Piston rings have one of the toughest jobs inside an engine. They’re slammed up and down between the ring lands thousands of times a minute, they’re subjected to searing temperatures and extreme pressures, and they’re constantly scraping back and forth against the cylinder walls. In spite of all of this, the rings are expected to seal combustion and vacuum, prevent blowby, control oil consumption, keep the cylinder walls lubricated, cool the pistons, and last but certainly not least, last almost forever (150,000 miles plus in a passenger car/light truck engine or up to 1 million miles in a heavy-duty over-the-road diesel)!
It’s a demanding list, yet most rings are up to the task and hold up pretty well — assuming the “right” rings are used for the application, the cylinders are finished properly and the rings are installed on the pistons correctly. Obviously, the ring sealing requirements of a high-revving racing engine or a hard-working diesel engine are much more demanding than those of a mild stock engine. So with that in mind, let’s look at some of the latest thinking as it applies to piston ring designs, materials and coatings.
With so many late-model engines running thinner, low-tension moly-faced ductile iron and steel rings, one might think cast iron rings are fading into history. They are at the OEM level, but it looks like cast iron rings will be around for a long, long time in the aftermarket. According to several ring suppliers, there is still a very strong demand for plain cast iron rings. The main reason is that cast iron rings cost less than more durable materials — and they hold up well enough in light-duty stock rebuilt engines. Even so, plain cast iron rings can’t provide the durability of a chrome or moly-faced ring set, or a steel or ductile iron ring set that is engineered for high output, late-model overhead cam engines.
The secret of using plain cast iron rings successfully is to thoroughly clean the cylinder bores after they have been honed. Plain cast iron rings don’t have a hard facing to resist wear, so they require a very clean surface. The cylinders must be washed and scrubbed with hot, soapy water to remove all traces of honing abrasive and metal residue from the surface.
Late-Model Ring Sets
Ring sets in late-model engines are running hotter than ever before. As rings move up higher and higher on the piston to reduce emissions, they are exposed to more heat. A decade ago, the land width between the top ring groove and piston crown was typically 7.5 to 8.0 mm. Today that distance has decreased to only 3.0 to 3.5 mm in some engines. This minimizes the crevice just above the ring that traps fuel vapor and prevents it from being completely burned when the air/fuel mixture is ignited (this lowers emissions). But the top ring’s location also means it is exposed to much higher operating temperatures.
The top ring on many engines today runs at close to 600° F, while the second ring is seeing temperatures of 300° F or less. Ordinary cast iron compression rings that work great in a stock 350 Chevy V8 can’t take this kind of heat. That’s why many late-model engines have steel or ductile iron top rings. Steel is more durable than plain cast iron or even ductile iron, and is required for high output, high load applications including turbocharged and supercharged engines as well as diesels and performance engines.
Under the top compression ring is the number two ring, which is the second compression ring. The number two ring assists the top ring in sealing combustion, and also helps the oil ring below it with oil control. Most second rings have a tapered face with a reverse-twist taper face. This creates a sharp edge that scrapes against the cylinder wall for better oil control. Some second rings’ designs have a “napier” style edge that has a squeegee effect as it scrapes along the cylinder wall. This helps reduce friction and oil consumption even more.
Second compression rings in late-model engines are still mostly cast iron because they don’t see as much heat as the top compression ring. On domestic engines, the first and second rings are moly-faced, but on many Japanese engines the rings are nitrided.
The third ring is the oil ring. This is typically a three-piece ring (though some are four-piece, two-piece or even one-piece) that helps spread oil on the cylinder wall for lubrication and scrapes off the excess oil to prevent oil burning. In three-piece oil rings, there are two narrow side rails and an expander that wraps around the piston. The expander exerts both a sideways and outward pressure on the side rails so they will seal tightly against the cylinder walls.
Over the years, rings have been getting smaller and thinner. Typical ring sizes today in domestic engines are 1.2 mm for the top compression ring, 1.5 mm for the second ring and 3.0 mm for the oil ring. Some are even thinner. The Buick 3800 V6 uses a narrow 2.0 mm thick oil ring.
The Japanese are going even smaller. Some Japanese engines now have 1.0 mm and even 0.8 mm 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.
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 focused on small diesel engines. The Europeans drive more diesel-powered cars than gasoline-powered cars because a diesel engine provides higher fuel economy. When low sulfur diesel fuel becomes available later this year in the U.S., it is expected to open up a whole new market for diesel-powered passenger cars in this country.
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 new composite coatings that combine ceramics, moly and other ingredients provide increased longevity.
Engine manufacturers have been going to smaller rings because the rings alone can account for up to 40% of an engine’s internal friction. Thinner rings exert less tension against the cylinders. This reduces friction and improves fuel economy. On high-performance racing engines, less friction means more usable horsepower. But low friction rings also require rounder cylinder bores, too. That’s why many late-model engines have torque-to-yield (TTY) head bolts and multi-layer steel (MLS) head gaskets. Both reduce bore distortion when the heads are installed on the block. On many engines, “torque plates” should be used when the cylinders are honed. Torque plates simulate the bore distortion that occurs when the cylinder head is bolted to the block. This helps produce a rounder cylinder bore when the block is honed.
Low tension rings also weigh less and reduce the reciprocating mass that pounds against the piston grooves with every stroke of the piston. But groove pound out and microwelding are still a concern because of the higher operating temperatures in today’s engines. To counteract this, some rings have a special coating on the sides to keep them from sticking as they bounce up and down in the piston groove. Some pistons also may have anodized or coated ring grooves to resist pound out.
Like stock rings, performance ring sets are also getting smaller and thinner. Reducing the tension on the rings not only cuts friction but also seals better and reduces blowby. This means a performance engine handles more vacuum in the crankcase with a dry sump oil pump to increase horsepower.
Many performance pistons today have ring lands that are very close to the top of the piston and use metric-sized ring sets. Ring grooves on many of these pistons are also machined to have a small vertical uplift to compensate for thermal expansion as the piston heats up.
Another trend has been to drill gas ports in the ring grooves behind the rings. Compression rings typically require 0.002” to 0.004” of side clearance so combustion pressure can blow around the ring and force it outward to seal against the cylinder. By drilling tiny gas ports in the back of the ring land, less side clearance is needed and ring sealing is improved. There is also less ring flutter at high rpm, which is where most performance engines spend a large percentage of their running time.
Pistons with vertical gas ports drilled from the crown of the piston work best in high rpm applications (such as drag racing). Pistons with lateral (sideways) gas ports work better for circle track engines.
Rethinking Ring Gaps
The old school philosophy of engine building said the end gaps on second compression rings could be tighter because the number two ring is not exposed to as much heat as the top ring. The new school of engine building says it’s better to open up the second ring gap a bit so pressure doesn’t build up between the rings and cause the top ring to lose its seal at high rpm. The result is better compression, better piston cooling and reduced oil consumption. Any pressure that builds up between the rings will blow down into the crankcase, keeping oil out from between the rings.
Getting rid of the end gap altogether also can improve sealing, cooling and horsepower. “Gapless” compression rings can reduce leak down and typically add 3-5% more horsepower with no other changes. Gapless rings are available in popular sizes with various wear-resistant face and side coatings. On some drag racing engines, the second compression ring can even be eliminated if a gapless top ring is used. Getting rid of the second compression ring cuts friction and adds horsepower, too.
Smoother, Flatter Rings
Another trend that seems to have additional benefits is the use of smoother, flatter rings and pistons with precision-machined grooves. One ring supplier says its racing 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.
With some low-priced pistons and ring sets, there is a certain amount of waviness that concentrates contact between the rings and lands. This encourages microwelding the groove pound-out at high rpm. Friction-resistance coatings on the sides of the rings and/or ring grooves in the piston can help prevent this from occurring.
Choosing the “Right” Rings
The right ring set can not only make more horsepower, but also improve the engine’s durability. Both are just as important on the street as on the race track. The best advice here is to follow the ring supplier’s recommendations. Use street rings on street engines, and performance ring sets on racing engines.
The type of ring materials and coating that work best in a given application will depend on the engine’s compression ratio, the type of fuel it is burning (gasoline, alcohol or nitro), how much horsepower per cubic inch the engine will 0hopefully make, and the engine’s rpm potential. For example, 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.
For high-boost turbocharged and supercharged engines, and engines using large doses of nitrous oxide to add power, ductile iron or steel top rings are a must. Many racers prefer to use nitrided rings made from steel wire because they can handle higher loads and thermal shock better than other materials. The nitriding penetrates into the metal and won’t flake off like other surface coatings. Another factor to consider is the type of racing. Off-road and dirt track engines often survive best with chrome rings that can handle dirt contamination better than moly-faced rings.
No ring will work well if the cylinder walls are not finished properly. Most ring manufacturers recommend a plateau finish, which typically involves a two-step honing process.
For plain cast iron rings in a stock motor, #220 grit silicon carbide honing stones are the best choice, followed by a honing tool or brush.
For moly-faced rings in a stock motor, 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, cork stones or a brush.
An average surface finish of 15 to 20 RA is typically recommended for moly rings. Anything less than 12 RA can result in glazed cylinders and the rings may not seat. If the surface is rougher than 20 RA, the rings and cylinder will scrub excessively as the rings seat.
For moly or nitrided rings in a performance motor, hone with #320 or #400 and finish with #600 stones, cork stones, a honing tool or brush.
If the cylinders are honed with diamond, they should be finish honed with a finer grit diamond, a fine grit vitrified abrasive or a honing tool or brush to plateau the surface.
Bore geometry is also important. In fact, many late-model blocks and most high-performance engines should always be honed with torque plates bolted to the block.
Crosshatch provides lubrication for the rings. Most engine builders prefer 30°, but some use as much as 45°.
Piston Ring End Gap Recommendations
Checking and adjusting the end gaps of the pistons rings is necessary when new rings or pistons are installed in an engine. The proper end gap assures a good combustion seal (minimum blowby) and allows adequate clearance between the ends of the rings so they do not butt up against each other and cause the rings to scuff.
End gap can be measured by placing the piston ring in the cylinder bore and inserting a feeler gauge between the ends of the ring. If the gap is too narrow (less than the minimum recommended clearance), the ends of the rings can be filed to increase the gap. File carefully so that both ends of the ring remain parallel to each other. In other words, don’t file at an angle. The ends of the ring must be square.
Warning: When measuring end gap, check the gap with the rings at the top and the bottom of the bore. If the bore has taper wear (bores typically wear most at the top), the end gap will be larger at the top and smaller at the bottom of the bore. Use the bottom position to set the end gap. If you use the top of a worn bore to set the end gap, the end gap will be too small when the piston reaches the bottom of the bore. The ends of the ring may hit each other causing the ring to bind and scuff. Cylinders that have more than 0.003” to 0.005” of taper wear should probably be bored or honed to oversize to restore proper piston and ring clearances. Refer to the engine manufacturer’s specifications for the maximum allowable taper wear.
Piston Ring End Gap Recommendations
Top Compression Ring — Most piston ring manufacturers recommend a minimum end gap of 0.004” times the bore diameter (0.016” for a 4” bore) for a stock engine. For a modified street performance engine, increase the gap to 0.0045” to 0.005” times the bore diameter (0.018” to 0.020” for a 4” bore).
Second Compression Ring — 0.005” times the bore diameter (0.020” for a 4” bore). For a modified street engine, increase the end gap to 0.0055” times the bore diameter (0.022” for a 4” bore). For a nitrous or blown racing engine, the top ring end gap should be opened up to as much as 0.006” or 0.007” times the bore diameter (0.024” to 0.028” for a 4” bore). For the second ring on a nitrous or blown motor, the recommended ring end gap is even wider: 0.0063” to 0.0073” times the bore (0.025” to 0.029” with a 4” bore).
The recommended ring end gap for oil rings regardless of the engine application is typically 0.015”.
Some racers believe that opening up the second end gap even more (say an additional 10%) can improve overall ring sealing by allowing trapped gases to escape before they blow past the top ring and cause ring flutter at high rpm (say above 5,000 to 6,000 rpm).
Note: These recommendations are rules of thumb only. Always follow the end gap specifications recommended by the piston ring supplier or engine manufacturer for the type of engine application.