How Does Stress Concentration Affect A Motorcycle Engine?

When we first took our new Kawasaki H1R to the races, we broke a steel cylinder stud every weekend. It broke right where the threads begin. And the reason for that is called stress concentration.

The cylinder head is held in place by four studs engaging with four sleeve nuts. They screw onto the studs, and you torque them up when you assemble the engine. There is a lot of tension in those studs.

The shank is nearly 8mm in diameter. In engineering school, we’re taught vertical lines of stress are trying to pull this stud apart as we tighten the nuts. All those lines of stress have to neck down to something more like 6.8mm or less in the threaded portion.

As those lines of stress suddenly neck downward, they concentrate right at the edges where the first thread begins. That stress concentration was breaking the studs. So my question was, what do I do now?

I could put new studs in and hope that they won’t break. Might have been a bad batch—a favorite theory. I decided to try the classical method, which is to turn down the thick part of the stud until it is as slender as the thin part so that the lines of stress all go straight.

That’s what I did. I took all the studs out and I turned the middle down to a smaller diameter using a round-nose tool so that the transition at the ends would be smooth. We never had another stud break.

Many highly stressed parts have very graceful, organic shapes. It’s no accident that trees flare out to their roots gradually. Nature has been experimenting with things like this for a billion years. We have to pack it all into our comparatively short lifetimes.

Kevin Cameron has been writing about motorcycles for nearly 50 years, first for Cycle magazine and, since 1992, for Cycle World. Kevin’s unparalleled experience and knowledge of the sport were—and continue to be—prompted by a lifetime of curiosity. His willingness to share that information with anyone who is willing to listen is likewise unique.

Kevin’s greatest strength lies in his ability to present complex subjects in simple terms with clarity and, often, humor. In this video series, shot in his home shop, Kevin draws upon his vast historical references to address modern-day questions. As Kevin has written, “Emotions bring us to engineering, but engineering then becomes a special way of confronting reality.”

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