Piston ring materials

The piston rings used in a race engine are primarily a seal, controlling the flow of oil upwards into the combustion chamber and the flow of blow-by gases into the crankcase. We don't want oil to get into the combustion chamber; in large enough quantities it can cause combustion problems, but even in small quantities where the oil is burned, the loss of oil can be serious, causing us to carry a supply of oil with us. On the road, this isn't a huge problem. In a race though, it means using a larger tank with more oil.

Equally, we don't want unburned compressed charge or burned combustion products to flow past the rings into the crankcases. Not only do we lose cylinder pressure, and therefore performance by doing so, we contaminate the oil. Oil dilution by unburned fuel thins the oil, and we end up with a substance that we hadn't intended to lubricate our engine with.

The correct choice of ring materials, heat treatment and coatings is critical to getting the best from an engine. Where you have adapted a production engine for racing, there is probably a mass of information available to you about what works well. For a bespoke race engine, the design engineer has a free choice. The most basic choice available to many people is whether to use cast iron or steel. For high-performance race engines, steel is generally the material of choice. However, cast iron can be more durable in certain situations owing to the fact that it often has graphite at the surface. One well-known engine builder says he finds that steel rings need more oil and a deeper valley in the cylinder honing compared to a cast-iron ring.

The choice of steels used for piston rings in engines ranges from very low-strength, low-alloy steels though stainless steels to high-strength, highly alloyed tool steels. Generally speaking, piston rings made from the steel grades with higher strength are used for more arduous applications.

While 'plain' steel materials of varying strength levels are widely used, it is also common for piston rings to be surface engineered to give improved sealing performance and wear behaviour. Nitrided steel rings are commonly used for racing applications, and both gas nitriding and plasma nitriding methods are used to provide the hard surface layer. Nitrocarburising is also used, but the 'case depths' are shallow compared to a nitrided part. It should be noted that nitrocarburising and nitriding are both used in the production of certain cast-iron piston rings as well.

Tool steel piston rings are generally used where the durability of other materials is insufficient. As a piston ring, they can be an expensive option, but where ring wear or breakage is the limiting factor for engine durability, they can reduce rebuild frequency. For a small extra investment in a piston ring made from a higher-specification material, the cost saving can prove to be substantial.

Tool steels combine toughness and wear resistance with high strength. These qualities also allow engineers to specify ever-smaller piston rings without sacrificing engine longevity. This is important as, in a well-optimised engine, the reciprocating mass savings are not limited only to the piston ring but also to the material around the ring grooves. The overall mass savings can be significant, and with the mass savings come performance and economy.

Piston ring development in racing is one item that benefits roadcars too; there are production engines in widespread use with piston rings that would have been considered pretty 'racy' in a production engine only 10-15 years ago.

Written by Wayne Ward