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Some performance pistons may be vented vertically...
Everybody wants thin low-tension rings to reduce ...
Matching Pistons And Rings
You must be realistic about the application and conditions the piston and rings will be under
By Larry Carley
Whether you are rebuilding a high-mileage engine or building a fresh engine from scratch, matching the pistons and rings to each other and the application is essential for a successful outcome. Rings are a wear component that are usually replaced during a rebuild. The pistons may be reused if they are not damaged, cracked or scuffed and the ring grooves have minimal wear. But if the cylinders have to be bored to oversize to remove wear, replacing the original pistons with oversize pistons is a must.
Many piston sets today are sold with rings. The piston supplier provides the rings based on the bore size of the engine and the piston set. But, as one ring manufacturer cautioned, the rings that come with the pistons may not always be the “right” rings for the application.
Everybody wants thin low-tension rings to reduce friction. The less outward pressure the rings exert against the cylinder wall, the less friction they create as the pistons move up and down. The reduction in friction does not create more horsepower but it does allow the horsepower that the engine produces to be used more efficiently and to generate more power at the flywheel.
Some compression rings today are less than .023˝, which is less than 0.6 mm, or about the thickness of the oil rails on a traditional three-piece oil ring. That’s not very thick, so if the cylinder bores are not almost perfectly round and straight with the proper surface finish, the rings may not seal very well. Lost compression means more blowby and lost power.
Low-tension 3 mm oil rings are commonly used on many pistons. Depending on the design of the ring and the materials from which it is made, many of these rings exert only around 10 lbs. of tension against the cylinder wall. Thinner rings (say 2.8 mm) exert even less pressure, maybe as little as 7 lbs. of tension. With good bore geometry and the proper bore finish, low-tension rings are perfectly capable of maintaining adequate oil control. But a lot of engines that are running these thin, low-tension oil rings often turn out to be smokers.
The Real Cause Of Ring Sealing Problems
When an engine burns oil, you have to blame somebody, right? The natural tendency is to blame the ring manufacturer. But, in most instances, the real problem is poor bore geometry or bore finish. Bore tolerances are absolutely critical when you’re using low-tension rings. If you’re using the same honing equipment and techniques that you’ve been using for the past 30 years to rebuild Chevy 350s with standard ring packs, you’re likely going to have sealing issues when you use the same equipment and techniques on a late-model engine or a performance engine with thin, low-tension rings.
Thin, low-tension rings like a plateau finish with a relatively smooth finish but adequate crosshatch to retain oil for proper ring lubrication. Typical surface finish for a normal plateau finish would be Rpk of 8 to 12 microinches, Rk of 25 to 35 microinches, and Rvk of 40 to 50 microinches. In a performance application, an optimized plateau finish should have a Rpk value of less than 12 microinches, Rk of around 20, and Rvk of around 40 microinches. If friction reduction is more important than longevity (say as in ProStock drag racing or NASCAR), an even smoother finish would be desirable with Rpk numbers in the 3 to 5 microinch range, Rk of 12 to 18, and Rvk of 20 to 25 microinches.
If you are not familiar with these values, here’s what they mean:
Rpk = Peak Height
Rvk = Valley Depth
Rk = Core Roughness Depth
Needless to say, if you are building high-end performance engines and do not already own a surface profilometer, you should invest in one. As for honing, you don’t necessarily have to use expensive diamond stones, but diamonds can provide a more consistent finish when used in equipment that has been designed from the start for diamond honing.
Even with the best honing equipment and techniques, you may still end up with a bore distortion problem that affects ring sealing. A seasoned block or one that has been cryogenically treated or vibrated will be more stable and less apt to shift than a new virgin casting. The loading on the head bolts can also cause distortion (which is why you should always use torque plates when honing).
Problems with coolant flow inside the engine can also affect bore distortion and ring sealing. Some lightweight blocks can also distort, depending on how the engine is mounted in the vehicle. Bore distortion can vary from almost nothing, up to a couple thousandths of an inch depending on what is causing it! With today’s tight piston-to-wall clearances, even .0005˝ of bore distortion may be too much for some engines.
Stiffer Rings May Help
If you want to use the thinner rings (like 1.5 mm top and second compression rings with a 3.0 mm or 2.8 mm oil ring), and you’re not ready to admit that you need (or can afford) new honing equipment, one alternative is to go with a slightly stiffer ring pack. Some ring manufacturers have thin “high tension” rings that apply 14 to 16 lbs. of tension with a 3 mm oil ring. Simply switching from the low-tension to the higher tension oil rings can often solve an oil burning problem.
Some racers don’t care if an engine smokes a little or not. Burning a little oil is no big deal if the engine is only making short runs down a drag strip or running a limited number of laps on a circle track or road course. Yet for an endurance engine or a street engine, it could prove fatal if the engine consumes all of the oil in the pan and starves to death for lubrication. You also have to ask yourself that if the engine is burning oil, is it also losing a lot of compression due to ring blowby? As we said earlier, lost compression is lost power.
Piston & Ring Sealing
For optimum sealing of the top compression ring, a barrel-faced steel ring is usually best, and the flatter the ring grooves in the piston the better. Cylinder pressure twists the ring down on the compression stroke so a barrel face experiences less friction and wear. To aid ring sealing, the piston may have an accumulator groove between the top and second compression ring. Some performance pistons may also be vented vertically or horizontally with gas ports to help ring sealing at high rpm. The extra pressure behind the top ring helps force it outward against the cylinder wall for a tighter seal.
In most high-performance engines as well as late-model high-output stock engines, there’s more heat in the combustion chambers. This increases the thermal shock and stress on the top compression rings as well as thermal expansion. Consequently, most performance ring sets as well as a growing number of stock ring sets are using steel top compression rings (though ductile iron plasma moly faced rings are still widely used in Top Fuel drag racing).
One major aftermarket supplier told us that they are now using SAE-9254 high-alloy carbon steel top rings in 40 percent of its stock replacement sets. Steel rings allow the radial wall thickness of the rings to be reduced to provide better groove seal, better bore conformability and less ring instability at high rpm. The higher tensile strength of steel compared to ductile iron means less bending and flexing under high loads, which reduces ring and piston groove side wear.
The reason why most rings fail is not because of detonation and breakage (though detonation can certainly damage rings). More often than not, rings fail as a result of abrasion destroying the ring surface. The abrasion comes from dirt and debris left inside the engine by the rebuilder (the most common cause according to one ring manufacturer), by airborne abrasives and contaminated lubricants (poor air filtration or air leaks into the crankcase) and by poor-quality cylinder finishes.
Though the use of steel rings has been expanding, ductile iron moly faced rings are a good option for naturally aspirated traditional street/strip engines like SB/BB Chevys and Fords. Ductile top compression rings can handle loads of up to about two horsepower per cubic inch. Beyond that, you should upgrade to steel rings.
The most popular design for the second ring these days is the napier style ring, often made of ductile iron rather than cast iron for added durability. The face of a napier ring does a superior job of scraping oil off of the cylinder wall when the piston travels down to help reduce oil consumption. If a piston is designed for a tapered second ring, it may have a J-groove cut in the lower second ring groove land. The groove creates a little shelf that allows oil to accumulate as it is scraped off the cylinder wall.
For some applications, though, a napier ring would not be the best choice. For engines that are running nitro or alcohol, or are using a lot of boost pressure, the second ring has to deal with more pressure than in a naturally aspirated gasoline engine. Consequently, there is more blowby to deal with so a ductile iron reverse twist ring or a barrel face top compression ring may be used in the second ring groove to improve sealing.
Matching Pistons And Rings
Pistons and rings obviously have to be dimensionally compatible with each other, which is why many piston suppliers include their own ring sets. The rings that are provided should have the proper back spacing and side clearances with respect to the ring grooves. They should also be the right size for the bore diameter. Even so, you don’t have to go with the rings that come with a particular piston set.
If you’ve been having oil control or sealing problems with the rings that come with a particular piston set, ask your piston supplier to provide a higher tension ring set. And if the piston supplier can’t provide the rings you want, shop the various ring suppliers to find a ring set that better seals the engine and meets your customer’s expectations.
As one ring manufacturer said, you have to be realistic about the application and what kind of operating conditions the piston and rings will be subjected to. When you’re building an engine for a bracket class, extremely thin low-tension rings may not be the best choice for this kind of racing. On the other hand, if the engine will be used in a heads up class, a thinner ring pack may provide the reduction in friction that can help win a race.
Like rings, pistons must match the application. You must have the correct diameter to fit the bore size of the engine, and the right piston height, compression ratio and wrist pin location so the piston will match the rods and stroke.
For circle track racing, rules often dictate what type of pistons you can and can’t use. Rules may prohibit the use of domed pistons or high compression ratios. For drag racing, compression ratios of 14:1 to as much as 16:1 are common today depending on the octane rating of the fuel. With a power-adder such as a turbo, blower or nitrous, you’ll want a stronger piston design with reinforced skirts to prevent the piston from collapsing. Some piston manufacturers use box-style reinforcing ribs to add rigidity.
Really high-output drag engines also need stronger thick-wall wrist pins to handle the loads. The wrist pins are often starved for oil, so a hard coating on the pins, such as physical vapor deposition diamond, can help the pins survive. Some racers are even doing away with the wrist pin bushing in the small ends of the rods so the rod can be thicker and stronger, or a larger wrist pin can be used.
Low-strength cast pistons are fine for everyday drivers but have no place in a high-output or high performance engine. You need stronger hypereutectic or forged pistons. Forgings can withstand the most punishment and help conduct heat away from the combustion chamber. But forgings typically require a little more clearance to compensate for increased thermal expansion.
For serious racing, the preferred alloy for forged pistons is usually 2618. This alloy is more malleable than 4032, which allows it to resist detonation better than 4032. It also has a higher coefficient of thermal expansion than 4032, so pistons made of 2618 aluminum require more wall clearance and make more piston noise while a cold engine is warming up. But 2618 lacks the longevity of 4032 so a set of pistons may last only a single season of racing before they have to be replaced. For a street application or an engine that has to last multiple seasons, forged pistons made of 4032 would be the way to go.
Piston weight is also something to be considered. Lighter is usually better when you are building a high revving engine or want quicker throttle response. But you don’t necessarily need lighter if the engine is stroked and cammed to be a low-rpm high torque motor. There’s nothing to be gained with lighter pistons except less stress on the rods and crank. If you do go with lighter pistons, keep in mind that changing the piston weight changes the bob weight of the piston and rod assembly. This means metal has to be removed from the crankshaft counterweights to balance the engine.
What about coated pistons? Many pistons today are available with some type of anti-scuff side coating. Coatings protect the piston against a dry start when the engine is initially fired up, and to provide some additional scuff protection if the engine overheats or starves for oil. Anti-scuff coatings provide an extra measure of protection but, are not absolutely necessary.
Anti-scuff coatings are typically quite thin, and can usually be ignored when figuring piston-to-wall installation clearances, unless the piston manufacturer tells you otherwise. Some coatings, though, are thicker to provide a “cushioning effect” that helps reduce piston rock that causes piston noise in a cold engine. These types of coatings may require some extra clearance for assembly.
Some pistons also have a hard anodized coating to provide extra wear resistance in the ring grooves.
For a list of piston and ring suppliers, click on our Online Buyers Guide at http://bit.ly/XvnTOT.