The split piston ring commonly used today was first invented by John Ramsbottom in the late 1800s. His invention immediately replaced the hemp style rings that were used in steam engines, and represent a quantum leap in performance capability. The advantages using this type of ring in the steam engine were overwhelming in terms of power, efficiency, and maintenance.
When you think of piston rings, have you ever considered that they are the smallest component of the internal combustion engine, yet have the largest responsibility? When you assemble an engine, you never really grasp what the piston ring is going to do during its lifespan, making the performance of this diminutive component even larger in reality.
The piston rings have three major tasks to ensure the engine makes consistent power efficiently. First of all, the ring must seal each cylinder effectively, without fail, for thousands–and sometimes hundreds of thousands–of miles before replacement. When the air and fuel mixture ignite in each cylinder, the ring must seal to the cylinder wall so the explosion can drive the piston down the bore. The piston ring–in reality a formed piece of wire–must also keep blow-by gases from entering the crankcase while containing the combustion explosion.
Secondly, the ring helps to transfer the heat from the piston induced by the explosion to the cylinder’s walls. The rings are the only contact between the cylinder bore and the piston, and this is the only way heat can be transferred into the cooling system from the combustion process.
Thirdly, and perhaps most importantly, the piston ring must control engine oil from entering the combustion chamber. Each cylinder bore is made to be much like an engine bearing. The hone scratches in the bore provide a pocket for oil to be trapped so the rings will have lubrication as they rotate and travel up and down in the cylinder. But all of the oil that gets splattered on the cylinder walls from the rotating assembly has to be scraped away to prevent oil from entering the combustion chamber, as oil that enters the combustion chamber can be detrimental to the combustion process by effectively lowering vehicle octane, potentially causing harmful repercussions.
Through the magic of research and development–and the trickle-down effect from OE manufacturing efforts–it seems as though every year piston rings become dimensionally thinner, yet engine performance improves. This applies to all applications, from custom racing pistons to off-the-shelf replacement pistons. If the piston rings have such an enormous job to do, then why are they becoming smaller? Are there any adverse effects that may arise in the future?
Extensive testing has shown that smaller ring widths have proven to be just as effective–and maybe a touch more so–than previous thicker versions. This is mainly due to the difference in the contemporary material being used for the piston ring compared to older, less efficient materials. In addition, differences in design and shape along with finish and coatings applied to the surface of the piston rings help to improve performance and reduce drag. These changes have proven to be more efficient, provide more power with less blow-by, and extend longevity. The best way to understand what is occurring in the piston ring world is to take a look back and understand where we came from. Thanks to input from Total Seal‘s Keith Jones, who also assisted us with the photography and diagrams for this article.
A popular material used for piston rings is cast iron, often referred to as grey iron. The biggest advantage in using cast iron to manufacture pistons rings is that it will not gall or scuff the cylinder bore. And as long as the cast iron ring is sufficient in size, it will provide adequate seal. If the operating loads are increased or the size is decreased for the application, then ring seal can become an issue. When cast iron is used for the top ring, it is usually coated with molybdenum or chrome to prevent bore wear. If cast iron is used for the second ring, no coating is applied. Cast iron material is very brittle; under a microscope, the grain structure of cast iron is rectangular and sharp. This is why if you were to try to twist a cast iron ring it will break because the grain structure is easily fractured. Cast iron is popular because it is somewhat cost-effective to manufacture. The drawback to its use is that several manufacturing steps are required for completion–and it’s not ideal for high-performance engines.
There are two primary methods in which a cast iron ring is made. The most common way is to take the desired outside diameter of the piston ring and form a mold. Then once the cast iron has been formed inside this cylindrical mold, the center of the mold is cut out to the inside piston ring dimension. To give an example, once the process is completed, you would have something similar to a gun barrel. Then each individual ring is cut from the mold sort of like slicing a loaf of bread.
The other way cast iron rings are formed is similar to the way a model car or truck is manufactured. When you open up a model car box, you find several sheets of plastic that have pieces formed that you break from the mold to extract the parts. Cast iron is poured into a mold much like the model car pieces, only in the shape of piston rings. When the process is complete the rings are snapped from the mold and final-machined for use. While cast iron rings may be affordable due to the cost of the material, they do require a lot of hands-on machining in order to be processed and finalized. Also, there is a lot of waste that has to be recycled once the finished product is achieved.
Ductile iron is another material used in the manufacture of piston rings; it has been around for quite a few years and is still common today. The forming process for ductile iron piston rings is extremely similar to that used to manufacture a cast iron ring. The composition of the material is taken from cast iron by extracting the carbon flakes–which is mostly graphite–and forming that material into a cylindrical mold to set the outside dimension. Then the inside dimension can be cut out. The rings can then be sliced from the “gun barrel” and heat-treated. Under a microscope, ductile iron has round nodular shaped grains that are very strong, unlike the grain structure for cast iron. If you were to take the ductile iron ring and try to break it you would find that it will only bend and twist into a pretzel shape. Ductile iron is twice as strong as cast iron and is used in high-output applications. Since most diesel engines are turbocharged, ductile iron rings were commonly used for their resistance to failure in high compression situations with high operating cylinder pressures.
The up and down motion of the piston keeps the keystone ring loaded in the ring groove of the piston and as a byproduct, also keeps the ring groove clean from the soot of the diesel fuel. The uniquely-shaped ductile iron keystone ring is not commonly used today, however. Because the use of Exhaust Gas Recirculation has become standard on nearly all internal combustion engines, when this shape is used, carbon packing tends to stick the ring in the piston groove, causing failure.
If you are not sure of what material your rings are made of, don’t try to bend them. An easy way to test them is to drop them on a table in your shop. If the ring makes a ringing sound it is ductile iron, and if it simply thuds onto the table, it is manufactured from cast iron.
Steel’s The Deal
Today–especially in high-performance and severe-duty applications–steel is used to construct piston rings. The advantages of steel rings are many: they are easier to manufacture, stronger and harder than ductile iron, and resist breakage especially in those demanding power-adder applications. The disadvantage? The materials are more expensive.
The manufacturing process for steel piston rings is simple; wire is cut from a spool of material measuring the desired proportions. There is no waste, and there are less steps from cutting to final product. Perhaps the best thing about using steel rings is that it can endure more heat stress from harsh environments and still hold its form without failure. And in high-RPM, low-tension, high-vacuum applications like NHRA Pro Stock and other naturally-aspirated racing classes, steel rings offer far better ring seal. The inside top surface will usually have a bevel which will help induce twist when the cylinder fires. The thin top ring is pushed down against the bottom of the top piston groove and gas pressure pushes the ring against the bore. Because the face of the ring is barrel shaped, as the piston travels down the bore the ring is in constant contact with the cylinder wall.
Steel Ring Details
In order for a steel ring to be compatible with cast iron cylinder bores, it must be coated with moly, chrome, PVD (Particle Vapor Deposition), or gas nitriding. Moly coatings are applied to the face of the ring. Moly offers a high resistance to scuffing, but also is porous which provides some oil retention.
Chrome is a very hard coating used in high load applications and is found often in dirt racing engines. The chrome coating can resist dirt impregnation and send the debris out the exhaust port. If you were to use moly coated rings in these applications, the dirt ingestion would be caught in the face of the ring because of porosity, and damage to the bore would result.
As a piston ring face application, PVD has become more popular in the last several years. PVD is a thin coating that is deposited on the ring using titanium or chromium evaporated by heat with a reactive nitrogen gas. This process will make the ring very hard, smooth, and temperature resistant.
Lastly, gas nitriding is a heat process that impregnates the ring with nitrogen which will cause the ring to case harden. This process hardens the surface somewhere around .001-inch deep; the cylinder bore will show signs of wear before the ring when gas nitriding is used.
Second rings are transitioning from cast iron to ductile iron and steel. Because the second ring scrapes most of the oil from the cylinder walls, steel rings for the second position are beveled on the underside to induce twist. As the piston goes down the bore the twist of the ring allows the tapered face to scrape the oil from the cylinder wall.
Napier rings–which use a hook-faced design–are also common for the second position. The hook pockets the oil as it is being scraped which allows the use of low tension oil rings in these situations. Second rings that are steel or ductile iron are not coated, as research has shown that coated second rings offer no advantages compared to an uncoated ring because the scraping action used to remove oil keeps them well lubricated.
Quick Assembly Tips
If you are assembling an engine and using steel rings, make sure to measure the free gap of the piston ring. Free gap is measured when you take the rings out of the box and lay them on a table. As an example, the gap in the piston ring laying on the table would be .600-inch. You install the piston ring in the engine and now the gap is .020-inch for your application. At freshen-up, the free gap now measures .500-inch which would be considered normal after the engine has been heat-cycled in competition. But, if the free gap measured .100-inch, then something is wrong with the air/fuel ratio or ignition timing because the ring is losing tensile strength and distorting due to too much heat.
Another tip is to debur and chamfer as little as possible when file-fitting piston rings. In addition, leave the edges as square as you can to offer better ring seal. Follow the manufacturer’s recommendations for the proper honing technique. Proper cylinder bore finish will offer the correct amount of oil retention for lubricating the ring material being used.
Using a piston ring set which is thinner than you ever thought possible is an easy way to free up horsepower in your performance engine. The trickle-down impact from current OEM technologies have proven to be a winner in this particular instance. These ring designs are not detrimental to performance; with proper break-in procedures they can be expected to last many thousands of miles in a street application with no harmful side effects, although we can’t promise the same if you’re hitting them with a couple of kits of nitrous oxide every week on your Saturday night trips into Mexico.