What Does The 2nd Piston Ring Do? Purpose And ...

Piston ring packages are as carefully engineered as any high-performance part, but the “middle child” might be the most misunderstood. Here’s a look at the science that goes into second ring design.

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Since the invention of the metal piston ring at the start of the industrial revolution (which, you could argue, finally made steam power impractical), there’s been constant innovation and improvement in cylinder sealing technology for these seemingly simple parts. The ring package has three primary goals: Keep pressure confined to the combustion chamber on both the compression and power strokes, transfer heat from the piston to the cylinder walls where it can be removed via air or liquid cooling, and control lubrication to limit oil consumption and unwanted emissions.

While it’s easy to look at the top ring or the oil ring at the bottom and intuitively understand their contribution to meeting these objectives, the second ring is more of a mystery. What’s it supposed to be doing, and why is it necessary? How do the materials used and physical properties of the second ring affect performance? To answer these questions, we turned to Wiseco’s Engineering Manager, David Back, for knowledge about piston rings.

For our first question, we asked Back whether the second ring had a role in containing compression or combustion gasses. “There was a time when bores were so bad in terms of surface finish, roundness, and so on, and ring materials were a lot worse, so that pistons used to have four rings; two for compression sealing, one for scraping oil, and one for pumping oil,” he explains. “The terminology hasn’t kept up with the technology. Referring to a contemporary second ring as a compression ring is a misnomer.”

So what’s the contribution of a modern second ring to combustion chamber sealing? Per Back, “Negligible. There have been SAE papers published that prove how enlarged second ring gaps actually increase top ring sealing and power. Combustion sealing is 100 percent the top ring’s job.” Combined with other piston features, the second ring’s role in this respect is to keep the pressure in the crevice space between it and the top ring as low as possible, giving any blow-by that makes it past the top compression ring a way to quickly escape to the crankcase.

“An accumulator groove works in concert with larger 2nd ring gaps,” Back explains. “In short, there will always be some combustion pressure leakage past the top ring due to secondary piston motion and cylinder cross hatch. Any pressure that makes it past the top ring tends to get trapped between the top and second ring, which then pressurizes the top ring from underneath which leads to ring flutter (especially at high RPM). The accumulator groove creates additional volume which decreases pressure. This is where Boyle’s law is applicable; volume and pressure have an inverse relationship, so increasing volume decreases pressure. Coupling this with larger second ring gaps provides a smoother transition of the trapped gas out of that space and reduces top ring flutter.” 

Because the second ring is specifically intended NOT to be a pressure seal, it’s often constructed quite differently from the top compression ring. Back says, “Many top rings have inside diameter bevels that cause them to twist opposite of the forces acting upon it in order to help keep it flat in the groove for better sealing. Second rings have a bevel opposite to that, so they actually twist the wrong way to help sealing.”

 So, having established that the second ring is most definitely not there to provide compression or combustion sealing, what about the second main objective of the ring package – transferring heat out of the piston and out to the cylinder walls, where it can be managed by the cooling system? It might seem like the relatively tiny amount of contact the rings make between the piston and the bore couldn’t possibly be a significant route for heat conduction, but it turns out to be the major provider. Per Back, “There are many variables here, but the rings transfer about 70 percent of combustion heat from the piston to the cooling system.”

The remaining 30 percent escapes via other routes, like radiation and convection cooling of the underside of the piston to the air inside the crankcase, conduction cooling through contact between the piston skirt and the cylinder bore, and heat carried away via oil splash from crankshaft windage. Some engines even employ oil squirters at the bottom of each cylinder bore that direct a spray of lubricant at the underside of the pistons specifically to aid in cooling.

Other sources of heat transfer notwithstanding, the ring package handles most of the load when it comes to keeping the piston at an acceptable operating temperature. Out of that previously mentioned 70 percent of total piston heat, “The top ring transfers 45 percent, the second ring 20 percent, and the oil ring 5 percent,” says Back. While the second ring definitely plays its part in this critical task, it’s still not the ring’s primary reason for being there.

As it turns out, the second ring has a lot more to do with lubrication control than the “oil ring” beneath it. “The second ring is what scrapes the oil,” Back explains. “The oil ring is what gathers it and pumps it away from the cylinder walls via oil return holes in the oil ring groove.” The second ring’s main function is to continuously remove excess oil from the bore – as the crank rotates, oil escaping from the pressurized bearings on the rod big ends is constantly thrown up behind the piston, coating the walls of the bore. 

On the downstroke, the second ring and the oil ring work in concert to clear all but a tiny amount of oil and return it down the bore to the sump. Back says, “The top rings will always receive latent lubrication by oil trapped in the cross-hatch of the cylinder walls.” It’s that microscopic texture on the bore that retains just enough oil to keep friction between the ring package and the cylinder wall to a minimum, while the second ring prevents too much oil from making it up past the top ring and into the combustion chamber.

Now that we understand each ring’s purpose in the package, we can see why different specific materials and ring cross sections are often used for the top and second rings. “The demands and intended function of the top and second rings are different, for sure, so the materials often are as well,” Back continues. “The overall best top ring material is steel. Now, granted, some steels are better than others, but as rings get smaller and specific output increases, the demands on the top ring (which sees the most abuse) are highest.”

Move down a groove on the piston, and the different job being performed places lower demands on the material being used. Per Back, “Many second rings in racing engines are still cast iron or ductile iron. The second ring is not under enough stress and temperature to necessitate steel.” The shape of the ring profile also has a significant effect on how efficiently it removes oil, as well as how much friction it introduces, and both the interior and exterior diameters have a role to play. “Bevels are on the inside diameter of the ring and dictate the direction the ring twists to aid in scraping,” Back says. Viewed in cross section, a beveled ring has one edge of the inside diameter cut at an angle – as Back points out, this encourages the ring to dynamically twist in the groove as it moves down the bore and focus additional pressure on the outside corner, in order to more efficiently sweep excess oil away.

“Taper, Napier and steps are all variations of the outside diameter shape,” he continues. The goal with all these profiles is to concentrate contact into a narrow band to increase the efficiency of the scraping action. As the name implies, a tapered outer profile is narrower at the top than at the bottom, while a stepped ring profile has what looks like a notch in the cross section, oriented toward the direction of travel on the downstroke. A Napier ring, named for the famed British D. Napier & Son engineering firm that originally developed the profile, is actually undercut at an angle or even hook-shaped on the outside diameter, further decreasing the contact area and providing space for scavenged oil to escape, away from the cylinder bore. “In order, the most efficient scraper is Napier, followed by step, followed by taper. Run a Napier if it’s available in your bore size and suits the groove in the pistons,” Back concludes.

What kind of a combination you are running will also influence the optimum choice for your ring package, including the second ring. Back advises, “Thinner second rings are more prevalent in dry sump engines pulling gobs of pan vacuum.” Because crankcase vacuum helps ring seal across the board, it’s possible to get the desired results without working the second ring quite as hard. “Naturally aspirated with no vacuum help should usually be 1.5mm or larger, while forced induction should err towards larger 1/16-inch rings,” he adds.

“Of course, this is all relative to bore size; you can almost think of it like a ratio of ring size to bore size,” Back cautions. “A big boost four cylinder engine will control oil just fine with a 1.2mm ring, while a 4.600-inch bore big block would be happier with a 1/16-inch ring. There are also substantial variables in crankcase efficiency when it comes to oil control. Modern engines with deep-skirted blocks, segmented oil pans, windage trays, and crank scraping/scavenging all have an effect on how much oil is thrown up into the cylinders. The more oil present, the harder the second ring’s job is.”

As you can see, second ring design and engineering is a complex subject, but fortunately the experts at Wiseco have the collective experience in all forms of high-performance engine builds to provide you with sound advice for your particular needs. While we can’t cover everything in a single tech article, we hope that what you’ve learned here will help you to better understand the ‘why’ behind a ring package’s specifications and take full advantage of the knowledge on tap from Wiseco’s staff when putting together your own combination.  

The split piston ring commonly used today was first invented by John Ramsbottom in the late 1800s. His invention immediately replaced the hemp style rings that were used in steam engines, and represent a quantum leap in performance capability. The advantages using this type of ring in the steam engine were overwhelming in terms of power, efficiency, and maintenance.

When you think of piston rings, have you ever considered that they are the smallest component of the internal combustion engine, yet have the largest responsibility? When you assemble an engine, you never really grasp what the piston ring is going to do during its lifespan, making the performance of this diminutive component even larger in reality.

The piston rings have three major tasks to ensure the engine makes consistent power efficiently. First of all, the ring must seal each cylinder effectively, without fail, for thousands–and sometimes hundreds of thousands–of miles before replacement. When the air and fuel mixture ignite in each cylinder, the ring must seal to the cylinder wall so the explosion can drive the piston down the bore. The piston ring–in reality a formed piece of wire–must also keep blow-by gases from entering the crankcase while containing the combustion explosion.

Secondly, the ring helps to transfer the heat from the piston induced by the explosion to the cylinder’s walls. The rings are the only contact between the cylinder bore and the piston, and this is the only way heat can be transferred into the cooling system from the combustion process.

Thirdly, and perhaps most importantly, the piston ring must control engine oil from entering the combustion chamber. Each cylinder bore is made to be much like an engine bearing. The hone scratches in the bore provide a pocket for oil to be trapped so the rings will have lubrication as they rotate and travel up and down in the cylinder. But all of the oil that gets splattered on the cylinder walls from the rotating assembly has to be scraped away to prevent oil from entering the combustion chamber, as oil that enters the combustion chamber can be detrimental to the combustion process by effectively lowering vehicle octane, potentially causing harmful repercussions.

Through the magic of research and development–and the trickle-down effect from OE manufacturing efforts–it seems as though every year piston rings become dimensionally thinner, yet engine performance improves. This applies to all applications, from custom racing pistons to off-the-shelf replacement pistons. If the piston rings have such an enormous job to do, then why are they becoming smaller? Are there any adverse effects that may arise in the future?

Extensive testing has shown that smaller ring widths have proven to be just as effective–and maybe a touch more so–than previous thicker versions. This is mainly due to the difference in the contemporary material being used for the piston ring compared to older, less efficient materials. In addition, differences in design and shape along with finish and coatings applied to the surface of the piston rings help to improve performance and reduce drag. These changes have proven to be more efficient, provide more power with less blow-by, and extend longevity. The best way to understand what is occurring in the piston ring world is to take a look back and understand where we came from. Thanks to input from Total Seal‘s Keith Jones, who also assisted us with the photography and diagrams for this article.

Material Selection

A popular material used for piston rings is cast iron, often referred to as grey iron. The biggest advantage in using cast iron to manufacture pistons rings is that it will not gall or scuff the cylinder bore. And as long as the cast iron ring is sufficient in size, it will provide adequate seal. If the operating loads are increased or the size is decreased for the application, then ring seal can become an issue. When cast iron is used for the top ring, it is usually coated with molybdenum or chrome to prevent bore wear. If cast iron is used for the second ring, no coating is applied. Cast iron material is very brittle; under a microscope, the grain structure of cast iron is rectangular and sharp. This is why if you were to try to twist a cast iron ring it will break because the grain structure is easily fractured. Cast iron is popular because it is somewhat cost-effective to manufacture. The drawback to its use is that several manufacturing steps are required for completion–and it’s not ideal for high-performance engines.

There are two primary methods in which a cast iron ring is made. The most common way is to take the desired outside diameter of the piston ring and form a mold. Then once the cast iron has been formed inside this cylindrical mold, the center of the mold is cut out to the inside piston ring dimension. To give an example, once the process is completed, you would have something similar to a gun barrel. Then each individual ring is cut from the mold sort of like slicing a loaf of bread.

The other way cast iron rings are formed is similar to the way a model car or truck is manufactured. When you open up a model car box, you find several sheets of plastic that have pieces formed that you break from the mold to extract the parts. Cast iron is poured into a mold much like the model car pieces, only in the shape of piston rings. When the process is complete the rings are snapped from the mold and final-machined for use. While cast iron rings may be affordable due to the cost of the material, they do require a lot of hands-on machining in order to be processed and finalized. Also, there is a lot of waste that has to be recycled once the finished product is achieved.

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Ductile iron is another material used in the manufacture of piston rings; it has been around for quite a few years and is still common today. The forming process for ductile iron piston rings is extremely similar to that used to manufacture a cast iron ring. The composition of the material is taken from cast iron by extracting the carbon flakes–which is mostly graphite–and forming that material into a cylindrical mold to set the outside dimension. Then the inside dimension can be cut out. The rings can then be sliced from the “gun barrel” and heat-treated. Under a microscope, ductile iron has round nodular shaped grains that are very strong, unlike the grain structure for cast iron. If you were to take the ductile iron ring and try to break it you would find that it will only bend and twist into a pretzel shape. Ductile iron is twice as strong as cast iron and is used in high-output applications. Since most diesel engines are turbocharged, ductile iron rings were commonly used for their resistance to failure in high compression situations with high operating cylinder pressures.

The up and down motion of the piston keeps the keystone ring loaded in the ring groove of the piston and as a byproduct, also keeps the ring groove clean from the soot of the diesel fuel. The uniquely-shaped ductile iron keystone ring is not commonly used today, however. Because the use of Exhaust Gas Recirculation has become standard on nearly all internal combustion engines, when this shape is used, carbon packing tends to stick the ring in the piston groove, causing failure.

If you are not sure of what material your rings are made of, don’t try to bend them. An easy way to test them is to drop them on a table in your shop. If the ring makes a ringing sound it is ductile iron, and if it simply thuds onto the table, it is manufactured from cast iron.

Steel’s The Deal

Today–especially in high-performance and severe-duty applications–steel is used to construct piston rings. The advantages of steel rings are many: they are easier to manufacture, stronger and harder than ductile iron, and resist breakage especially in those demanding power-adder applications. The disadvantage? The materials are more expensive.

The manufacturing process for steel piston rings is simple; wire is cut from a spool of material measuring the desired proportions. There is no waste, and there are less steps from cutting to final product. Perhaps the best thing about using steel rings is that it can endure more heat stress from harsh environments and still hold its form without failure. And in high-RPM, low-tension, high-vacuum applications like NHRA Pro Stock and other naturally-aspirated racing classes, steel rings offer far better ring seal. The inside top surface will usually have a bevel which will help induce twist when the cylinder fires. The thin top ring is pushed down against the bottom of the top piston groove and gas pressure pushes the ring against the bore. Because the face of the ring is barrel shaped, as the piston travels down the bore the ring is in constant contact with the cylinder wall.

Steel Ring Details

In order for a steel ring to be compatible with cast iron cylinder bores, it must be coated with moly, chrome, PVD (Particle Vapor Deposition), or gas nitriding. Moly coatings are applied to the face of the ring. Moly offers a high resistance to scuffing, but also is porous which provides some oil retention.

Chrome is a very hard coating used in high load applications and is found often in dirt racing engines. The chrome coating can resist dirt impregnation and send the debris out the exhaust port. If you were to use moly coated rings in these applications, the dirt ingestion would be caught in the face of the ring because of porosity, and damage to the bore would result.

As a piston ring face application, PVD has become more popular in the last several years. PVD is a thin coating that is deposited on the ring using titanium or chromium evaporated by heat with a reactive nitrogen gas. This process will make the ring very hard, smooth, and temperature resistant.

Lastly, gas nitriding is a heat process that impregnates the ring with nitrogen which will cause the ring to case harden. This process hardens the surface somewhere around .001-inch deep; the cylinder bore will show signs of wear before the ring when gas nitriding is used.

Second rings are transitioning from cast iron to ductile iron and steel. Because the second ring scrapes most of the oil from the cylinder walls, steel rings for the second position are beveled on the underside to induce twist. As the piston goes down the bore the twist of the ring allows the tapered face to scrape the oil from the cylinder wall.

Napier rings–which use a hook-faced design–are also common for the second position. The hook pockets the oil as it is being scraped which allows the use of low tension oil rings in these situations. Second rings that are steel or ductile iron are not coated, as research has shown that coated second rings offer no advantages compared to an uncoated ring because the scraping action used to remove oil keeps them well lubricated.

Quick Assembly Tips

If you are assembling an engine and using steel rings, make sure to measure the free gap of the piston ring. Free gap is measured when you take the rings out of the box and lay them on a table. As an example, the gap in the piston ring laying on the table would be .600-inch. You install the piston ring in the engine and now the gap is .020-inch for your application. At freshen-up, the free gap now measures .500-inch which would be considered normal after the engine has been heat-cycled in competition. But, if the free gap measured .100-inch, then something is wrong with the air/fuel ratio or ignition timing because the ring is losing tensile strength and distorting due to too much heat.

Another tip is to debur and chamfer as little as possible when file-fitting piston rings. In addition, leave the edges as square as you can to offer better ring seal. Follow the manufacturer’s recommendations for the proper honing technique. Proper cylinder bore finish will offer the correct amount of oil retention for lubricating the ring material being used.

In Conclusion

Using a piston ring set which is thinner than you ever thought possible is an easy way to free up horsepower in your performance engine. The trickle-down impact from current OEM technologies have proven to be a winner in this particular instance. These ring designs are not detrimental to performance; with proper break-in procedures they can be expected to last many thousands of miles in a street application with no harmful side effects, although we can’t promise the same if you’re hitting them with a couple of kits of nitrous oxide every week on your Saturday night trips into Mexico.

Contact us to discuss your requirements of Piston Ring Material. Our experienced sales team can help you identify the options that best suit your needs.