Piston rings must be carefully selected and finely tailored to piston and cylinder bore for high-performance and race engines. If not, you risk decreased performance at best, and catastrophic engine failure at worst. Total Seal Piston Rings has built its company around helping enthusiasts find the right piston ring for their engine combination and application.
The ring selection process is pivotal. Builders are assembling a wide range of naturally-aspirated, forced-induction, alcohol-fueled, and other special-application engines. Thus, one ring does not fit all engines … not even close. In fact, the piston model, engine block and cylinder head combination, and power output needs to be well-defined, so you can match the ideal ring set to the engine package.
Piston rings are made out of the finest materials, exhaustively engineered, and meticulously constructed. These thin circular strips of metal travel up and down the cylinder bore several thousand times per minute. In addition, they are subjected to combustion chamber temperatures of 1,100 degrees or more. For a production engine, these rings are expected to endure more than 150,000 miles of service. No small feat indeed.
Rings must seal the cylinder bore to the piston, so the vacuum is maintained, cylinder pressure is maximized, oil blow-by is prevented, cylinder walls are lubricated, and oil consumption is controlled. Engine builders and owners alike, demand a lot from piston rings, therefore, it’s imperative to choose wisely.
When it comes to selecting the right ring package for your engine build, there is no simple straightforward formula. You need to do research to determine the piston rings that are best suited for your engine build. To do so, several crucial factors must be weighed in order to make the best selection. The ring set you select should have the ideal combination of ring thickness, ring tension, ring design type (Napier or barrel-faced), and the most important factor is often material construction.
As a guideline, the more horsepower an engine produces, the more heat it will produce. As a consequence, an engine typically requires a thicker ring made of higher-end material and higher tension. By comparison, a stock Chevy Gen I small-block will run fine on cast-iron rings with moderate tension because it produces lower internal engine temperatures.
Most of the piston companies sell rings with their pistons … they’re basically giving you the cheapest thing that they can put their hands on. – Keith Jones
According to Keith Jones, technical specialist with Total Seal, “Probably, the biggest mistake I see today is most of the piston companies sell rings with their pistons. The problem is they’re basically giving you the cheapest thing that they can put their hands on. Not all of them do, but a lot of them do. The ring that’s sold with the piston is simply sold by a bore size for a particular engine. I see a tremendous amount of failures due to that. The rings being supplied with the piston are simply not right for that particular customer’s application. It may work in a naturally-aspirated engine, but when an engine is a twin turbo with 40 pounds of boost, you have an immediate failure.”
In order to arrive at the correct ring choice, you need to define the entire engine package, because all the parts in the engine are interdependent, and one component affects another. As Jones puts it, “The pistons, rings, and cylinders all have to work together. These are all links in the chain, and if you do something to one of them, that affects the other. There’s a lot of things that affect good ring seal in a high-performance engine. It’s a fairly complex situation.”
Stainless steel with chrome nitride and steel reside at the top of the hierarchy of high-performance rings. These rings withstand high-combustion chamber temperatures and provide better heat transfer than ductile or cast-iron. In addition, these SAE 9254, 440, and M2 steel alloy offer a higher-tensile strength than ductile iron. This translates in enhanced rigidity and less flexing for race and extreme-high-performance engine builds. But these materials are not required for all engine builds and can be overkill. Carbon steel that’s nitride coated is also a popular option for high-performance builds. Steel nitride is a common choice for many racing applications, and some steel rings are coated with plasma molybdenum.
According to Jones, “Most companies service gas nitride, which is a way of making an inexpensive steel ring work. Chrome nitride is better, but it’s more expensive. We have multiple grades of steel top rings ranging from [SAE] 9254 to 440, up to M2 tool steel. We use a 440 stainless steel and then the ring is chrome-nitride coated, which in our view, is the best way to do it.”
Hardened-ductile iron ring sets are a popular choice for moderate high-performance engine builds and slot in between steel and common ductile iron rings. These are commonly used for naturally-aspirated applications. In addition, ductile iron provides better flexibility and heat resistance than cast-iron rings. But keep in mind, there’s a massive mosaic of material and coating combinations, and it often takes some research to find right set.
A barrel-faced steel ring is often selected for plenty of high-performance and race builds because of its improved sealing. Higher cylinder pressures and high heat chamber temperatures often twist the ring in the piston groove. The barrel-face design produces less friction than a Napier design. Piston design also becomes a factor as gas ports vent vertically and horizontally to enhance ring sealing. The extra pressure behind the top ring helps force it outward against the cylinder wall for a tighter seal.
The Napier-style design is often used for the second ring which provides oil scraping on the cylinder wall and minimizes oil consumption. If a piston is designed for a tapered second ring, a J-groove is often notched in the lower second ring groove land. This notch provides an area for oil to collect and then the oil ring can scrape it off the cylinder wall.
The Napier-style second ring is not a suitable choice for high-boost, forced-induction, alcohol, or nitro engines because it has to support high cylinder pressures in these applications. This ring-type often twists and deflects more, and as a result, the engine suffers more oil blow-by.
For several years, the prevailing wisdom has been to select the thinnest rings with the least amount of tension so cylinder wall friction can be kept to the absolute minimum, but a balance needs to be achieved. Currently, 0.6mm thick compression rings and 3mm oil rings are common in many different builds. For most street engine builds, you are better off not going this thin, but rather, you should go with a slightly heavier and thicker ring in order to maintain piston seal.
According to Jones, “When we get into really thin rings, that’s not the ring to use for every application. Everything comes back to the application. What are you building? How big is the ring? What is the ring made out of? What is the material? There is no simple answer.”
He added, “The thinner the ring is, the less friction it has, which is a very good thing. But when you get into engines with a turbo, blower, or nitrous, you’re dealing with more heat. To make more horsepower, you have to make more heat.”
You can opt for a thinner ring package, such as 1.5mm top and second compression rings while the oil ring is 3mm. With rings on the thinner range, you can select higher-tension rings about 14 to 16 pounds. But of course, you need to be sure that these rings can handle the heat-load of the engine, because sacrificing low friction for poor sealing is not a viable trade-off.
Piston ring material, thickness, and tension must be matched or suited for the cylinder wall. Stock GM, Ford, and Mopar blocks from the 1960s and 1970s tend to be on the softer side, while the new high-performance blocks from World Products, Dart, RHS, have a much higher density and hardness, and there is a wide range of block hardness.
The piston ring material must be compatible with the block material so that the rings quickly seat to the cylinder wall and provide a positive seal to maintain cylinder pressure. Thus, the cylinder walls must be properly prepared for piston rings. Hence, accurate and proper cylinder honing is a must to realize ultimate ring sealing, and therefore, the best cylinder pressure. Professional engine shops are equipped with a PAT gauge that determines cylinder wall surface roughness. High-performance engines should yield cylinder roughness measurements of about RPK 8-12, RK 20-30, and RVK 30-50.
Ask the Experts
In the space of this brief article, I was not able to provide a definitive guide to selecting piston rings for every conceivable engine platform and application. When selecting rings, you should consult a qualified source, so you thoroughly and completely consider all the relevant factors.
As Jones states, “If they are not confident in what they’re doing, reach out to us. We’re here for this, we’re racers, we’re gearheads. When it comes to piston rings, make us your first call and not your last. In many cases, we have talked to customers about XZY pistons and they come with XYZ brand of rings. It’s just not right for the application and they’re having all of these problems. The owner spends all this money because the engine is being taken back apart. It just wasn’t the right selection to begin with.”