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Piston Rings and Sealing
By Larry Carley
Though piston rings haven’t changed much in recent years, they are still just as important as ever for sealing compression, reducing blowby and emissions and controlling oil consumption. The changes that have occurred in ring designs, materials and cylinder bore refinishing techniques have been more evolutionary than revolutionary. Subtle refinements over the years in rings and cylinder bore refinishing techniques have created a generation of engines that can easily go 150,000 miles or more in most passenger car and light truck applications, and up to a million miles in big heavy-duty diesel trucks before a ring job is needed.
Better ring sealing and longer lasting engines are a bonus for vehicle owners. But for aftermarket ring suppliers and engine rebuilders, it’s been a curse. Fewer traditional engines are being rebuilt today than a decade ago. The bright spot has been performance engine building and other niche markets ranging from subcompact diesels to restorations. The performance market continues to show steady growth year after year in spite of a sluggish economy. Racers and hobbyists are spending money on engines for their racecars and restorations, but not necessarily their daily drivers.
Less traditional engine repair volume means more competition for the engines that are being rebuilt. Consequently, some engine rebuilders are substituting less expensive plain cast iron rings for moly rings and chrome rings to improve their profit margin and/or competitiveness. Likewise, some ring suppliers have felt compelled to add cast iron ring sets for engines that were not originally equipped with these type of rings because their competitors have them and the market is demanding them.
Tod Richards of Dana/Clevite Engine Parts says Dana has been adding new cast iron ring sets to its product line this year to cover applications that never had these kind of rings. "Cast iron ring sets are what many of our customers want," says Richards. Richards notes that the decision to use cast iron rings by some engine builders is being driven by price competition in today’s market."
Cast iron rings obviously won’t last as long as moly faced rings in hard-working late model engine applications, but how long they last often depends more on the quality of the engine build than the type of ring that’s used. Richards says cast iron rings will work well but only if the cylinders are properly cleaned after honing. This means scrubbing the bores with hot soapy water to remove all the honing residue and abrasives from the surface of the metal.
Aftermarket ring manufacturers are also having to add more new ring sets for newer applications, too. "Like other ring manufacturers, we have to follow the OEM lead and keep up with changing ring sizes. That means more new ring sets as rings continue to shrink in size," says Richards.
Rings are also running hotter than ever before. As rings move up higher and higher on the piston to reduce emissions, they are seeing more and more heat. The top ring on many engines today run at close to 600° F, while the second ring is seeing temperatures of 300° F or less. Ordinary cast iron compression rings that work fine in a stock 350 Chevy V8 can’t take this kind of heat. So that’s another reason why many top rings are now steel or ductile iron rather than cast iron.
More Changes Coming?
Scott Gabrielson of Federal-Mogul/Sealed Power says things seemed to have "settled down" for awhile as far as new ring designs and materials are concerned with the domestic engine manufacturers. "Passenger car and light truck engines built for the North American market are mostly using moly faced 1.2 mm top rings, 1.5 mm second rings and 3.0 mm three-piece oil rings. We’re seeing more steel top rings but the second compression rings are still mostly cast iron with a reverse-twist taper face. It’s a combination that is working well so there’s no pressing need to change things – for now."
There are some exceptions. The current Buick 3800 V6 uses a narrow 2.0 mm oil ring. Gabrielson also says some second rings now have a "napier" style face. The napier design has a small notch in the bottom face of the ring to improve oil control and sealing as the ring scrapes against the cylinder wall. The napier design is also used with a positive twist to improve its sealing characteristics.
Though ring designs appear to be stable here for the moment, such is not the case in Japan and Europe. The Japanese are continuing to shrink ring sizes with some late model engines now using 1.0 mm and smaller top compression rings. The Japanese don’t use moly facings but prefer gas nitrided rings for added longevity. North American ring manufacturers say nitriding is too expensive and moly works better because it is porous, holds oil and is more scuff resistant. Even so, nitriding remains popular in Japan.
Jack Bishop of NPR America (Nippon Piston Rings) says the Japanese want to push fuel economy to the max. Their current goal, he says, is to build engines that get 100 kilometers to the gallon (that’s about 62 mpg, which is light years beyond the current U.S. CAFE requirements of 27 mpg for passenger cars). One way they hope to achieve their goal is to reduce internal engine friction by shrinking ring dimensions even more. The Honda Civic that’s currently being sold in the Japanese market has tiny 0.8 mm top compression rings.
"The Japanese are also working on variable tension rings that change tension as engine load and temperature change. Variable tension rings reduce friction when the engine is cold, then stiffen up at higher loads and rpms to maintain a good seal.
"The Japanese are also looking at titanium nitride surface treatments for new production rings. Right now, titanium nitride rings are available for high-end racing engines but they are expensive. However, the rings are super hard and experience almost zero wear. The goal, says Bishop, "is to get manufacturing costs down so they can be used in production engines within the next two or three years."
The Europeans, by comparison, use a mix of ring facings: moly, chrome and nitride. Like the Japanese and domestic OEMs, they, too, are using smaller and smaller rings. But much of the ring development work that’s going on in Europe today is now aimed at small diesel engines. The Europeans are buying more diesel-powered cars than gasoline-powered cars because of the diesel’s higher fuel economy. They don’t have the same emission regulations as we do, so diesels are a popular engine there.
Diesel engines run leaner and hotter than gasoline engines, so hard, durable ring facings are needed to provide good longevity. Moly works well in diesels, but engineers are also working on developing new composite facings that combine ceramics, moly and other ingredients.
One of the big changes that will impact the longevity of diesel rings in the years ahead are new emission regulations that require exhaust gas recirculation (EGR) on heavy-duty trucks. Some engineers say diesel engines that were capable of going a million miles before EGR will be lucky to go half that distance because of the higher combustion temperatures – which is good news for diesel engine rebuilders. Time will tell if such predictions hold true.
Performance Ring Sets
Performance engine builders are always pushing the envelope and are the most demanding of all ring customers. They want rings that can withstand extreme abuse while also providing the best possible seal. Reducing blowby increases horsepower so a tight seal is absolutely critical to winning races.
A lot of the cutting edge ring and piston technology that’s being used in racing today will eventually find its way into production engines. The piston and ring sets that are found in many production engines today were considered racing parts less than a decade ago, so it’s logical to assume such things as "gapless" rings and exotic coatings may be in OEM engines before long.
Keith Jones of Total Seal Piston Rings says eliminating the end gaps in the compression rings can improve horsepower by as much as 10 percent depending on the application. "Our gapless rings have been very popular with racers, but we also have conventional rings, too, and offer both types with various coatings."
"We have steel rings down to 0.6 mm size in both gapless and conventional designs. Our ‘Diamond Finish’ rings are manufactured to within 50 millionths of an inch flatness and parallelism, with a finish that is typically 4 Ra microinches or less. This allows tighter assembly tolerances for better performance."
Jones says Total Seal’s most popular face coating material is "C23," which has a coefficient of friction of 0.1 (three times better than moly) and won’t flake off like plasma moly. It also works well with hard blocks and nickel silicon carbide-lined cylinders. Total Seal also offers a "C72" titanium coating, "C33" chrome nitride coating and conventional moly coatings as well, plus a "D47" side coating for the top and bottom of its steel Diamond Finished rings to reduce groove friction and microwelding.
"We’re the Burger King of piston rings. We’ll do rings anyway a customer wants them," says Jones.
The key to choosing a particular ring design and coating, says Jones, is to identify an engine’s primary function in life. If an engine is a street/strip application, chances are it will spend 90 percent of its time on the street. For this kind of engine, street rings would work better than an all-out racing ring. Of course, it all depends on the compression ratio and whether the engine has a blower, turbo and/or nitrous oxide (in which case racing rings would be better).
"If you don’t know which type of rings to use, call us and we’ll help you figure it out," says Jones.
Vern Schumann of Schumann’s Sales & Service, Blue Grass, IA, says ring selection for performance engines depends on three things: compression ratio, the type of fuel (gasoline or alcohol), and horsepower. Schumann says plain cast iron rings should never be used in an engine that burns alcohol because alcohol cuts lubricity. Coated rings are a must with alcohol.
Gas nitrided steel rings manufactured from coil wire are best for turbocharged and blown engines, says Schumann, and especially those that run nitrous oxide for an extra power boost. He says nitriding penetrates into the surface of the metal and alters its chemical makeup. Because of this it can handle thermal shock much better than any add-on facing material and won’t flake off under load.
"One of the biggest misconceptions that’s out there is that moly faced steel rings are racing rings. Welded moly steel rings work great on the street but won’t hold up like nitrided steel top rings," says Schumann. "Nitrided rings are stronger, provide better heat transfer, and won’t flake from thermal shock. In five years, I think most racing engines as well as many street performance engines will be running nitrided rings instead of moly."
Schumann explains that the different coil steel wire used in rings provides different tensile strengths. "The coil steel wire we use in our rings has a tensile strength of over 200,000 psi with zero porosity. Other alloys commonly used to make moly rings are typically 50,000 to 55,000 psi. Ductile iron, which we recommend for the second ring if the compression ratio is over 11 to 1 or the engine makes more than 400 hp, is rated at 70,000 to 80,000 psi. But ductile is typically two to eight percent porous, which reduces heat transfer and cooling. Ductile iron must be used with a coating, otherwise it smears the cylinder walls."
Schumann says the biggest mistake any engine builder can make is to use cheap rings with racing pistons. The rings should be steel or ductile iron so they don’t fail. Otherwise they are likely to break, and when that happens you can kiss the piston and the motor goodbye.
Cylinder Bore Refinishing
As a rule, engine builders should follow the cylinder bore refinishing guidelines by the ring manufacturer. But like every other aspect of engine building, opinions differ as to what techniques work best in any given situation.
Federal-Mogul’s Gabrielson says a "plateau finish" is the optimum bore finish for today’s moly-faced rings. A plateau bore finish is what all types of rings eventually produce when they are fully seated, so the closer the bore can be prefinished to a plateau-like condition the less the rings and cylinders will wear as the engine breaks in, the better the rings will seal right from the start, and the longer the rings will last.
For moly rings, Gabrielson recommends a two-step honing process: first hone with a conventional #280 grit silicon carbide vitrified abrasive, then finish by briefly touching the bores with a #400 grit stone or giving them several strokes with an abrasive nylon honing tool or brush.
If the cylinders are honed with diamond, Gabrielson says to follow up with finer grit diamond, a fine-grit vitrified abrasive or a brush to finish the bores. Diamond stones are fast and long lived, but they are more aggressive than silicon carbide and create more tear outs and other undesirable residue on the surface. Because of this, a rough diamond honing procedure should always be followed up with another operation afterwards to finish the surface.
Equally important is bore geometry. Gabrielson says engine builders have to be especially careful about oil control on late model engines. He says the block should always be honed with torque plates if the manufacturer recommends doing so to minimize bore distortion that can cause blowby and prevent the rings from sealing properly.
"The bores have to be straight and round. Make sure you keep the Ra finishes within factory specifications, too, which is typically in the 10 to 15 Ra range on many late model engines."
Jeff Welsh with Peterson Machine Tool says #220 grit silicon carbide honing stones are the best choice for plain cast iron and chrome rings, #280 grit is best for moly-faced rings, and #320 to #400 grit is best for moly rings in a racing application. To finish the bores, Welsh recommends using a brush hone or flexible brush in a drill.
"The main advantage of finishing the bores with a flexible brush in a drill is that you can run the drill backwards. The honing stones usually run clockwise so if you brush in the opposite direction (counterclockwise) it will do a much better job of deburring the surface. No more than 15 strokes should be necessary to produce a high quality finish."
Welsh also says crosshatch is important. Some people want 30 degrees and others as much as 45 degrees. "Thirty is probably best. We now offer an inverter with our honing cabinet so the operator can vary the spindle speed as well as stroking speed to achieve as much crosshatch as he wants."
Michael Mohondro of Rottler Manufacturing says most ring manufacturers call for a bore surface finish of 10 to 20 Ra microinches (see sidebar on page 35). "If the bores are honed with #325 to #400 diamond stones, the finish will usually be in the 22 to 24 Ra range. If the bores are then finished with a brush, they usually come down to about 18 Ra which is just about right."
Mohondro says some OEMs are using a much coarser grit of diamond to increase valley depth in the bores for better oil retention and ring break-in. He says International Harvester is using #140 to #170 diamond stones to hone their cylinders, then finishing with #600 stones to plateau the surface. This leaves a surface finish of 10 to 14 Ra but with 60 to 100 Ra of valley depth to retain oil.
"Most racers are also using #600 diamond stones to plateau the cylinders after they have been honed to get a really smooth finish," says Mohondro.
Tim Meara of Sunnen Products Co., says there are a variety of ways to achieve a plateau finish. You can use conventional abrasives, cork bond stones, a plateau honing tool or a two-step diamond honing process.
"Sometimes the cycle time dictates the type of process or stone that’s used. If an engine builder wants a fast cycle time, he may use a coarser grit stone to rough hone, then follow up with a finer stone to plateau finish the bore.
"Typically, most production engine rebuilders are using #320 or #400 grit diamond stones today, followed by brushing using a #180 grit PHT tool."
Meara says Sunnen has recently introduced a new honing head for its CK21 that holds both diamond stones and brushes in the same tool. This allows a user to hone with diamond. The diamond stones then retract and the brushes extend to finish the cylinder without having to change anything.
One change Meara says he’s seen lately among some race engine builders is a desire to increase the "Rvk" numbers (valley depth) in the crosshatch to improve oil retention.
Another issue is how to minimize bore distortion when the engine is running. Torque plates have long been used to simulate the bore distortion that occurs when the cylinder heads are installed on the block. Honing the block with torque plates installed results in rounder holes and better ring sealing. But temperature is also a factor that’s hard to duplicate.
Mart Jeltema of K-Line says the type of plateau finishing procedure he recommends depends on the engine and type of honing equipment an engine builder is using. "What kind of honing machine are they using or are they honing with a drill? What Ra finish are they trying to achieve, and what kind of finish are they getting before they attempt to plateau the cylinders?" he asks
To achieve a plateau finish, Jeltema recommends using a brush: either the rigid style that mounts in the honing head holders or a spaghetti style bristle brush in a hand-hone (K-Line sells both types). He says it usually takes about 10 to 15 strokes in each cylinder to plateau the finish. The improvement is generally about 10 Ra points on the surface finish.
Ed Kiebler, a consultant for Winona Van Norman says he still recommends a 15 to 20 Ra finish for moly rings. Anything less than 12 Ra can result in glazed cylinders and the rings may not seat.
"You can use various methods to get there. Winona Van Norman still uses conventional vitrified abrasives. A #280 grit stone will give you the right finish, but it should be followed with a plateau honing tool that loads into the hone head – not a bottle style brush. The soft honing tool does not exert enough pressure against the surface to change the overall Ra finish but it will do an excellent job of removing all the torn and folded metal you don’t want on the surface. It makes a huge difference in ring seating and oil consumption."
Kiebler says diamonds can produce good results, too, provided they are used in a hone head with at least eight stones and are followed up with a brush for 20 to 30 seconds. A set of #400 grit diamond stones will produce a finish that is similar to #280 vitrified carbide stones.
Analyzing Cylinder Bore Finishes
(from a presentation by Michael Mohondro, Rottler Manufacturing)
For many years, cylinder bore finish has been analyzed by using the roughness average (Ra) parameter as a primary means. This measurement is very effective in determining the "smoothness" of a cylinder wall after it has been finish honed, although it is not enough to fully determine if a cylinder has been finished properly. Monitoring the cylinder finish is very important for many reasons, with the importance being to find such a method that makes rings seat faster and last longer.
One method that can be used to analyze bore finish is fax film. Fax film analysis provides a qualitative determination of torn and folded metal, burnishing, pull outs, and hone crosshatch.
A more detailed method of analysis are the Rk parameters of measurement. The Rk parameters directly analyze the bearing characteristics of a cylinder over a given sampling length. These measurements are graphically illustrated on the Abbott-Firestone Bearing Curve. This type of analysis also provides a qualitative determination of torn and folded metal, burnishing, pull outs, and hone cross hatch angle.
The Ra parameter can have many different forms, while still maintaining the same value. This is simply because Ra is only an average. The same Ra number can represent three very different surface finishes though each has the same average roughness. This being the case, it becomes very important to look at additional parameters when analyzing the surface finish. These include the Rk value, the Rpk and Rpk values, and the Rvk and Rvk values.
The Rk value refers to the bearing area that exists after the rings have seated. Rk is the height of the cylinder wall profile after the highest peaks and lowest valleys have been removed.
Rpk monitors how much material must be removed from the cylinder wall before the rings have seated, providing a good seal (reducing effects such as blow-by). Inconsistent high peaks (Rpk) are filtered out of this equation, and are not as important because they immediately are "knocked off" upon starting the engine.
Rvk refers to the valleys in the cylinder bore finish, filtering out the lowest extremes. This is a very important characteristic, indicating the surface’s oil retention qualities. Typically, a high Rvk value is very acceptable and indicates that the cylinder bore is effective in holding oil across its surface.
The plateau level is commonly described by the ratio of Rk to Rvk. This is a direct comparison of the bearing surface roughness to the valley depths.