Just because you’ve been building engines all your life doesn’t mean you know everything there is to know about pistons and rings. We asked various aftermarket piston and ring suppliers what type of questions commonly arise when a customer is buying a set of pistons or rings for an engine.

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1.  If you are ordering custom pistons for an engine, get the rings first and then order your pistons.  Depending on the diameter of the pistons and bores, off-the-shelf rings may or may not be available for your application.

If the application will require custom rings, your ring supplier will need as much information about the engine as you can give them. In addition to obvious things like bore diameter and desired ring thickness and ring tension, they may also want to know what type of fuel the engine will burn (gas, alcohol, nitrous or nitromethane), if the engine is blown or turbocharged, and what type of racing the engine will be used for.  All of these factors will influence the type of rings that will perform best in your application.

Most importantly, listen to their advice. Like your favorite jewelry store, manufacturers and suppliers are the ring experts. They know which materials and which ring designs and facings will best match your application.

Once you’ve determined which rings will work best in your motor, you can then go to your piston supplier to work out the details of your custom pistons. The piston supplier can then determine how deep and wide the ring grooves should be to match the rings you will be using.

2.  The top compression ring needs a certain amount of gas pressure behind it to seal the ring face tightly against the cylinder bore surface. If gas pressure can’t get behind the ring to push it out against the cylinder wall the ring will seal poorly and leak compression.

Side clearance between the ring and piston groove is easy to measure with a feeler gauge. Most rings for performance applications require .001 to .˝ of side clearance.  Some original equipment engines may allow as much as .002 to .˝ of side clearance for the rings in their grooves. It all depends on the application.

Diamond-lapped rings, which are smoother and flatter than most standard rings, can often be installed with somewhat tighter clearances. One ring manufacturer recommended about . to .˝ of clearance for its diamond-lapped top compression ring, and .001 to .˝ of side clearance for its diamond-lapped second compression ring.

Some piston manufacturers make pistons that are machined with such a high degree of accuracy that they say you can run as little as .˝ of side clearance with a good set of rings!

However, if the rings fit too tightly in the grooves, there may not be enough gas flow to get behind the rings to push them out against the cylinder wall, resulting in ring flutter, a poor seal and loss of power.

At the recent Advanced Engine Technology Conference that preceded last year’s Performance Racing Industry show in Indianapolis, George Bryce of STAR Racing gave a seminar on big bore high RPM engine building. He said most engines will start to lose horsepower at high RPM because of ring flutter long before they experience valve float. He recommend using the lightest rings possible to reduce weight, and adding gas ports (vertical or horizontal) to the pistons to help keep the rings sealed.  Adding gas ports can allow you to use somewhat tighter ring side clearances.

Bryce also recommends using pistons that have an accumulator groove between the first and second ring. Gas trapped in the accumulator groove will help maintain ring seal at high RPM. Another recommendation is to use a shallower crosshatch in the cylinder bores if you are building a high revving engine.

Gas ports in the pistons can help route combustion gas behind the top ring for a better seal, but it’s a trick that’s used primary on the race track, not the street. Vertical gas ports tend to clog with carbon, which may result in uneven gas pressure behind various areas of the top ring. Some engine builders say they prefer to use horizontal gas ports or small horizontal “D-shaped” cutouts machined into the top ring groove land around the circumference of the piston to get more even pressure behind the top ring.

3.  Ring back clearance can also affect ring sealing. As combustion pressure pushes down on the top ring and flows behind it, it quickly fills the void behind the ring to push the ring outward against the cylinder wall. If the void under the ring is too small, there may not be enough flow to achieve good ring control and sealing.

Ring back clearance is a little more difficult to determine because it requires some math to figure out. The space behind the ring is calculated by subtracting the ring radial wall thickness from the root depth of the ring groove in the piston. Or, you can measure the inside diameter of the ring installed in a cylinder bore, and subtract that from the outside diameter of the bottom of the ring groove in the piston.

Let’s say you have a ring that is .170˝ thick and a piston groove that is .195˝ deep. Subtract the ring thickness from the groove depth and you get .025˝ of space behind the ring.

The important point here is to make sure you have at least .005˝ of back clearance between the underside of the ring and the bottom of the piston groove, and preferably .007˝ for good gas flow and ring sealing.

According to lab tests performed by one piston and ring manufacturer, running a larger back space clearance behind the top ring seems to have no negative effect on ring sealing. Combustion pressures are so high and push gas behind the rings so quickly that the volume of the void behind the rings doesn’t matter.

This means you can safely run thinner steel rings in place of cast iron rings on the same piston and get better sealing even if the back spacing is greater. Low tension steel rings are about a third stronger, a third lighter and much more conformable to the cylinder walls than cast iron rings, so you end up with a better seal, better high RPM control and longer ring life.

4.  A lot of people worry excessively about ring end gaps. The end gap provides clearance so the ring can expand as it gets hot without the ends butting together and causing a problem. Some engine builders worry that if the top ring end gap is too wide, they will lose compression through the gap causing blowby and loss of horsepower.

A “gapless” style top ring is one way to eliminate an “open” gap altogether. But even gapless rings have recommended end gaps for the upper and lower sections of the ring. The end gaps will depend on the application, and should be staggered 180 degrees from one another to assure the best possible seal.

Regardless of the type of rings used, you obviously need a certain amount of end clearance to accommodate thermal expansion. The more heat the engine produces, the greater the end gap that’s recommended. Common recommendations for top ring end gaps in a 4-inch bore include .016 to .018˝ for moderate performance, .018 to .020˝ for drag or circle track racing, .028 to .030˝ for drag nitrous, and .024 to .026˝ for supercharged or high boost turbo.

Typical second ring end gap recommendations for a 4-inch bore would be .020 to .024˝ for moderate performance, .022 to .024˝ for drag or circle track, .028 to .030˝ for nitrous drag, and .024 to .026˝ for blown or turbocharged.

Common signs of insufficient end clearance and butting end gaps are scuffed ring faces, damaged rings and cylinders, and shiny areas on the butt ends of the rings.

Ring end gap is measured by inserting a ring into a cylinder bore about an inch from the top, and using a feeler gauge to measure the clearance between the ends of the ring. If the end gap needs to be widened, you file or grind the ends of the ring. The ends of the gap should also be carefully deburred with a fine stone before the rings are installed.

A common recommendation is to run a somewhat wider end gap on the second compression ring if you are building a performance engine (about 1.25 times wider). Testing has shown that a slightly larger second ring end gap usually improves top ring stability and sealing at higher engine speeds by venting excessive pressure that builds up under the top ring. But this recommendation can vary depending on the application.

The second ring functions primarily as an oil control ring in most naturally aspirated engines. Because of this, you can usually open up the second ring end gap a bit with no adverse effect on compression or power in a high revving engine. But in a stock engine or one that is primarily a low revving, high torque motor, the second ring end gap can often be the same or even a little less than the top ring to minimize blowby. On a blown or turbocharged engine that is running a lot of boost pressure, the second ring may be used more to control compression than oil consumption. On this type of application, a tighter end gap on the second ring can help minimize blowby.

5.  Ring seating depends on the cylinder wall finish, whether or not the rings have been lapped at the factory, and the type of facing material on the rings (plain, chrome, moly or nitride). Always use the type of honing stones and procedure recommended by your ring supplier. A plateau finish is always best and allows almost instant ring seating within minimal break-in and wear.

Always use a break-in oil when breaking in the rings on a new engine, and change the oil after the initial dyno tuning session. Also, varying the engine speed and load during the break-in period is important for rapid ring seating.

6.  Piston geometry is important not only when choosing off-the-shelf pistons, but also when ordering custom pistons.  Most pistons have a barrel shape that changes from the top to the bottom of the piston skirt. The hotter the application, the more barrel the piston may need to not scuff in its bore.

In addition, piston skirts are not round but oval to allow for expansion without scuffing.

7.  The thickness of the ring lands is also important. Since pistons are exposed to extreme heat from combustion, extra clearance is added to each land to allow them to expand closer to the bore size without scuffing. In general, the closer the ring land is to the top of the piston, the smaller in diameter it will be to provide the necessary clearance. One exception is the third ring land (above the oil ring groove), which is often reduced in diameter more than necessary to aid in transferring oil off the cylinder wall and into the oil ring groove.

8.  Selecting the proper wrist pin and proper thickness is a critical component. If the wrist pin is too thin, the piston will be exposed to additional stress which can cause piston cracking, pin bore wear, or even failure.

For more Piston Wear Ringinformation, please contact us. We will provide professional answers.

Look closely at the piston pin bore  for dark gray wear near the lock and wrist pin or excessive pin wear near the rod as indicators of insufficient wall thickness. If either of these are present, you need a thicker pin.

A simple tip to get the most out of your existing wrist pin is to ensure the side clearance between the piston pin boss and small end of the connecting rod is not excessive.

The location of the wrist pin in the piston is also important. Most stock engines have the pin slightly offset to reduce piston slap and noise. But in a race engine, a center-mounted pin will be more stable at high RPM.

9. Modifying piston valve reliefs is often necessary with oversized valves and high lift cams. You can make such modifications, but it may be safer to have your piston supplier make the changes. It’s important to measure the thinnest point to the piston undercrown or ring groove to make sure there is adequate thickness prior to cutting the piston. In some cases, it isn’t possible to measure this without cutting a piston in-half or computer modeling. Most piston suppliers recommend a minimum thickness of .050˝ under the thinnest point of the valve relief.

10.  Diesel pistons are not the same as those for most gasoline engines. Diesel engines are ultra high compression, high heat engines that demand a lot from pistons.  Compression ratios are typically in the 16:1 to 20:1 range, which improves thermal efficiency and fuel economy but creates more pressure.

Most diesel pistons are cast aluminum with a steel insert for the upper ring groove to prolong ring life. Lighter weight billet or forged aluminum pistons are also available for racing, with most being anodized to improve life and wear resistance.

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.

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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