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.
The humble, hard-working piston ring dates back to 1854 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
When it comes to ring selection you must also consider piston choice in the planning. Ideally, you will go with matched pistons and rings from the same source. Total Seal, as one example, is strictly in the piston ring business. Once they understand what you have for pistons they can set you up with the correct ring package, and that includes custom ring sets.
This is your typical box-stock ring package for a street engine, from left to right: ductile iron, cast iron, and a flexible non-alloy cast iron for the oil rings. Ductile iron is used for the top compression ring because it is flexible and more resistant to heat. Cast iron is used for the secondary compression ring, which also controls the oil film and isn’t as exposed to the extreme heat as the top ring. The oil ring package consists of an expander and two rails, which is designed to wipe oil down the cylinder wall.
The top compression ring is generally made of ductile iron with a molybdenum face. Molybdenum can take the heat better in most applications yet it is porous enough to hold oil for improved lubrication. Because the top ring is barrel faced, it rolls with piston action to maintain solid contact with the cylinder wall, yet with less friction.
The secondary compression ring with the reverse-bevel, taper-face, plain cast-iron second ring has long been the approach for this location. Heat is not really a problem in the second groove, which means there has been little need for exotic materials or coatings. Today, most secondary rings continue to be made from cast iron or even ductile iron in some applications. This ring is about 85-90 percent oil control and only 5-10 percent compression control, so to better manage the oil, there’s a definite trend toward the Napier ring, especially with LS engines.
The bottom ring package consisting of an expander and two rails is for oil control exclusively. Oil ring tension accounts for about 40 percent of total engine friction, with the oil rings alone accounting for 50 percent of the ring pack friction. The key to reducing ring tension is the ring’s radial depth in the ring groove. If you maintain the traditional SAE-standard 0.190-inch depth, you still need higher-tension oil rings. However, by decreasing radial depth to around 0.140-0.150-inch with a ring groove machined accordingly, tension can be reduced because the overall oil ring assembly is more flexible, conforming to the bore.
Confused about which piston ring goes where? Ed Law of Total Seal tells us it’s in the shape and in the sound. Ductile iron top rings have a barrel face and no dimple. When gently dropped onto a hard surface, the ductile iron ring makes a “ringing” sound.
The secondary compression ring sports a tapered face and is made of cast iron. When gently dropped onto a hard surface, the cast iron ring makes a dull “thud” instead of a ringing sound.
Another means to identifying the secondary compression ring is this dimple or a laser mark, which always faces up, according to Ed Law.
Ductile iron (left) versus cast iron (right). Identification is in the flex. Ductile iron rings are flexible and can take the extreme heat on top. Cast iron rings are, by contrast, brittle and they break if you try to bend them.
More extreme applications with supercharging, turbocharging, or nitrous get a tougher ring package. Up top (left) is generally an AP stainless steel with a PVD coating. The secondary ring (both on right) is roughly 85-90 percent oil control and just 5-10 percent compression according to Speed Pro’s Scott Gabrielson. Scott adds if you undercut the bottom of the ring, it exposes more of the endgap back into the ring groove, which opens up the flow area, providing a reservoir for oil being carried back to the pan.
To better manage oil, there’s a definite trend toward the Napier (hooked or claw-shaped) second ring as shown here. In fact, most GM LS engines arrive from the factory with Napier rings. The Napier ring creates a reservoir for the scavenged oil to flow through.
Here’s a close up of the AP stainless ring and its ultra-hard and durable surface designed for extreme duty applications.
Total Seal’s TNT centrifugally cast compression rings are heat treated to provide the most uniform grain structure of any martensitic ductile iron ring. They’re engineered for extreme abuse such as nitrous, turbocharged, supercharged, and high cylinder pressure applications.
Here’s a closer look at the TNT compression ring from Total Seal.
Total Seal’s Diamond Finish compression ring offers axial tolerances of +/- 0.000050-inch; improved sealing between piston and piston ring; optional PVD coatings engineered to match cylinder material and minimize friction losses; custom axial thickness down to 0.6mm; and are available in gapless, conventional, or Napier style.
Total Seal’s gapless top compression ring is an industry benchmark because it works so well. It simply seals better than a conventional piston ring thanks to its two-piece design. As a result, it delivers increased horsepower and torque, longer engine life, improved consistency, longer ring life, improved oil control, reduced internal friction, greater intake vacuum signal, broader torque curve, and increased manifold vacuum.
The Total Seal gapless ring is a two-piece affair where two interlocking rings work together to achieve a perfect seal against the cylinder wall.
For as long as engine builders have been building engines, there has been the debate over how to properly install piston rings. JGM Performance Engineering prefers to use a ring expander to carefully install rings. They tell us the expander causes less ring distortion.
The oil ring expander rides between two rails, which wipe oil down the cylinder wall.
After the expander is installed, oil control ring rails are rolled onto the piston and into the groove as shown.
Another approach to compression ring installation is shown here by Mark Jeffrey at Trans Am Racing in Los Angeles, where compression rings are gently rolled into the ring groove. Jeffrey assures us it doesn’t cause ring distortion.
We like these custom-sized piston ring compressors, which are available from Summit Racing in a wide variety of sizes. If you’re a home garage engine builder, we suggest the purchase of an adjustable ring compressor. If you build a lot of engines, the custom-sized ring compressor is a great investment.
Cylinder wall finishing has had to follow the technology curve much as piston and ring design has through the years. Your finished cylinder bore must be round, straight, and void of taper. Rings used to wear in to compensate for any slight bore irregularities. Today’s high-tech piston rings are so hard they just don’t wear like older rings did. Today’s engine blocks are also harder, so the final finish has to be spot on. Your machinist must also use a torque plate for honing. No exceptions.
Although manufacturers make pre-gapped rings, it is strongly suggested you check ring endgaps and adjust the gap as necessary.
Pistons, rings, and grooves must receive liberal doses of SAE 30 weight engine oil or assembly lube during installation. Saturate the grooves with oil and lube cylinder walls accordingly.
Cylinder bore sizing should be checked using a dial-bore gauge. Any other means promises you an inaccurate reading. The dial-bore gauge promises you a spot-on reading.
In the end, piston and ring selection and fitment is all about cylinder sealing and oil control. It is also about getting the right cylinder wall finish to further enhance sealing and oil control.
Photos by Jim Smart