How To Choose the Best Piston Ring for Your Application

By Mark Houlahan 6/12/ Share Add Article To List

Piston Rings Have a Tough Job, Choosing the Wrong Ones Will Make That Job More Difficult

There has been a lot of advancement in piston ring technology over the last few decades. Piston ring materials, coatings, edge profiles, and even ring thickness have all seen great improvements in oil control, sealing, and wear. Of course, these enhancements in ring technology only work when they are used in the proper manner. A basic street engine built for a cruiser will use a much different ring package than a 1,000 horsepower turbocharged engine. There are many decisions to be made when choosing the right set of piston rings for your engine build. While some piston kits include rings, often the higher you go up the performance ladder the rings become a separate purchase decision.

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There is no one “best” ring package on the shelf. Determining the engine’s use, power level, compression, type of fuel, and of course any power adder, are all factors in choosing the proper ring package. The decision should factor in proper sealing, wear, and durability so that your engine produces maximum power with minimal blowby and proper oil control, all with a ring package that will wear appropriately for the intended use with minimum friction loss. What follows is a breakdown of modern piston ring materials, ring types, coatings, and more that will help you determine what type of piston rings are indeed best for your build. If you’re having the short block assembled by an engine builder, then obviously we suggest following the ring package guidelines that they have for your reciprocating assembly.

What Are Piston Rings Made Of?

When it comes to piston ring material types there are a few ring materials no longer used or only used in specialty applications now. Currently the most common piston ring material types for automotive engines are cast iron, ductile iron, and steel. While steel does have the highest tensile strength, don’t count out cast iron or ductile iron rings for the right applications. For example, if you’re performing a basic “hone and ring” job to drop back into your daily driver there is no need for the added expense of ductile iron or steel rings.

What Is the Benefit of Different Piston Ring Materials?

• Cast Iron: Fragile piston ring material properties, best used for stock engine builds due to low tensile strength. Low cost, great option for a budget rebuild.
• Ductile Iron: Stronger piston ring material properties with double the tensile strength of gray iron rings. Better option for performance engine builds.
• Steel: Better still in tensile strength and fatigue strength over ductile iron rings. Better option for boosted and nitrous applications. Used in narrow ring width applications for better sealing and less blow-by.

What Are the Different Types of Piston Rings?

Now that we’ve discussed piston ring material composition it is important to explain how many types of piston ring are commonly used. Modern pistons feature three different types of piston rings. Starting from the top of the piston you have the top compression ring. This is the primary ring that seals the piston to the combustion chamber wall. Below this ring you have the second or intermediate compression ring. This ring backs up the top ring by sealing the combustion chamber while also aiding in heat transfer and scraping oil from the cylinder wall. Finally, you have the oil control ring at the bottom, which has the piston ring function of controlling the amount of oil delivered to the combustion chamber wall for lubrication and cooling. Know that you can have different top and intermediate compression piston ring material selection in various ring packages, such as a ductile iron top ring with a cast iron intermediate compression ring.

Top and Intermediate Compression Ring Types:

• Conventional Ring: Top and 2nd rings with gaps that can be set for various uses (N/A, nitrous, etc.). This style of ring is often file-fit by the engine builder to a specific final specification. See the section on ring gaps below for more details.
• Gapless Top Ring: Provides increased horsepower and crankcase vacuum, used mostly on N/A engine applications to help fill the cylinder due to better ring seal. You want the gapless ring as close to the intake valve as possible. We offer Total Seal Gapless rings for your engine project build.
• Gapless 2nd Ring: Preferred ring for turbo or supercharged applications as well as boxer engines. With a turbo or blower helping to fill the cylinder the gapless 2nd ring is utilized to keep heat and contaminants out of the oil pan. A gapless top ring can be used in boosted applications as well but is certainly more effective in N/A setups.
• Gas Ported Top Ring: Increases horsepower by improving ring seal. The gas ported top ring features lateral gas ports machined into the top of the ring, which adds the benefits of gas porting to any piston. Works for both street and competition engines.

Oil Control Ring Types:

• One-Piece Oil Control Rings: Rarely used today, they are like a compression ring where the tension against the cylinder wall is taken from the ring’s cross section. A U-shaped design, the groove in the center moves excess oil back to the crankcase. Available with various ring profiles.
• Two-Piece Oil Control Rings: A coil spring is placed into the oil ring groove of the piston first and a special oil control ring is then placed over the coil spring. The spring provides the tension of the oil ring to the cylinder wall. Available with various ring profiles.
• Three-Piece Oil Control Rings: A pair of support rails with an expander between them for rail tension. The expander pushes the two rails, which act as scrapers, against the cylinder wall to remove engine oil and return it to the crankcase. This is the most used oil control ring design today.

Oil Ring Tension: When ordering piston rings, you often have the option of choosing the type of oil ring tension you desire for your engine build specs. You can choose from standard tension, low tension, and high tension oil ring offerings.

• Standard Tension: Varies by oil ring thickness, so a standard tension 3/16 oil ring is not the same tension as a standard tension 3.0mm oil ring. The thicker the oil ring, the higher the standard tension for that size.
• Low Tension: Also varies by oil ring thickness but does not always drop below the next size down in oil ring. For example, a low tension 3/16 oil ring is 15 lb/ft while a standard tension 3.0mm oil ring is 12 lb/ft. Utilized correctly, a lower tension oil ring increases horsepower and extends cylinder bore life.
• High Tension: Also varies by oil ring thickness, but inversely. For example, a high tension 3.0mm oil ring is 15 lb/ft and a standard tension 3/16 oil ring is 23 lb/ft. High tension oil rings are recommended for boosted and nitrous applications to reduce motor oil related detonation.

What Are the Different Types of Piston Ring Coatings?

Piston ring coatings are applied to the face of the ring (the side of the ring where it contacts the cylinder wall) to improve durability and lower friction. These coatings also provide faster break in. No longer do you have to drive 500 careful miles to break in your piston rings. With modern coatings they can break in quickly and provide a long service life. Ring coatings do affect piston ring price a bit, but we feel the added expense is well worth it for a modern performance engine build.

• Uncoated Cast Iron: Very soft for an easy break-in but doesn’t offer good durability.
• Hard Chrome Coating: Very hard for good durability but is very difficult for break-in with lower scuff resistance.
• Plasma Moly Coating: Rings bed in faster with higher scuff resistance, normally use a ductile iron base ring. Not for use with nitrous applications as the moly coating can fracture and break off the face of the ring.
• PVD Coating: Physical Vapor Deposition coatings provide a lower coefficient of friction, better adhesion and increased hardness compared to other coatings. Ideal for boosted and nitrous applications.

Are There Different Types of Piston Ring Profiles and How Are They Installed?

When we talk about piston ring profiles, we are referring to the outer edge of the ring that seals to the combustion chamber wall. Different profiles, or faces, are used for varying reasons, including increased sealing, greater oil control, and more. These ring profiles are often hard to see clearly, which is why all manufacturers mark their rings with a dot or the word “top” on the ring face so that the ring profile can be installed in the proper direction. This does not mean it is the top ring on the piston, but the orientation of the ring itself. Always install rings with the dot or “TOP” facing up.

• Square Face: Seals well but has higher wear, eventually wearing to a barrel shape, used on top ring.
• Barrel: Best sealing properties with longer life/lower wear, used on top ring.
• Taper Face: Used on 2nd compression ring, usually 2-4 degree taper of ring face to help scrape oil off cylinder wall.
• Napier: Groove machined under 2nd compression ring to improve oil removal from the cylinder wall.

The top compression ring will usually be a barrel face, while the second ring will often be a taper face or Napier face ring. The reason for the different profiles is to optimize the performance of the ring for the job it must perform.

How Do I Know What Size of Piston Ring I Need?

A piston ring’s diameter is directly proportional to the cylinder bore. If an overbore of the cylinder has occurred, then the proper piston ring size (and piston) must be ordered to properly fit. For example, a standard 4.00-inch bore that has been machined .030-inch to remove wear or wall damage will now require both 4.030-inch pistons and rings. A file to fit ring is +.005 over the bore size to allow the fitting of a tighter end gap in performance engines.

What Is the Standard Piston Ring End Gap?

End gap is usually specified by the ring manufacturer, but most fall back on the general rule of thumb of .-inch of ring gap per inch of bore diameter (for example, a 4.00-inch bore naturally aspirated engine would take a .018-inch top ring gap). Second rings are usually gapped at .006-inch per inch of bore. Again, for a naturally aspirated engine. The goal here is to have enough gap that as the rings are exposed to the combustion chamber’s heat that the ring end gap provides enough room for ring expansion without the ring ends butting up against each other, which will cause ring scuffing and even breakage. A piston ring end gap filing tool is the proper way to file both ends of the piston ring equally.

Boosted applications require larger ring gaps due to the increased combustion chamber temperatures these engine combinations see. Finally, some ring manufacturers spec the second ring to be gapped between .005-.010 more than the top ring to aid in preventing gas buildup between the top and second rings. Ultimately, we suggest going with the ring manufacturer’s specifications, for the ring material you’re using and the application. Be sure to watch our video on piston ring gap placement (clocking) for more details on proper ring installation.

Are Thicker Piston Rings Better?

Traditional piston ring sizing has been in fractional inch measurements. You’ll typically find top and 2nd rings in 5/64-inch, 1/16-inch, or .043-inch sizes, with oil rings typically in the 3/16-inch size. Modern engines moved to metric ring measurements of 1.5mm to 1.0mm for top and second rings with 3.0 to 2.0mm oil rings. These ring thicknesses have been the norm for decades, but moving to a thinner ring package has shown several advantages. With custom pistons, you’ll find types of piston rings as thin as .5mm (.020 inch). The thinner rings provide some great benefits, including increased horsepower and torque while reducing weight and compression height. Significant power gains can be had from utilizing thinner, modern rings and piston designs. While it has been more critical to use the proper piston ring installation pliers on thicker rings, we highly recommend that you use the same tool on thinner rings as well. The only types of piston rings that are OK to be “spiraled” onto the piston are the oil ring’s top and bottom rails. Never spiral the compression rings onto a piston.

As you can see, piston ring materials and piston ring function are just as critical to a successful engine build as the camshaft specs, cylinder head flow, and other major engine building decisions that you must make. We hope this guide has helped you understand what your piston ring options are and what is best for your build. If you have any questions on the types of piston rings your engine build should use, simply give our techs a call for expert assistance or reach out to your engine builder.

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Why piston ring selection and endgap are so important to proper break-in, power, and longevity

What makes piston ring selection and ring endgap so important to engine life and power? What's more, what makes that first engine firing so critical to proper break-in and longevity? Those first few minutes an engine runs sets the stage for power, efficiency, and engine life. In an ideal world, all engine builds would include a dyno pull for proper engine break-in. However, if you're like most of us you can scarcely afford the engine build let alone the added expense of a dyno session. On the dyno, an engine builder gets to load the engine and lean on the throttle hard to seat piston rings and bearings, which is more challenging to do on the open road.

However, engine break-in is getting ahead of our story. Piston and ring selection is where it begins. Before we get into piston ring types, we need to talk about what piston rings do. Though the job of piston rings is obvious—cylinder sealing and oil control—another equally important function is carrying heat from the piston to the cylinder wall via direct contact. Oil also carries destructive heat away. As piston rings have become thinner for friction reduction, efficiency, and more power, heat transfer from piston to cylinder wall has become more critical and decidedly harder on pistons.

If you're planning high-performance driving, piston ring selection is as critical as piston selection. Selection depends upon how your engine will be used. There are three basic piston types: cast, hypereutectic, and forged. Cast pistons tend to be old-school and were original equipment in most cars and trucks back in the day. Hypereutectic pistons are high silicon cast—chosen for their hardness and cost. They don't cost much more than cast and they're more durable. Forged pistons are very rugged and can withstand a lot of abuse. They're the only choice for high-performance applications. The downside to forged pistons is expansion rate. They require greater piston-to-cylinder wall clearances because they employ aggressive expansion rates. As a result, they're noisy on cold-start with a very predictable rattle that gives way to quiet as the engine warms.

Ring width is the first consideration in ring selection, which depends on how you intend to use your engine. If you're going racing or conducting a fuel economy challenge, you want thinner piston rings, which create less friction and consume less energy. More conventional piston ring dimensions (wider) are appropriate for weekend cruisers and daily commuters because they wear better and carry greater loads. You're going to get more life out of them.

Back in the day, standard ring packages were a 5/64-inch top compression and secondary rings followed by a 3/16-inch oil ring package. These dimensions apply to a ring's thickness. The 5/64-inch thickness (0.078-inch) called for a significant amount of pressure against the cylinder wall for proper sealing. Piston ring manufacturers call this radial tension. The downside to this is friction. The most friction in any piston ring package is created by the oil rings. However, the combined friction of all three rings is significant. It robs the engine of power and efficiency.

Automotive engineers eventually realized a thinner piston ring package would reduce internal friction and improve efficiency. Factory engines came with a 1.5mm/1.5mm/3.0mm piston ring package beginning in the s for reduced friction. As piston rings have become thinner, the amount of radial tension required to seal it against the cylinder wall is greatly reduced. This occurs because as we reduce the total surface area of the ring touching the cylinder wall, the radial tension can be reduced to produce the same amount of load on the ring.

By reducing radial tension on the cylinder wall with a thin piston ring, we're also reducing friction generated as the ring rides against the cylinder wall. We improve power by reducing internal friction in multiple cylinders. It can also be safely said thinner piston rings seal better, which means blow-by past the rings (lost power) is reduced. This means more cylinder pressure (heat energy) is captured above the pistons, rewarding us with greater sums of power. Little by little, the aftermarket performance industry has been offering us a greater selection of piston and ring combinations.

Total Seal piston rings from Summit Racing Equipment offer a greater lineup of thin, low-friction ring packages for the progressive engine builder, with Ultra-Thin Advanced Profile series piston rings with 0.9mm top and second rings with a 2mm oil ring. According to Summit Racing, replacing a typical 1/16/1/16/3/16-inch thin-ring package with the Total Seal Ultra-Thin 0.9/0.9/2mm combination reduces internal friction by as much as 90-percent. It is this advanced ring technology that gives us both performance and efficiency.

Advanced piston ring technology does not come cheap. Total Seal's Classic 1/16-inch ring set from Summit for a 4.030-inch bore costs just over $100. Total Seal Gapless Secondary rings cost just under $400 per set. Ring spacers run costs even higher at nearly $500.

For performance applications, it is more about choosing the proper piston configuration and compression ratio along with selecting an optimized ring package. This must first begin with the selection of the right ring material. Ring widths and type can be chosen once the material has been selected. JE Pistons, as one example, offers a huge selection of ring materials, which at first can be overwhelming. First iscarbon steel, which is a much more malleable material than traditional cast iron and can handle higher temperatures without losing temper and is better able to withstand detonation.Cast iron, by its very nature is brittle and not as strong as hardened forged steel. Hardened steel top rings perform so well that even automakers are using more of them these days in production engines for durability. Steel makes more sense if you're planning boost, nitrous, or excessive amounts of compression because it tolerates the extremes better than iron.

Chrome-facedwere popular at one time, but have lost favor and aren't used much anymore by engine builders. The problem with many chrome-faced rings was they were extremely hard and difficult to break-in due to their hardness. What's more, they didn't handle detonation very well. It is suggested you not use them.

Some rings are offered with a tough plasma molybdenum coating in addition to gas nitride for durability.Steel nitridetop rings are a good choice for performance applications, however, are not always optimum because they can be expensive. JE Pistons offers a great many ring options if you're on a budget.Hardenedis a good option for street performance engines and a step up from traditional cast iron with magnesium added to the grey iron to improve ductility because ductile iron is more flexible. It is less likely to break. In fact, ductile iron is roughly twice the tensile strength of grey iron and flexes instead of breaking when subjected to high stresses. This makes ductile iron a terrific top ring when you're concerned with cost. JE Pistons tells us ductile iron rings are offered with a plasma molybdenum (moly) face coating to make them more compatible with iron cylinder walls.

JE Pistons adds itsPremium Racering pack is a great example of a ductile iron top ring that uses the more modern plasma-moly inlay technology, which produces an extremely hard, porous, wear-resistant surface that retains oil and improves lubrication while at the same time reducing internal friction. The plasma is applied by spraying the ring with an alloy powder containing Chromium, Molybdenum, and Nickel along with other elements into a small channel in the ring face. Using extreme heat, powdered metal turns into a molten spray that offers adhesion characteristics that reduce the potential for flaking and failure. This approach means faster break-in and better cylinder sealing.

The second piston ring doesn't deal with the intense heat and pressure the top ring does. JE Pistons tells us its Plasma Moly ring package is an excellent choice calling for a carbon steel nitride top ring with a ductile iron second. A less expensive version of this combination is theSportsman Seriesring package that involves a plasma-sprayed ductile iron top ring combined with a grey iron second ring, which is a more affordable ring package.

Oil rings in the third groove are simpler, with most ring packs offering carbon steel for the two sealing rings. The expander in the middle of the two rings may vary but the main question you want to ask yourself is—how will the engine be used? Once you've made your ring material selections, you can move on to the next level of decision making in terms of top and second ring design, face styles, radial thickness, and perhaps any special treatments such as lapping and/or ultra or critical finish steps. This entire process is all aimed at optimizing ring seal and capturing cylinder pressure above the piston where it can be translated into power.

If you're looking to save money and aren't going racing, Summit Racing Equipment and Speedway Motors offer a broad selection of Speed Pro cast, hypereutectic, and even forged pistons for virtually every Detroit-born engine out there. Hypereutectic pistons are a nice compromise over forged and cast if you're just going cruising. Summit's tech staff can advise you best on what to select along with just the right ring package for the piston you have selected.

Contact us to discuss your requirements of piston ring manufacturer. Our experienced sales team can help you identify the options that best suit your needs.