Machined Grooves’ Role in Motorcycling

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

George W. Patchett was one of Britain’s “wizards of tune” who made their way by consulting with various motorcycle manufacturers. In the 1920s he had raced for Brough and McEvoy—makers of big V-twin-powered bikes—and for FN in Belgium. Then in 1930 he began work at Jawa that was ended by the outbreak of World War II. His escape from Czechoslovakia took place at the last moment and seems to have been rather exciting.

Returning to England, he found work at Sterling Armaments Co., where in 1944 he designed a submachine gun intended as an improvement upon the famous but crude Sten. Prototypes were favorably received but accuracy was a problem and so was “the mud test.”

Patchett’s response to jamming of the Sterling’s blowback bolt was to provide its OD with helical grooves, such that a particle of grit trapped between bolt and receiver would be swept into one of the grooves, allowing the bolt to continue to cycle freely.

Ultimately the Sterling was accepted by the British army as its standard postwar submachine gun, serving for many years. Have a look at the British TV series “Last Post,” about the Aden insurgency, and you will see men with Sterlings

A few years ago I had the pleasure of speaking with an engineer employed by a piston manufacturer. He described the modern cylinder wall preparation called “a plateau finish” in much the same terms as the success of Patchett’s helically grooved bolt. A plateau hone begins with a fairly coarse crosshatch hone, followed by use of a finer set of stones. The result is a myriad of small, smooth plateaus surrounded by the deeper scratches left by the coarse hone. The smooth areas support the piston and its rings.

Modern cylinder wall preparation is called a plateau finish. (Kawasaki/)

“Those deeper scratches protect the piston from micro-seizures,” the engineer said. “If the beginnings of seizure occur on a given plateau, it can’t progress because the seizure debris is immediately swept into one of the scratches.”

When in the 1960s I first looked into Yamaha’s vertically split TD twins, I saw an evolution in the design of the spring detent provided to positively hold the gearbox’s shift plate in gear. This took the form of V-shaped indents in the OD of the plate, into which a spring-backed 10mm steel ball would click at each shift, preventing accidental rotation of the plate.

Unfortunately, the ball didn’t work very well; it made only circular line contact with the bore that held it and its spring, and the resulting high local pressure sometimes produced excessive resistance, making shifting notchy.

The next design was a tubular and round-ended plunger that fitted into the same bore as had the previous ball detent. Because side-pressure on the detent plunger was now spread over the length of the plunger, side-force during shifting no longer jammed it. Shifting improved.

But particulate debris from gearbox wear could and evidently did get between the plunger and its bore, occasionally causing the plunger to stick and no longer hold the shift plate in its six positions (first, neutral, and ratios 2, 3, 4, and 5).

The next year’s model had a new detent plunger, now with George Patchett–like spiral grooves in its OD, into which particulates would be swept, making the detent much more resistant to jamming.

None of these applications was devised by a computer or recovered from a vast database by AI. They were the result of ingenious thought by actual humans.

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