clear the underhead radius on the bolt in all circumstances. The main part of the hole is pretty plain; we simply need to ensure that the surface finish is acceptable and that it is as perpendicular as possible to the split face of the rod.

In the other part of the hole, it is common not to have the thread extending all the way to the split line. One reason for doing this is to prevent the thread pulling at the split line. Anything that prevents a thin edge of material being present will help to avoid the possibility of a fatigue crack starting; by ‘burying’ the high stress associated with the first engaged thread, we increase the durability of the rod. Having a greater distance between the bolt head and the first engaged thread allows the shank to be a little longer and thus less stiff. This can improve the durability of the bolt. Where the bolt enters the thread, the geometry of the hole entry can affect the stress concentration factor.

Chamfers have been shown to improve the distribution of load in fastener threads by transferring some of the load from the first thread to the next few less highly loaded threads. The thread form itself is very important. There are thread forms with increased root and crest radii for both male and female parts; this can improve durability by decreasing the stress concentration at the major diameter of the female thread.

Beyond the thread engagement, the run-out into a plain counterbore at the end of the bolt is an important feature. To run the thread all the way until it breaks out onto the surface of the rod is likely to lead to fatigue failures in highly stressed rods, so a counterbore is provided which should have a radius at its base.

Written by Wayne Ward

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