How Do Motorcycle Piston Rings Work?

Cycle World Technical Editor Kevin Cameron answers your motorcycle engineering and mechanical questions

The split piston ring was invented in 1852 by a locomotive superintendent named John Ramsbottom. Before that time, pistons either had oakum packing—loosely twisted fibers impregnated with tar or similar—or nothing at all. When James Watt, the pioneer of the steam engine, came back from a new boring mill, he was very pleased with the cylinder he had seen produced.

“It doth not differ,” Watt said, “from a true cylinder in any part by more than the thickness of an old shilling.” We’ve achieved better precision in recent years.

The piston ring does the job of pressing hard enough against the cylinder wall to achieve the basis of a seal. If you fitted the entire piston that tightly, the friction would be overwhelming. The trend in recent years has been for piston rings to become thinner. It was quite common when I was a boy for piston rings to be 3mm thick. Now they’re under 1mm. The reason for that is obvious: The less area you have pressing firmly against the cylinder wall, the lower the friction.

When the combustion process is started by the spark and the pressure in the cylinder begins to rise, the gas flows into the piston ring groove, across the top of the ring, and presses against the inside of it uniformly, inflating the ring against the cylinder wall.

The rate at which that gas flows into the ring groove is controlled by how much clearance there is between the ring and the groove. The fact that the ring is pressed firmly down against the bottom of the groove and out against the cylinder wall by combustion gas is attested by the fact that the wear surface—that which is pressed against the cylinder wall—is shiny and the top tends to collect deposits.

In a two-stroke, the top piston ring does most of the job of sealing. The second ring is there to help with the job of sealing as the top ring ages. In some cases, it can also be a heat-transfer ring. The pistons of large marine diesels have rows of rings. When they tried to do away with those rings, the pistons got hot.

In the case of a four-stroke piston, there will be a top compression ring, a second compression ring, and then an oil scraper, which is there to make sure the amount of oil that stays on the cylinder wall after the piston has passed by is quite limited so it doesn’t get into the combustion chamber. You can see how things used to be by watching an old movie: Pre-war automobiles all smoked. Scraper-ring technology was not highly developed in those days.

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.”