Advancements in piston ring technology over the past few decades have significantly improved materials, coatings, edge profiles, and ring thickness, enhancing oil control, sealing, and wear. However, these advancements only perform optimally when used correctly. A basic street engine for cruising requires a vastly different ring package than a 1,000-horsepower turbocharged engine. Selecting the right piston rings for your build involves numerous considerations. While some piston kits include rings, higher-performance builds often require rings as a separate purchase.
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There's no single “best” ring package. Choosing the right one depends on factors such as engine use, power level, compression, fuel type, and any power adders. The ideal ring package ensures proper sealing, durability, and minimal friction loss while maximizing power and oil control. It must also wear appropriately for the engine's intended use.
Below is a breakdown of modern piston ring materials, types, and coatings to help you select the best rings for your build. If a professional is assembling your short block, it's always wise to follow their recommendations for your reciprocating assembly.
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.
Modern pistons utilize three distinct types of piston rings, each serving a specific function. At the top is the top compression ring, the primary seal between the piston and the combustion chamber wall. Below this is the second or intermediate compression ring, which supports the top ring by enhancing combustion chamber sealing, aiding heat transfer, and scraping excess oil from the cylinder wall. Lastly, the oil control ring at the bottom regulates the oil delivered to the cylinder wall for lubrication and cooling.
It's worth noting that different materials can be used for the top and intermediate compression rings within various ring packages. For example, a package might feature a ductile iron top ring paired with a cast iron intermediate compression ring.
- Conventional Ring: Features gaps that can be adjusted for various applications (N/A, nitrous, etc.). This style is often file-fit by engine builders to meet specific final specifications. Refer to the ring gap section below for further details.
- Gapless Top Ring: Designed to enhance horsepower and crankcase vacuum, this type is mainly used in N/A engine applications to improve cylinder filling through better ring sealing. For optimal performance, position the gapless ring close to the intake valve. Total Seal Gapless rings are available for engine project builds.
- Gapless 2nd Ring: Ideal for turbocharged, supercharged, or boxer engines. With forced induction aiding cylinder filling, the gapless 2nd ring is effective in keeping heat and contaminants out of the oil pan. While a gapless top ring can also be used in boosted setups, it performs best in N/A applications.
- Gas Ported Top Ring: Enhances horsepower by improving ring seal. This ring features lateral gas ports machined into its top, bringing the benefits of gas porting to any piston. Suitable for both street and competition engines.
- One-Piece Oil Control Rings: Rarely used today, these function like compression rings, with cylinder wall tension derived from the ring's cross-section. Featuring a U-shaped design, the center groove directs excess oil back to the crankcase. They are available in various ring profiles.
- Two-Piece Oil Control Rings: Consist of a coil spring placed into the piston's oil ring groove, followed by a specialized oil control ring over the spring. The spring provides tension to press the ring against the cylinder wall. These are also available in various profiles.
- Three-Piece Oil Control Rings: Composed of a pair of support rails with an expander in between to provide rail tension. The expander pushes the rails outward, allowing them to act as scrapers to remove excess oil from the cylinder wall and return it to the crankcase. This design is the most commonly used today.
When selecting piston rings, you can specify the desired oil ring tension to suit your engine build specifications. The options include standard tension, low tension, and high tension oil rings.
- Standard Tension:Oil ring tension varies by thickness. For instance, a standard tension 3/16 oil ring has a higher tension than a standard tension 3.0mm oil ring. Generally, thicker oil rings have higher standard tension for their size.
- Low Tension: Although also thickness-dependent, low tension does not always fall below the next size down in oil ring tension. For example, a low tension 3/16 oil ring might measure 15 lb/ft, while a standard tension 3.0mm oil ring measures 12 lb/ft. Proper use of lower tension oil rings can increase horsepower and enhance cylinder bore longevity.
- High Tension:High tension values also depend on thickness but operate inversely. For example, a high tension 3.0mm oil ring might measure 15 lb/ft, while a standard tension 3/16 oil ring could reach 23 lb/ft. High tension oil rings are ideal for boosted and nitrous applications, helping to mitigate oil-related detonation risks.
Piston ring profiles refer to the outer edge of the ring that seals against the combustion chamber wall. Various profiles are designed for specific functions, such as enhanced sealing or improved oil control. While these profiles can be hard to distinguish visually, manufacturers mark their rings with a dot or the word "TOP" to indicate the correct installation orientation. Note that this marking shows the correct orientation of the ring itself, not its position on the piston. Always install rings with the dot or "TOP" facing up.
- Square Face: Provides excellent sealing but experiences higher wear over time, eventually wearing into a barrel shape. Typically used for the top ring.
- Barrel Face: Offers the best sealing properties with lower wear and longer life, commonly used on the top ring.
- Taper Face: Found on the second compression ring, this profile features a 2-4 degree taper to assist in scraping oil off the cylinder wall.
- Napier Face: Features a groove machined under the second compression ring to enhance oil removal from the cylinder wall.
The top compression ring is generally a barrel face design, while the second ring often features a taper or Napier face. These varying profiles are selected to optimize each ring's performance for its specific function.
The piston ring diameter must match the cylinder bore. If the cylinder has been overbored, you'll need to order the corresponding oversized piston and ring set. For instance, a standard 4.00-inch bore machined to 4.030 inches will require 4.030-inch pistons and rings. File-to-fit rings are sized +.005 inches over the bore diameter, allowing precise adjustment of the end gap for performance applications.
End gap specifications are usually provided by the ring manufacturer, but a common guideline is . inches of gap per inch of bore diameter for the top ring in naturally aspirated engines. For example, a 4.00-inch bore engine would require an .018-inch top ring gap. Second rings typically have a slightly larger gap, about .006 inches per inch of bore diameter.
The end gap must accommodate ring expansion due to combustion heat, ensuring the ring ends do not touch, which could lead to scuffing or breakage. Using a piston ring gap filer is essential for evenly filing both ends of the ring to achieve the proper gap.
Traditionally, piston rings have been measured in fractional inches, with common sizes being 5/64-inch, 1/16-inch, or .043-inch for top and second rings, and 3/16-inch for oil rings. Modern engines, however, have transitioned to thinner, metric-sized rings, such as 1.5mm to 1.0mm for compression rings and 3.0mm to 2.0mm for oil rings. Custom pistons can use even thinner rings, down to .5mm (.020 inches).
- Increased Horsepower and Torque: Reduced friction from thinner rings contributes to better engine performance.
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- Reduced Weight and Compression Height: Modern piston and ring designs allow for more efficient engine operation and lighter components.
While thicker rings require piston ring installation pliers for proper handling, thinner rings benefit from the same careful installation to avoid damage. Only the oil ring's top and bottom rails should ever be “spiraled” onto the piston; compression rings must always be installed with dedicated tools to ensure accuracy and prevent deformation.
Engine builders should always verify that rings are available before ordering custom pistons. “Small runs of rings get very expensive,” said our source from Total Seal. “It’s going to cost me as much to make the rings as it was for you to buy the pistons.”
No matter if the job is done by a seasoned engine builder or a complete novice, small mistakes can have big consequences when it comes to selecting and installing piston rings. After all, the rings have many jobs to do, from maintaining a tight seal under immense pressure to managing heat inside the cylinder to scraping excess oil off the walls and leaving just enough to lubricate the piston on its way up.
How piston rings work, and how important they are to an engine’s operation, is common, Engine Building 101 knowledge. Yet even in the performance world, misconceptions and outdated information about these key components keep cropping up. We spoke with experts on the subject to find out what builders should be doing to get the most out of their rings.
This first bit of advice seems like common sense, but time and time again Keith Jones of Total Seal Piston Rings in Phoenix, Arizona, is greeted by horror stories from customers come Monday morning—after the fact when the damage is already done.
“Make me your first call, not your last,” he said. “We can sort all these problems out before you start.”
There is so much bad information, especially on the Internet, that Jones said when in doubt, consult the manufacturer.
For example, when using a power adder, do the rings that came with the pistons look too skinny? Give the sales rep a call and double-check that they’re the right ones for the application.
Jones also strongly recommended checking to make sure rings are available before ordering custom pistons. “Small runs of rings get very expensive,” he said. “It’s going to cost me as much to make the rings as it was for you to buy the pistons.”
Don’t take the engine building tips passed down from your granddad as gospel. A lot has changed with piston rings in the past few decades, as Dan Begle of MAHLE Aftermarket in Farmington Hills, Michigan, can attest.
“Years ago, performance development drove the OEMs, but now it’s the other way around,” he said. “As engines are becoming more efficient and performance-oriented, that technology trickles down to the performance world.”
With that in mind, thinner modern steel rings can often do the same job, if not better, than a thicker ring of an older design. “You can get by with a thinner ring using a better material,” Begle said, “so you can carry more heat out even with less thickness. You can run a 1 mm ring and pull more heat out than a 1/16th-inch cast ring.”
But old habits die hard. “There are many people still using late s technology in their engines, which is like choosing a Nokia brick when you could have an iPhone,” said Total Seal’s Lake Speed, Jr. “People still buy 5/64th rings, which is like a rotary dial !”
With so many different piston ring base materials and face coatings available, choosing the correct rings for an engine can be an intimidating task. Two materials—ductile iron and cast iron—can be crossed right off the list, as they’re outdated and surpassed by carbon steel rings in their ability to dissipate heat and hold up to high cylinder pressures.
Another type of ring to eliminate is chromoly, because it doesn’t exist. That persistent misnomer likely originates from a conflation of chrome, a coating once used for performance rings, and moly (short for molybdenum), a coating still used today. Jones cited this common misconception as yet another example of the misinformation that pervades the industry. To this day he encounters customers who insist they have chromoly rings.
A real coating to consider is chromium nitride (CrN), also known as physical vapor deposition (PVD). It’s a solid choice for top rings as the coating is low-friction, wear-resistant and easy on cylinder walls.
“Even experienced engine builders can benefit from learning what ring and face coatings are out there,” said Begle. “Technology has changed so much. Just because you ran that ring before doesn’t mean it’s the latest and greatest. There are better options out there.”
Our sources suggest going back to the basics to fine-tune engine-building techniques. Gapping rings, for instance, is an easy-enough procedure, but there’s so much riding on getting it right that it’s worth taking a second look at the process.
Use a good squaring tool to ensure the ring is square in the bore before measuring the gap. When adjusting it, only file one side of the end gap from outside to inside. “Don’t put big chamfers on them. You’re not trying to put a bevel on it,” said Jones. “Just lightly debur the edge.”
Begle agreed that slow and precise is the best approach. “Getting the gap symmetrical and parallel in the bore is key,” he said. “People can get overzealous with deburring. When building an engine, you want to be on the cautious side.”
Another thing to get right is the cylinder hone. “Honing is as important as making the right ring selection,” said Jones. “A ring cannot fix a bad cylinder wall.”
One thing Jones sees far too often is people using the wrong lubrication to break in rings. “An engine needs friction to break in,” he said. “You have to make heat and pressure to seat the rings. If you use an oil with a high amount of friction modifiers, you’re over-lubricating.”
Use a dedicated break-in oil, he advised. This generally means a low- to no-detergent oil with a low total base number (TBN), a measurement of the number of alkaline additives in an oil. Look for additives like moly and calcium in low numbers, as they reduce friction. One additive he recommended is Zinc Dialkyl Dithiophosphate (ZDDP).
Another pro tip from Jones: “Make sure you get a proper fuel map before you start the engine. A fresh piston and ring package will not tolerate fuel wash. An overly rich fuel mixture will wash the lubrication right off the cylinder walls.”
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