In addition to varying in thickness and diameter, piston rings now come in several different materials, styles, and coatings to maximize performance across various engine environments.
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Not too long ago, all piston rings were essentially the same: heavy, thick, and made exclusively from cast iron. Today, performance engine builders have a wide range of material choices for piston rings and finishes. This evolution is driven by the need for stronger ring materials that can withstand the harsh environments of higher cylinder temperatures and pressures.
Before diving into materials, it's crucial to understand the responsibilities of a piston ring. While sealing combustion pressure is the primary goal, an equally important function is to transfer heat from the piston to the cylinder wall. As rings become thinner, this heat transfer function becomes even more critical. Additionally, the rings must minimize the amount of oil that enters the combustion space. Contrary to popular belief, the second ring's main task is not as a backup for sealing cylinder pressure but as an oil control mechanism, sweeping up the remaining oil from the cylinder wall not removed by the oil ring.
For a performance engine, selecting the right piston involves not only choosing the proper configuration and compression ratio but also selecting an optimized ring package. This selection process largely depends on how the engine will be used and starts with choosing the appropriate ring material. Once the material is finalized, ring widths and designs can be determined.
The first material is carbon steel, which is much more malleable than cast iron. It can handle higher temperatures without losing its temper and is better suited to withstand detonation. Cast iron, by nature, is very brittle and not as strong as gas-hardened forged steel. Gas nitride top rings perform exceptionally well in a wide range of applications, which is why even original equipment manufacturers (OEMs) now invest extra money in this ring material for production engines.
While carbon steel may seem like a generic material, it is an excellent choice for high-cylinder-pressure applications such as nitrous, turbocharged, supercharged, and high-rpm engines.
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Steel nitrided top rings are an excellent ring in terms of durabilty and strength. They are formed from forged steel for strength and ductility, and the nitriding process makes them extremely hard and detonation resistant.
Chrome faced rings were a popular option a few years back, but now have also succumbed to progress and are rarely employed in modern performance engines. Many chrome facings had issues due to their extreme hardness, making them difficult to break in properly. They often suffered from flaking or cracking when exposed to detonation. As a result, most dirt circle track applications have transitioned to steel nitride ring packages. Some steel nitride rings also feature a plasma molybdenum coating in addition to gas nitride.
Steel nitride top rings are an excellent choice for street performance applications. However, they may not be necessary from a cost standpoint for every engine.
Once you've chosen your ring materials, you can proceed to select the design features for the top and second rings, including face styles, radial thickness, and any special treatments like lapping or critical finish steps. The goal of this process is to optimize ring sealing and ensure that all cylinder pressure is effectively contained within the combustion space for maximum performance.
Piston ring materials must possess the following characteristics in order for the ring to perform its functions under the most severe operating conditions. Moreover, these characteristics must be retained over the engine's entire operating life; not just when it is first brought into service!
Cast iron has been and still is a key piston ring material. However, engine design has continued to evolve. To meet the ever-increasing performance demands of modern engines, piston ring materials have been changing as well. In many applications, steel rings have replaced iron rings because:
Let's compare the properties of cast iron and steel.
Typical Steel Material Material Tensile Strength