Picky Little Details

[Hugh Janus]

Kevin Cameron has been writing about motorcycles for nearly 50 years, first for Cycle magazine and, since 1992, for <em>Cycle World</em>. (Robert Martin/)

This week I discovered that an old 250 twin engine under a bench in my shop is from a YDS-1 Yamaha, made 60 years ago. Because I had just been installing cylinder studs in a later engine, I looked more closely at the older engine’s studs. They looked really fat to me, so I fetched the Vernier caliper. What a difference. The slender studs I’d been setting into a 1964-65 crankcase measured 7.15mm (0.281 inch), but those in the older YDS-1 engine were 7.95 (0.313 inch). Why?

There is always a story behind such detail changes. Experience with other cylinder studs over the years makes me pretty sure that Yamaha adopted slender studs for later engines because it had suffered some breakages with the fatter ones.

How can that be? You’d think the fatter studs should be stronger, but in fact the reverse is the case. When studs break, they nearly always do so at the root of the first thread. The nominal diameter of the root of an 8 X 1.25mm external thread is 6.445mm, so at that point there is a sudden reduction in cross section (by about 1/3) from the 7.95mm stud shank, down to the small thread root. In a highly stressed part like a cylinder stud, any sudden change of cross section concentrates stress there. The stress in the shank of the stud concentrates at the root of that first thread. There, the slightest lack of surface smoothness is easily transformed by cyclic stress into a crack.

In 1970 studs broke on our Kawasaki H1R 500 triple roadrace engine. Although Kawasaki soon supplied H1R owners with improved replacement studs, I decided to try what I’d seen in mechanical engineering texts—to reduce the stress concentration by machining the shanks of the studs to a diameter slightly smaller than their thread root diameter—just over 6mm. To my delight, not one of those machined-down studs ever broke in service. Why not? Because, without the original stress concentration at the root of the first thread, the fatigue life of the stud became much longer. When the improved studs arrived from Kawasaki, they looked just like what I’d seen in the textbooks.

Makers of other highly stressed parts have had to learn the same lesson: expect fatigue cracking wherever there are sudden changes of cross section, notches, sharp edges, or holes. This is why pistons and connecting-rods—subject to reversing stress thousands of times per minute—are given the smooth, graceful shapes they have.

Motorcycle manufacturers are organizations of humans who, like ourselves, learn from experience. As Mr. Honda liked to say, we learn more from failure than from success. When we succeed, we go to the party with everyone else. When we fail, we stay up late trying to understand how to do better.

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