If asked what you wanted for piston rings in your engine build, would you know how to answer? Piston rings have the most important job in your engine. We will get arguments on this one because there really are no unimportant parts in your engine. However, what makes piston rings so important is what they do.
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The humble, hard-working piston ring dates back to when a man named John Ramsbottom demonstrated the friction-reducing value of piston rings along with the sealing and cooling benefits. Piston rings improved efficiency. In those days, it was more about steam engines and less about internal combustion.
Piston rings provide cylinder and combustion chamber sealing, which keeps heat energy contained where it belongs, above the piston. Any heat energy that escapes past the piston rings is lost power—period. Heat energy contained above the piston goes to work making power at the crankshaft. Piston rings also carry destructive heat into the water jacket via the cylinder wall to control heat and prevent piston meltdown.
What people want most from piston rings is cylinder sealing along with low tension to achieve less friction and better efficiency. It is challenging to get both. We live in an age of skinny, low-tension compression rings—sometimes as narrow as 0.023-inch, or 0.6mm. This works if you have perfectly honed cylinder walls. If you don’t, rings tend to distort and you’re not going to get optimum cylinder sealing.
Proper ring selection means understanding ring function, material, piston design, and bore dynamics. Pistons, rings, and cylinder bores must have a perfect marriage to function properly. Proper engine break-in is critical to endurance and reliable ring function. The type of piston ring you choose depends on how you intend to use your engine. Mild street performance engines call for a more “vanilla” ring package than supercharged, turbocharged, or nitrous-fed engines. Racing engines demand a much tougher ring package on par with what’s used for supercharged, turbocharged, or nitrous engines.
Which ring you choose boils down to how much heat and force you intend to impose on them. If your engine is bone stock as delivered from the factory, you’re probably not going to want to hear this. A box-stock engine is equipped with ductile iron and cast-iron piston rings. This means your rings are not going to like a supercharger or that occasional nitrous blast because stock ductile and cast-iron rings can’t always stand the heat and pressure associated with forced induction or squeeze.
If you’re opting for nitrous or forced induction, you’re going to need a top compression ring capable of withstanding the heat and pressure associated with these elements. This calls for high-end materials according to Ed Law at Total Seal. Ed suggests an AP Stainless top ring with PVD (Physical Vapor Deposition) for forced-induction and nitrous applications.
Total Seal’s high-performance piston ring sets include an AP Steel top ring that has been coated using PVD-applied C-33 chromium nitride anti-friction coating for greater efficiency. The C-33 coating is easy on cylinder walls while the steel top ring still has the ability to handle extreme pressures. Napier secondary rings and three-piece stainless oil control rings come standard with the AP Stainless Steel Ring Set.
As a rule, pistons and rings are generally sold in sets unless you’re reusing old pistons or are choosing a different type of ring than the manufacturer provides. Manufacturers such as Federal-Mogul Speed Pro from Summit Racing Equipment sell pistons and rings as sets for your convenience. This makes piston and ring selection a no-brainer for the average enthusiast. Just look at what the manufacturer suggests for the type of driving you intend to do and refine your decision from there.
An important consideration as to how well the piston rings seal is the hone of the engine block. Your machine shop should have a PAT gauge to accurately measure the final hone’s surface roughness. Total Seal says typical values (measured in microinches) for general performance applications should be around RPK 8-12, RK 20-30, and RVK 30-50. Is your local machine shop capable of this caliber of work? Not all of them are. If a machine shop can finish late-model Ford or GM stockers with their thin rings to maintain original emissions compliance and factory tolerances, the answer is likely affirmative. Confirm this when you drop the block and pistons off. CHP
Photos by Jim Smart
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.
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?
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:
Oil Control Ring Types:
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.
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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.
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.
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.
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.
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.
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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