By Rob Kinnan

You will get efficient and thoughtful service from Dongya.

Illustrations: Steve Amos

In a conventional application, there are three rings per piston. The top ring is called the compression ring. Its purpose is to seal the combustion gas pressure, transfer heat from the piston to the cylinder wall (and therefore the water jacket) and control oil consumption. The second ring from the top is also called a compression ring, and it functions basically as a backup for the top ring. The bottom ring is called the oil ring, and it is actually made up of three different components, which are described later.

The How it Works series has up to this point been an explanation of the automobile's basic operating systems, from the engine's four-stroke cycle all the way to the differential. We've intentionally glossed over the individual parts and minute details to instead look at the whole picture. But now that we've covered the major components, we're going to get increasingly technical and start focusing on the individual parts, which make all the difference in how these systems work to make a car go fast. Last month's How It Works explained the hows and wherefores of piston design. This month's installment will run rings around that subject—literally—as we discuss how piston rings work and why an engine needs them.

A piston's function is to draw in the air/fuel mixture, compress it into a more dense mixture and then transfer the heat energy and pressure of the combustion process to the crankshaft. The piston has to be a little smaller than the cylinder bore so that it can move freely. The difference in size means that there is a small gap between the piston and the cylinder wall, and the gap allows pressure to leak past the piston, which detracts from the piston's functions and its ability to transfer the heat of combustion to the engine block and water jacket. The piston rings seal the gap and help transfer heat but still allow the piston to move freely up and down in the cylinder.

Just as in nearly every engine component, there are many different types of rings to suit every situation and purpose, and to a newcomer it can be confusing. The following should give you enough confidence to bench race with the best of them when the subject turns to piston rings.

This chart, provided by Childs & Albert, shows the function of each ring in a three-ring package. The top ring is mostly responsible for sealing combustion-chamber pressure, but still does a bit of oil control. The second ring actually functions more as oil control yet seals off what pressure makes it past the top ring. The oil ring only controls oil flow to the cylinder walls and does not hold any pressure.

By virtue of its location on the piston, the top ring is subjected to a more extreme environment than the second ring, so the materials and coatings are different for those two rings. In the past, rings were made of cast gray iron or chrome-plated stainless steel, but most modern top rings are now ductile iron, which is more durable, heat resistant and less prone to cracking under detonation. A commonly used coating for the top ring is molybdenum (moly for short). The second ring is still the traditional cast iron, which works fine because the second ring is farther down the piston and shielded by the top ring from all the commotion in the combustion chamber, so it sees lower operating temperatures and pressures. Thus, it doesn't have to be as durable as the top ring. Cast iron is also more economical than ductile iron. Some severe racing applications have coated ductile iron rings in both the top and second groove, but for a normal street application it's expensive overkill.

Looking at them from the ends, the top- and second-groove piston rings are each shaped differently. The outside edge of the top ring is normally barrel-shaped to seal out combustion pressure, whereas the second ring, since its major responsibility is oil control, has a tapered face that scrapes oil off the cylinder walls on the downstroke. This taper is very slight, usually only a few degrees. Both rings also have a bevel on the internal diameter, which we'll explain next.

The opposite effect, negative twist, is often used on the second ring. In that situation, the bevel is on the inside bottom of the ring, so the ring twists in the opposite direction, sealing better at the bottom outside edge and at the top inside edge of the ring. The advantage of that is that the twisting helps the second ring scrape oil off the cylinder walls on the downstroke.

Another type of ring is called the Dykes ring. The Dykes ring (fop) is a gas pressure-loaded ring shaped like the letter L. The thinner part of the ring seats in the piston. When combustion pressure gets high, it gets trapped in the ring groove and pushes against the L-shaped portion, forcing it out against the cylinder wall. The advantages of that type of ring are higher sealing capabilities and lower tension—since the ring uses combustion chamber pressure for sealing, it doesn't need nearly as much diametric tension, tension that robs horsepower through friction. A Dykes ring usually has only about 4 pounds of tension as opposed to 9 to 10 pounds for a standard-type ring. It is commonly used on endurance and high-output drag-racing engines. The disadvantage of a Dykes ring is that since it has no built-in twist, it doesn't do a very good job of oil control, and therefore is not a good ring for a street engine.

Piston rings seal against the cylinder wall in a couple of ways. Think of the ring as a spring. As it's compressed to fit in the cylinder, it wants to spring back outward. That outward force is known as diametric tension and helps seal the space between the piston and the cylinder bore. But rings also seal by twisting. A bevel cut into one side of the ring determines which direction it twists. A top ring has a bevel on the inside top edge of the ring as shown in the illustration. When the ring is compressed (to fit in the bore), the bevel makes the ring twist in its seat on the piston and seal at the inside bottom of the ring and at the outside top of the ring. That is known as positive twist and works with the diametric tension to seal not only at the ring's face but also inside the groove. It also enhances oil control on deceleration, which is usually incorporated into the top ring.

Gas porting is a method used by some drag racers to get Dykes ring performance out of a standard ring. Tiny holes are drilled in the top of the piston and lead down into the top ring groove. During the compression and power strokes, combustion-chamber pressure goes through those holes and pressurizes the backside of the top ring, forcing it out against the cylinder wall for a tight seal. The bad part about gas porting is that during a high-vacuum situation, such as during deceleration, oil gets sucked through those holes directly into the chamber, leading to detonation. The ports also subject the ring to extreme heat, which drastically shortens its life. Those are real problems on a street car, but it's not a big deal on a drag-race motor that only operates under low-vacuum, wide-open throttle and is torn down often. Many engine builders still use gas-port pistons, but using a Dykes ring does the same thing for less expense and much greater durability.

Conventional piston rings have a gap so that they can be spread apart enough to be installed on the piston and to prevent the ends of the ring from touching, which will instantly destroy the ring. The gap is small (only about .004 inch per inch of cylinder bore), but it's there nonetheless and presents an escape path for cylinder pressure and allows for expansion of the ring due to heat. That is partly where blow-by comes from—combustion-chamber pressure that leaks past the rings and into the crankcase. Childs & Albert is one company that is trying to do away with the gap, and its Zero Gap Second ring is the result. An overlapping step makes it much harder for the combustion pressure Io get past the ring, theoretically making more power with less blow-by. Due to the complex shape, those ductile iron rings don't currently have a moly coating, so they are used in the second groove only. Check out next month's story on this package.

The bottom ring is called the oil ring and its job is just that, to make sure just enough oil gets on the cylinder walls to lubricate the other rings but not so much that oil gets into the combustion chamber. An oil ring is actually three rings: two thin rails and a central expander ring. The chart shows that the oil ring (standard tension) accounts for nearly 50 percent of the frictional lossesin the entire engine!For that reason, there are three levels of tensions available in oil rings: high tension, standard tension and low tension. High-tension oil rings are used only on engines that have high piston-to-wall clearances, high rod/stroke angles and extreme rpm, such as Top Alcohol engines. Standard-tension rings are the most common for production cars, but recent technology has allowed low-tension oil rings to control oil almost as well as standard-tension rings and still benefit from less parasitic frictional loss.

OTHER STUFF YOU OUGHTA KNOW

RING GAP—When a piston ring is installed in a cylinder, there must be a sufficient gap between the ends of the ring to prevent the ends from touching. As the engine gets hot, ports expand and the gap gets smaller. If the ring ends butt together, the ring is instantly destroyed and, chances are, so is the piston and cylinder wall. It's always better to have too large a gap than too small.

RING SEATING—You've probably heard someone say, "The rings haven't seated yet." Nearly all modern piston rings are lapped in at the factory and do in fact completely seat themselves to the cylinder wall in about the first five minutes of engine operation, assuming the cylinder was properly machined.

RING ROTATION—The rings don't stay put once they're installed on the piston and in the cylinder. They actually rotate around the piston as the engine is running. The amount of rotation depends on how the cylinder was honed: A steeper crosshatch angle means more rotation.

RING SIZES—Rings come in different widths for different applications. The reasons for thicker or thinner rings are twofold: lower tension and therefore friction, or because the piston only has room for a certain thickness ring package, such as when the pin is very high on the piston. The most common ring sizes are 5/64 inch, 1/16, .043, 1.5 mm, 1.2 mm and 1 mm for the top ring; 5/64, 1/16, .043 and 1.5 mm for the second ring; 3/16, 4 mm, 1/8, 3 mm and 2 mm for the oil ring assembly.

INSTALLATION—Piston rings shouldneverbe installed dry or without lubrication. Engine oil can be used in moderation, but a better choice is a special lube made by the ring manufacturer (Childs & Albert has one) or automatic transmission fluid.

TWO-RING PACKAGES—In an effort to remove as much friction as possible, some race-only engines use only two rings per piston. It doesn't seal very well and is definitely not recommended for the street, but if does dramatically cut down on frictional losses,

MOTORTREND and HOT ROD's rich magazine history and legacy dating back to is something highly valued by its longtime readers, and that's why we've invested deeply to make the content available to you in a modern and accessible format. In the interest of transparency, these magazine articles are presented as originally published, without modification, and may contain content that does not reflect the company's contemporary values and standards.

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.

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.

If you want to learn more, please visit our website Piston Ring Manufacturer.

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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