Piston rings seal the cylinders and keep pressure in the combustion chamber. Stock piston rings are typically plain cast iron or moly faced iron. Performance rings may be chrome, ductile iron or steel with various facings or coatings. The ring sealing requirements of a high-revving performance engine are much more demanding than those of a 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
Over the years, rings have been getting smaller and thinner. Typical ring sizes today are 1.2 mm for the top compression ring, 1.5 mm for the second ring, and 3.0 mm for the oil ring. Many are even thinner, with top compression rings only 1.0 mm thick, and 2.0 mm thick oil ring.
Engine manufacturers have been going to smaller piston rings because the rings account for up to 40% of an engine's internal friction. Thin, low tension rings reduce friction and improve fuel economy. They also weigh less and reduce the reciprocating mass. However, low tension rings cannot tolerate much distortion in the cylinder bore, and the thinner cross section of the rings does not conduct heat as efficiently as thicker rings.
Pistons are cooled partially by heat conduction through the rings to the cylinder walls, by oil splash from underneath and by the incoming air/fuel charge. The use of thinner, low tension rings reduces heat transfer via conduction causing the piston to run hotter. With a standard F-132 alloy piston, hotter means more thermal expansion and a need for greater clearance between the piston and cylinder to prevent scuffing, which is exactly the opposite of what's desired in today's engines to reduce blowby, emissions and noise.
Most engines today have very tight piston-to-wall clearances (.001 in. or less) to minimize blowby and reduce piston rock. The more stable the piston, the better the rings can maintain a tight seal. Close tolerances also make for a quieter running engine, especially after a cold start when clearances are greatest).
Anti-scuff moly-based coatings are used on the sides of many stock and performance pistons. Skirt coatings not only protect the piston, but also allow tighter clearances between the piston and cylinder to reduce piston rocking and blowby.
To reduce hydrocarbon (HC) emissions, the rings in many late model engines have been moved up closer to the top of the pistons. 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. Relocating the rings closer to the top of the piston reduces the size of the crevice just above the top compression ring. Fuel vapor that enters this area during the piston's compression stroke often fails to burn completely when ignition occurs. A few droplets of fuel may not seem like much, but every drop adds up when it comes to meeting hydrocarbon emission requirements.
Moving the rings up on a piston reduces emissions, but it also exposes the top ring to more heat from the combustion chamber. The reduced thickness of the land area between the first groove and top of the piston also weakens the piston, increasing the risk of piston failure. Hypereutectic pistons have the extra strength needed to resist failure in this critical area, and the hard silicon particles provide a wear-resistant surface that prevents the top ring from pounding out the groove. Wear resistance is also improved in the skirt and wrist pin areas, too.
On some pistons (GM 3800 supercharged V6, for example), the crown and top ring groove are also anodized to improve durability and resistance to microwelding. Microwelding occurs when high combustion temperatures cause tiny particles of aluminum to melt on the piston and redeposit themselves on the ring.
Relocating the top compression ring higher on the piston also means the ring itself must be made of a more heat resistant material. The top rings on many engines today run at close to 600 degrees F, while the second ring is seeing temperatures of 300 degrees 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 of these late model engines have ductile iron or steel top rings. Both materials are more durable than plain cast iron, and are required for high output, high load applications including turbocharged and supercharged engines as well as diesels and performance engines.
If you are rebuilding an engine, therefore, you should use the same type of replacement rings that were originally used in the engines (or better). If old rings are steel, replace them with steel rings, not cast iron. If the old rings are cast iron, you can replace them with cast iron, ductile iron or steel. You can always upgrade, but you should not downgrade to a cheaper replacement ring.
How can you tell what type of rings are on a piston? Twist the ring after it has been removed. A steel ring will bend while a cast iron ring will break.
Cast iron rings are still a popular choice for many older engines as well as "economy" rebuilds that are suitable for light duty, everyday driving. Most late model applications, though, require more durable rings such as ones faced with chrome or moly. For applications where an engine is subjected to higher loads and operating temperatures, moly faced rings usually provide the best wear resistance. Molybdenum also provides great scuff resistance and is porous so it retains oil to keep the ring lubricated. Moly also has a higher melting point than chrome or plain cast iron, which enables it to survive under the harshest operating conditions.
Chrome provides improved scuff resistance over plain cast iron, but not quite as much as moly. Yet chrome rings are a good choice for engines that are operated in dusty environments because chrome is very dense and won't trap and hold contaminants like moly can. And compared to plain cast iron, chrome rings can cut wear by more than 50%
Plain cast iron rings and chrome faced rings both require a slightly rough surface finish #220 grit honing stones are recommended), while #280 grit stones are usually recommended for moly faced rings.
Nitrited rings, which are used in many Asian engines, also provide improved scuff and wear resistance. But nitriding involves the use of cyanide, which poses environmental concerns for ring manufacturers. So moly and chrome are the preferred facing materials in North America.
Replacement rings, in most instances, should have the same or better type of facing material than the original to maintain the durability and scuff resistance that was originally designed into an engine. Substituting plain cast iron rings in an attempt to hold down costs will sacrifice those advantages.
To improve ring sealing, some late model engines such as Ford 4.6L and GM LS engines use a wider end gap on the second ring. The end gap on the second ring is 1.5 to two times that of the top ring. The actual specification may range from .006 to .013 inches greater than the top ring depending on the application. The theory here is to treat the rings as a dynamic rather than static assembly.
When the combustion pressure over the top ring is greater than the pressure between the top and second ring, it forces the top ring downward and outward to seal against the piston groove and cylinder. But if pressure builds up between the two rings, it can prevent the top ring from sealing and increase blowby.
One trick racers have long used to improve ring sealing is to pull vacuum on the crankcase with a dry sump oil system. This helps maintain the pressure differential at the top ring by pulling air out from under it. But for an everyday street engine, a dry sump oil system is too expensive. So another way to maintain the pressure differential is to open up the end gap of the second oil ring. A wider end gap provides an escape route for blowby gasses that get past the top ring. This prevents pressure from building up so the top ring will continue to provide maximum sealing.
On some pistons, an "accumulator groove" is machined into the piston between the top and second ring to increase the volume of space between the rings. The accumulator groove helps reduce the buildup of pressure until the blowby gases can escape through the end gap in the second ring.
What some people don't understand is that the second ring is not really a compression ring but an oil control and vacuum ring. Many second rings have a reverse-twist, taper face design that allows the ring to glide over the cylinder wall during the piston upstroke. When the piston reverses direction, the sharp edge of the ring is forced out against the wall and acts like a squeegee to wipe off the excess oil. At the same time, the second ring seals against the wall so the piston can pull as much vacuum as possible on the downstroke.
On many late model engines, the second ring is a taper faced, corner groove, positive twist design. The lower outside diameter of this type of ring has a scraper groove that collects oil and also acts like an accumulator to reduce inter-ring pressure. On many European engines, the second ring has a shallow groove along the lower outside diameter. This is called a "THG" or "Napier" type ring.
When reverse twist taper face rings are used, the piston usually has a step groove on the third piston land to form a reservoir for the oil that's scraped off, and to add volume to the area between the second ring and oil ring to control inter-ring pressure. The step groove is not needed if the second ring has a scraper groove or Napier groove on the underside of the outside diameter.
As for oil rings, most North American and Asian vehicle manufacturers are using a traditional three-piece oil ring. The Europeans, on the other hand, prefer a one-piece oil ring with an expander behind it.
Installation is just as important as the type of pistons or rings that are used in an engine. One of the most common installation errors is installing one or more rings upside down. If only one second oil ring is accidentally installed upside down in a V8 engine, it can double the engine's oil consumption! If every second ring on all the pistons are reversed, the engine will have an unquenchable thirst for oil - which may be mistakenly blamed on improper ring break-in, seating or cylinder wall finish.
Rings are usually marked with a dot, which must always face up. If no mark is provided, rings with a bevel on the inside diameter must be installed with the bevel facing up. On rings with no mark and a groove on the outside diameter, install the rings with the groove toward the bottom of the piston.
Another common error is spiraling rings onto a piston. This will usually deform the ring which can affect ring rotation and seating. Always use a ring expander. Position the ring on the expander and expanded just enough to it can be slipped onto the piston. Do not over extend the ring.
Position the rings so the end gaps are staggered 180 degrees apart. This will reduce blowby when the engine is first started.
Ring and groove depth should be compared prior to installing the rings to make sure the rings are the correct ones for the piston. If shallow groove rings are installed on a deep groove piston, the rings will tend to pop off the piston before the piston is installed in the engine. If rings designed for a deep groove piston are installed on a shallow groove piston, the rings will bottom out and jam against the cylinder.
Piston ring side clearances and end gaps should always be measured after the rings have been installed to make sure the rings fit the grooves and cylinders correctly.
Piston rings can be damaged during installation if the lower lip of the ring compressor is nicked, rolled over or bent. For this reason, the underside of the ring compressor should be checked frequently for damage.
Finally, adequate ring and cylinder lubrication is essential to provide for proper ring seating and protection when the engine is first started. Scrubbing out the cylinders with hot soapy water after the block has been honed and before it is assembled is an absolute must to remove all the abrasives and other contaminants that can damage new rings.