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 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 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 new second ring designs are now using a "napier" style edge that has more of 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.
LOW TENSION PISTON RINGS
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. Even so, nitriding remains popular in Japan.
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 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 percent of an engine's internal friction. Thinner rings exert less tension against the cylinders. This reduces friction and improves fuel economy. And 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.
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. And the pistons themselves have been improved to make them more heat resistant and durable.
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 - 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? or less) to minimize blowby and reduce piston rock. The more stable the piston, the better it is able to maintain a tight seal. Close tolerances also make for a quieter running engine, especially after a cold start when clearances are greatest). To keep clearances to a minimum in such circumstances, an alloy with a lower coefficient of thermal expansion is required.
Many pistons today are made of a "hypereutectic" alloy for this reason. Hypereutectic pistons have a coefficient of thermal expansion that is about 15 percent less than standard F-132 alloy pistons. Consequently, hypereutectic pistons can be installed with a much tighter fit - up to .0005? less clearance may be needed depending on the application. Hypereutectic pistons with moly coatings on the side can handle even less clearance.
The tensile strength of hypereutectic pistons and conventional cast pistons is about the same, but the high temperature fatigue strength of hypereutectic alloys is better than either cast or forged alloys. This eliminates ring pound out problems in the top oil ring groove and the need for an iron insert.
On some pistons (GM 3800 supercharged V6, for example), the crown and top ring groove are 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 stick to the ring.
RACING PISTON RINGS
SMOOTHER, FLATTER PISTON RINGS
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. Many late model blocks and most high performance engines should always be honed with torque plates bolted to the block to simulate the distortion created by the cylinder head bolts. Crosshatch provides lubrication for the rings.
Most engine builders prefer 30 degrees, but some use as much as 45 degrees.
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