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Piston Ring Technology: Stock and Performance

Let's look at some of the latest thinking as it applies to piston ring designs, materials and coatings.

By Larry Carley

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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 a 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.

STOCK RINGS
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 among engine rebuilders. 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.

A plain cast iron ring that's designed for an older pushrod V8 or V6 can give an engine rebuilder a cost advantage over a higher priced ring set. Even so, plain cast iron rings won't provide the durability of a chrome or moly-faced ring set, or a 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 finish honed. Plain cast iron rings don't have a hard facing to resist wear, so they require a very clean surface. The cylinders should be washed and scrubbed out with hot soapy water to remove all traces of honing abrasive and metal residue from the surface. Using a plateau finish will allow the rings to seat almost immediately to extend life and reduce blowby.

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 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 RINGS
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. 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.