Pistons & Rings, Larry Carley, Automotive Rebuilder, December 1998

With each new generation of engines come changes that are designed to reduce emissions and extend durability. Key among these have been changes in piston and ring materials.

Most vehicle manufacturers today are building passenger car engines that, with proper maintenance (regular oil changes), are capable of going well over 100,000 miles – with some OEMs aiming for a service life of up to 200,000 miles!

Many of these engines won't last that long under real world driving conditions because hose failures, overheating, detonation, head gasket failures, etc., will cut their potential service life short. But regardless of how long an engine lasts, in many cases the time will come when it can be rebuilt. So it's important to be aware of the piston and ring changes the OEMs have made so, if desired, the engine can be rebuilt to the same emission and durability standards as the original motor.

Obviously, it isn't necessary to rebuild an aftermarket passenger car engine to last another 100,000 to 200,000 miles. But with respect to emissions and performance, there should be little or no difference. Many states now have I/M 240 "enhanced" loaded mode emissions testing on a dyno, so emissions are essential to prevent test failures. An engine that lacks adequate oil control can fail for excessive hydrocarbon (HC) emissions as well as shorten the life of the catalytic converter.

Piston changes
One of the "tricks" that the OEMs are using to reduce hydrocarbon emissions is to position the top compression ring closer to the top of the piston. This reduces the volume of the dead space between the piston and cylinder wall above the top ring that traps unburned fuel and contributes to incomplete combustion.

But moving the ring up places it closer to the combustion chamber and exposes it to higher operating temperatures. Consequently, the rings need to be thinner to reduce inertia and made of tougher materials such as ductile iron or steel so they can withstand the heat. If ordinary gray cast iron rings are used in such an application, it would greatly increase the risk of ring breakage.

Another consequence of moving the top ring up is that it reduces the thickness of the piston land area above the ring, which also increases the risk of the piston cracking if the engine experiences detonation. For this reason, many of the newer engines today have hypereutectic pistons instead of ones made of ordinary cast aluminum

Hypereutectic alloys are much tougher than the alloys used in ordinary cast aluminum pistons. Hypereutectic pistons (which are also cast, not forged) have a very high silicon content ranging anywhere from 16-22%. The excess silicon forms little hard spots in the alloy, giving the piston improved wear and scuff resistance at high temperatures. However, the added hardness also makes the pistons much more brittle than ordinary cast pistons, which means hypereutectic pistons must be handled more carefully.

Another change is that many pistons are getting shorter as the OEMs reduce deck heights for shorter blocks and reduced hood clearance. To maintain torque output, longer rods are used – which requires moving the wrist pin higher on the piston. This leaves less room for the rings, which means smaller rings must be used and crowded together more closely.

Reducing the thickness of the piston lands also increases the risk of breakage, so that's another reason for using hypereutectic pistons. The harder hypereutectic pistons experience less ring groove wear and do not need inserts as some pistons do to prevent land pound out.

Many of today's engines are also running much closer piston-to-cylinder wall clearances to reduce blowby emissions. Some clearances are .001" or less, which leaves little room for error – or overheating. A tighter fit means there is less room for thermal expansion, which is yet another reason why the OEMs are using hypereutectic pistons in many of these engines.

Hypereutectic pistons expand less than ordinary cast aluminum alloys, and CNC machining of the piston profile allows piston-to-bore clearances to be reduced. This also eliminates the need for steel struts inside the piston to control thermal expansion, which reduces piston weight and complexity. Eliminating the steel struts inside the piston also eliminates a potential source of trouble that may, under certain circumstances, lead to piston cracking and failure.

Federal-Mogul's H100CP flat top performance hypereutectic piston (right), and its similar design H631P piston (left) which is targeted more towards the budget oriented enthusiast. The H100CP is completely CNC machined from Federal-Mogul's own FM244 alloy consisting of 16% silicone for extreme surface hardness. Ring grooves provide for a reduction in ring weight and enhanced cylinder seating at high rpms. The H631P piston features CNC machined skirt and pin areas and a dome of as-cast FM244 alloy. The piston will easily accommodate less expensive traditional service replacement ring sets. Good sealing and dependable performance are provided at rpm levels found in street-oriented performance applications.

"Some of the newer engines such as the Ford 4.6L V8 are using coated pistons to reduce the risk of scuffing," said Russ Hayes, product design engineering manager, Federal-Mogul Corp. "The graphite based coating protects the piston against both dry starts and galling if the engine overheats.

"If you get an air pocket inside the cooling jacket, it only takes two or three minutes to form a hot spot that can gall the piston. This is a real concern in marine applications because the risk of piston galling is probably three times that of automotive applications. So we offer a graphite coated skirt on our aftermarket replacement pistons for the Ford 4.6L as well as other applications including the 350 small block Chevy.

"When rebuilding these late model engines, rebuilders must follow the OEM design. You can't rebuild them cheaply or make substitutions," Hayes continued. "But that doesn't mean you can't improve upon the OEM design. When we make our placement piston for one of these new applications, we can usually produce a better design at less cost.

Hayes said one of the drawbacks of having the top ring so close to the combustion chamber is that it reduces ring life. "If the distance is less than about. 0175" to .020", the top ring runs hotter and requires a larger gap," he said. "The top land is also weaker which makes it more vulnerable to cracking if there's detonation. So on some of our aftermarket pistons, we place the rings further down the piston in a more normal location to improve durability.

"We also destroke our aftermarket pistons .010" to .020" depending on the application to compensate for overboring and deck resurfacing. Destroking the piston maintains the stock compression ratio and reduces the risk of detonation."

Hayes said another thing rebuilders should be aware of is that not all hypereutectic pistons are the same. The composition of the alloy can vary, and the silicon must be the right size and evenly distributed in the aluminum matrix. If the alloy is not mixed right, the silicon can clump together forming hard spots in the piston. These hard spots concentrate stress and increase the risk of cracking.

"Using bottom of the barrel pistons to save money will get you into trouble on these late model engines," warned Hayes. "Not only do you need a quality design and alloy, but also proper machining with diamond tooling to get the correct profile and finish."

"We try to provide the aftermarket with pistons that meet OE criteria," offered Keith Sulprizio, general manager of United Engine and Machine. "When a new engine comes out, the National Engine Parts Manufacturers Association (NEPMA) provides its members with the OE design specifications. Our quality planning group then reviews the information and comes up with a design that produces the best replacement product."

Sulprizio said that although his company offers both hypereutectic and conventional cast aluminum pistons, many of its aftermarket rebuilder customers want a less expensive replacement alternative for engines that came originally equipped with hypereutectic pistons. So where demand warrants it, his company offers an alternative.

"We also are doing some coated pistons – often at the request of our rebuilder customers who want the added protection provided by some type of skirt coating," said Sulprizio.

Forged pistons
Though forged pistons are used primarily in heavy-duty diesel truck applications and few OEM passenger car engines today, they remain a popular alternative for aftermarket performance and marine engine rebuilders. The main attraction of forged pistons is their ductility, which allows them to withstand punishment that would destroy ordinary cast pistons.

The forging process eliminates porosity in the metal, which makes it denser and stronger. The type of alloy used in forged pistons is also up to 600% more ductile than that used in conventional and hypereutectic cast pistons. The result is a stronger piston that is well suited to racing and other equally punishing environments.

Forged pistons also run 18% to 20% cooler than cast pistons because the metal conducts heat away from the combustion chamber more quickly. This reduces the risk of detonation – but the trade-off is greater thermal expansion in the piston. Consequently, forged pistons require greater installed clearances which increases cold start noise and blowby.

The best advice from the piston manufacturers is to replace same with same, or better. If a late model engine has hypereutectic pistons (or in some turbo motors, forged pistons), they should be replaced with the same type of piston to maintain engine reliability.

Rings
"The ring sets in many of today's new engines look like the race sets from the 1980s," said Daniel Wilkinson, vice president of applications engineering, Perfect Circle/Dana Corp.

"Rings are getting thinner and narrower to both reduce inertia and to improve sealing by allowing greater conformability to the cylinder bore. Most top rings today are 1.2 to 1.5mm, with some Japanese rings as small as 1.0mm or even less.

Most oil rings are now in the 3.0mm size, and provide much less tension. The oil ring alone used to account for up to 25% of an engine's internal friction, but today the number is down around 12-18%.

"One area where we're going to see big changes is in ring coatings," said Wilkinson. "Back in the 1950s, chrome was the most common type of facing material for rings. Moly was introduced in the 1960s, followed by plasma spray moly coatings in the 1970s. We think chrome plating will be going away because of environmental reasons.

The Japanese use nitriding to increase the wear resistance of their ring sets. Nitriding does not have the scuff resistance of plasma spray moly, but it gets rid of the chrome. We're developing nitrided oil rings now for some OEM applications," said Wilkinson.

Gas nitriding, which should not be confused with the black phosphate coating that is currently used on most rings to prevent rust during shipping and storage, is a heat treatment process that impregnates the surface of the metal with nitrogen in order to harden the surface of the metal. This makes the rings very hard – about 68 on the Rockwell C scale for improved wear resistance.

"One of the advantage of the plasma spray process is that different materials such as chrome carbide can be mixed with the moly powder to produce different ring characteristics," said Wilkinson. "Different coatings can also be developed for diesel engines which are popular in Europe, or even alternative fuels such as hydrogen.

"We can manufacture rings that don't wear (themselves), but they wear the cylinder bore. So we have to balance ring and bore wear to come up with the best overall solution."

Regardless of the type of ring facing that's used, the proper bore finish is required for a good initial seal. Most aftermarket ring manufacturers say the bore finish should be less than 20 RA, with some recommending 14 to 18 RA as the "ideal" range.

"To achieve the proper cylinder bore finish, rebuilders need measuring equipment that can measure more than just RA," said Wilkinson. "They also need to measure RVK, RK and RPK."

RA is the roughness average and should be .25 to .50 microns or less, according to Wilkinson. RVK is the average depth of the valleys and should be .75 to 1.50 microns. RK is the average roughness of the core, and should be .63 to 1.25 microns. PRK is the average height of the peaks, and should be .25 to .50 microns.

To achieve these kinds of numbers, various honing processes may be required depending on the type of rings used. Scott Gabrielson, ring design engineer for Federal-Mogul, says for moly rings, he recommends a two-stage honing process: honing the bores with a #220 or #280 grit stone to within .0005" of final size, then polishing the bores with a #400 grit stone or flexible abrasive brush to plateau the surface.

This procedure can produce a bore surface with plenty of bearing area to support the rings, adequate crosshatch to hold oil for ring lubrication, and no sharp peaks to wear the new rings. A plateau finish also provides an instant seal to minimize blowby and ring break-in.

For chrome or nitrided ring sets, Gabrielson says the cylinder bores can be finished with a single stage process using #220 or #280 grit stones. For these applications, he does not recommend plateauing – but he does say the stones should be run a little longer at reduced load to finish the bores.

Tougher materials
The increased demands on today's engines means more OEMs are using ductile and steel top compression rings. As mentioned earlier, the tougher materials are needed to withstand the pounding and heat.

Gray cast iron is an adequate ring material for most older passenger car applications. But the change to thinner low tension rings in newer engines, and the relocation of the top ring closer to the top of the piston, has dictated the use of ductile iron or steel top compression rings in many applications.

Gray cast iron is a brittle material that can easily break if mishandled. Rings made of this material may also break if the engine experiences heavy detonation. Ductile iron (also called "nodular" iron), on the other hand, has a different microstructure with rounded grains instead of rectangular grains. This allows the metal to bend without breaking so it can withstand detonation in high load engines. It also makes the metal about twice as strong as gray cast iron.

Chrome or moly faced ductile iron 1.5mm top compression rings have been used since the early 1980s in many turbocharged engines, and are now used in many late model domestic engines with the new piston and ring configurations. Gray cast iron rings are still common in aftermarket ring sets, but many premium ring sets now have ductile iron top rings.

Another ring material that is seeing greater use in new engines is steel. Twice as strong as ductile iron and four times as strong as gray cast iron, steel can provide the durability and toughness needed for the most demanding top ring applications. Steel rings have a tensile strength in the range of 240,000 psi, which compares to 180,000 psi for ductile iron and 45,000 psi for gray cast iron. Hardness can vary depending on the alloy and heat treatment, but is generally in the 44 to 53 HRC range compared to 38-40 HRC for ductile iron and 22-23 HRC for gray cast iron.

Like ductile iron, steel is not compatible with cast iron cylinder walls so it must be coated with either chrome or moly, or nitrided. Most of the steel rings currently in production have a width of 1.0 to 1.2mm. Such rings are found in many late model Japanese engines as well as Ford's 4.6L V8 and Buick's 3800 V6. Steel rings are usually barrel faced, having contoured outside diameters which give the ring a center contact with the cylinder wall.

Though the best advice here is to follow the OEM lead and replace ring sets with ones made of the same material (or better), several ring manufacturers said steel and ductile iron rings are virtually interchangeable. If a steel replacement ring is not available for a certain application that uses steel as original equipment, a ring set with ductile iron top rings can be substituted.

The type of replacement rings a rebuilder chooses will ultimately depend on what the customer expects from an engine and how much they're willing to pay. For low-cost rebuilds, some rebuilders may prefer ordinary gray cast iron rings. That's why economy ring sets are so popular with rebuilders.

Federal-Mogul's Gabrielson says there are even some instances where the OEMs have gotten away from steel and gone back to gray cast iron. The use of knock sensors and more precise engine control systems on these engines has reduced the risk of detonation to the point where the added durability of steel is no longer necessary.

Reducing blowby
Another change that may be coming in OEM ring designs is to use something to seal the gap in the top compression ring (some type of overlapping end gap). Though you might think the reason for doing this would be to reduce blowby from the combustion chamber into the crankcase, the real reason is to eliminate reverse blowby from the crankcase back into the combustion chamber.

When the piston goes down on the intake stroke, vacuum in the combustion chamber can pull oil and other vapors from the crankcase into the cylinder – especially during cold cranking and the first five to 10 minutes of engine operation. Sealing the gap in the top compression ring to prevent this from happening can lower start-up emissions 10% to 27%.

Installation tips

One point all the ring manufacturers emphasized is the need for cleanliness in the cylinder bores. Honing leaves a lot of debris in the cylinders, which if not removed by washing and scrubbing with hot soapy water can ruin a new set of rings.

When oversized pistons are being installed, measure the pistons, then bore and hone the cylinders to size to obtain the proper skirt clearance. To determine the required amount of piston clearance, measure the diameter of the piston across its major axis (crosswise or 90 degrees to the wrist pin) just below the pin centerline with a micrometer. Then add the skirt clearance recommended to this dimension to get the required cylinder bore size. Another method to check clearance is to insert the piston into the cylinder and measure piston skirt-to-bore clearance with a feeler gauge. When installing ring sets on pistons, make sure the rings are placed right side up (refer to the manufacturer's markings) using a ring expander. Rings should not be twisted or spiraled into the piston grooves as doing so can deform the rings and prevent them from sealing properly. Bending also increases the risk of breakage with gray cast iron rings.

When the engine is first started, it must be held at a fast idle for 30 minutes. If the vehicle can be driven, alternately accelerating and coasting in low gear can help seat the rings. The engine must not be allowed to idle for any length of time during the critical break-in period because the cylinder walls and rings rely solely on splash lubrication. If the proper break-in procedure is not followed, the piston skirts and rings may scuff and glaze the cylinder walls, resulting in excessive blowby and oil consumption.