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Gasket Technology: The Science of Sealing
In spite of the fact that castings have gotten thinner, valvetrain components have gotten smaller and engines are more highly stressed than ever before. Today’s crop of high tech, overhead cam bi-metal engines have turned out to be much longer lived than their heavy, cast iron predecessors from the 1970s and ’80s. The OEM design standard today is 100,000-miles-plus durability, which requires improved manufacturing techniques, better quality control, closer assembly tolerances and new gasket designs and materials that can ‘go the distance.’
Gaskets have played a significant role in making today’s generation of engines the most robust ever built. A "robust" design is one that holds up well over the long haul – and that certainly describes many of the new gaskets that are now being used to seal everything from cylinder heads to valve stems to valve covers and oil pans.
In recent years, there has been a significant shift away from composite head gaskets with nonasbestos facings to those with expanded graphite facings in many new engines. At the same time, many late model Japanese engines, as well as Ford’s 4.6L V8 and modular V6 engines, and some Chrysler engines, have switched to "rubber coated embossed" (RCE) or "multi-layer steel" (MLS) head gaskets.
Graphite head gaskets
Though roughly three times as expensive as nonasbestos facing materials, graphite has excellent cold sealing properties and provides the lubricity needed to handle the difference in thermal expansion between cast iron blocks and aluminum cylinder heads (aluminum expands at twice the rate of cast iron). Graphite can also withstand high temperatures and is an "anisotropic" material, which means it can draw heat away from hot spots to reduce thermal stress and loading.
But in spite of its many advantages and popularity, graphite has also turned out to be the Achilles Heel for some engine applications. The original OEM head gasket on the General Motors 2.3L Quad Four was a graphite design that had a high failure rate at around 50,000 miles. The failure usually occurred around the combustion flange as a result of metal fatigue.
The premature failure problem was solved by redesigning the gasket’s combustion flanges and adding a silicone coating to improve sealing. Some aftermarket replacement gaskets for the Quad Four have a Teflon coating to reduce fretting and fatigue which can contribute to the failure problem on this engine. Others do away with the graphite altogether and use a coated nonasbestos composite facing material to seal the engine.
A more recent example of an OEM graphite head gasket with a high failure rate is the Ford 3.8L V6 in 1995 Ford Windstars and 1994/’95 Taurus and Sables. Ford recently extended its warranty coverage on this gasket to five years or 60,000 miles because of coolant leakage problems that in some instances have caused engine failure due to overheating.
Eric Roline, head of research and development at Detroit Gasket, said his company is using composite engineered gaskets to replace many of the "problem" OEM graphite gasket applications. "Our composite replacement gaskets cost about half as much as a graphite gasket, and they don’t come back," said Roline. "We’ve had no problems with the composite materials in these applications."
"Graphite is a good material, but we don’t think its the answer for everything," said Marty Novil, chief engineer, aftermarket, for Federal-Mogul‘s Fel-Pro product line. "We prefer to use a toolbox of materials and technologies to select the best gasket materials and designs for a particular application."
One of the drawbacks of graphite, said Novil, is that it must be protected to withstand exposure to oil over the long term. And although graphite has good sealability, it can crush and extrude. Consequently, some of Federal-Mogul’s aftermarket replacement gaskets for OEM graphite applications are nonasbestos composite materials on solid or perforated steel cores. However, others are graphite or MLS material designs.
Novil said Federal-Mogul will be introducing several new solid core graphite gaskets for intake manifold and water pump applications soon. The new gaskets are steel coated with a layer of graphite about .005" to .010" thick. The design provides the core strength of solid steel with the sealability and conformability of graphite.
Other aftermarket gasket suppliers are much more bullish on graphite. Some have introduced graphite replacement gaskets for engines that came originally equipped with nonasbestos composition head gaskets or even some MLS head gasket applications. Graphite has also become a popular choice for high performance applications, too.
"We’re adding more and more graphite head gaskets to our product line, including ones for some of the older cast iron engine applications," said David Sills, sales manager for ROL Gaskets. "It’s a great material, especially for bi-metal engines. But you can’t use it with a solid core because there’s no way to glue it to the steel. You have to use a perforated core.
"One of the limitations of using a solid steel core with a composition material is that the bonding agent may delaminate if the gasket gets too hot. So we see a move away from solid core designs back to a perforated, clinch type of core construction."
Sills said another trend he sees in both graphite and composition replacement gaskets is a greater availability in thicknesses. "We offer some gaskets in three different thicknesses, e.g., .038", .048" and .051", so rebuilders can compensate for head and block resurfacing," he said.
Bob Anderson, gasket product team leader for McCord Gaskets, said his company is trying to use graphite "every way possible" for aftermarket applications because graphite conducts heat so well, pulls heat away from hot spots and doesn’t require a special surface finish as MLS gaskets do.
In addition to head gaskets, Anderson said graphite is now being used in some intake manifold applications, as well as exhaust manifold gaskets. Anderson said exhaust manifold gaskets have long been an ideal application for graphite because of the high temperatures encountered in this area of the engine. But intake manifolds are a good application, too, because of the movement that may occur between the head and manifold.
Tim Golema, aftermarket sales engineer for Victor/Reinz, Dana Corp., said graphite doesn’t act like a standard composition material because it is very sensitive to how much it can be compacted. Consequently, the latest design/manufacturing focus is on controlling the density of the graphite facing material during its manufacture.
"The durability of a graphite head gasket can be determined by carefully controlling how much the graphite is compacted on the perforated steel core," Golema said. "The secret is fine tuning the amount of compression by determining density versus bolt loading."
Golema said Victor Reinz is currently evaluating a new coating material for graphite head gaskets that promises a significant improvement in sealing, as well as long term durability. "Like other gasket manufacturers, we’ve used Teflon and other coatings on graphite over the years," said Golema. "But this new coating material surpasses everything else, hands down. It allows 10 times as much movement of the head across the surface of the gasket without the head grabbing and pulling the graphite.
"We’ve tested the coating on a new all-aluminum GM engine and were astonished at the results," Golema continued. "With other coatings, the head gasket would start to leak combustion pressure after 1000 cycles and 30 to 40 hours of testing. But with the new coating, we could go 7,000 cycles with no leakage whatsoever at 1000 lbs. of nitrogen pressure."
Golema said the new coating material should be in production soon, and may be applied across the board to all Victor/Reinz aftermarket graphite head gaskets in the next couple of years.
As mentioned earlier, the other trend in engine sealing is to go to a rubber-coated, embossed multi-layer steel head gasket. MLS head gaskets are virtually bullet-proof and have proven themselves to be extremely durable.
Several gasket manufacturers said the future trend is away from graphite and composition gaskets at the OEM level to more MLS head gaskets. But to do so, an engine often has to be redesigned to alter the loading on the head bolts.
The OEMs like multi-layer steel gaskets for several reasons. MLS gaskets don’t take a compression set and lose their ability to seal over time like composition gaskets do. Durability can be as high as 150,000 miles plus. Their solid-core construction also provides added strength and reinforcement to resist blow-outs, and the rubber coating provides good cold sealability.
The typical MLS gasket may have three to seven layers of steel. The outer layers are usually stainless spring steel, embossed and coated with a thin layer (.001" to .0015") of nitrile rubber or Viton. The inner layers provide added support and thickness. The embossed multi-layer construction also reduces the load on the head bolts, which, in turn, reduces bore distortion for less blow-by and lower emissions.
The only drawbacks of the MLS design are its cost – which may be three times or more that of a conventional composition gasket – and the need for a very smooth (20 to 30 RA) surface finish.
It isn’t difficult for OEMs to achieve high quality surface finishes on blocks and heads using precision multi-head cutters and long-lasting superabrasives. But by the time these engines reach the point where head work or other internal engine repairs are needed, the smooth, flat surfaces are often gone.
Many machine shops still don’t have the right kind of refinishing equipment or the expertise to reproduce a high quality surface finish necessary for an MLS gasket. Consequently, there’s a concern that the aftermarket may have problems rebuilding some of the engines that use MLS gaskets.
Some say it’s better not to attempt to resurface a head on an engine that has an MLS gasket unless a shop has the proper equipment and know how to achieve the required high quality surface finish. Others say it may not be necessary to use an MLS replacement gasket.
Some gasket manufacturers have introduced replacement gaskets for the Ford 4.6L and other MLS applications that are not an MLS design. Victor/Reinz, for example, has a Teflon coated graphite "Nitroseal" gasket for the Ford 4.6L which it says is more accommodating of surface finish and surface irregularities than the OEM gasket.
The issue of cost and surface finish are crucial ones for the aftermarket when it comes to replacing OEM MLS gaskets. Yet, new MLS gaskets are being introduced into the aftermarket as cures for some "problem" engines that were not originally equipped with MLS gaskets.
Federal-Mogul, for example, recently introduced its Fel-Pro aftermarket MLS gasket for the hard to seal Toyota 5VZFE 3.4L V6 truck engine. The original Toyota head gasket is a graphite design that tends to fail prematurely because of motion between the castings in the motor. Federal-Mogul’s replacement for the engine is a four-layer MLS stainless steel gasket. A special coating on the gasket allows it to handle surface finishes as rough as 70RA.
Another new MLS aftermarket application is Federal-Mogul’s Fel-Pro replacement gasket for the Dodge/Plymouth Neon engine. The OEM composition gasket reportedly has been experiencing a high failure rate due to an oil hole leakage problem brought on by excessive movement between the head and block. The MLS replacement gasket solves the leakage problem and is coated so it will seal with the original factory finish.
One long term trend in new MLS gasket technology, according to Victor Reinz’s Golema, is to reduce the number of layers in the gasket to help reduce the cost of manufacturing. "We see the OEMs reducing the thickness of MLS head gaskets as well as the number of layers from an average of four down to three or possibly even two," he said. "We also see a move to limit the coating to only those areas of the gasket that really need it. This would be primarily around the cooling holes."
Reinforced head gaskets
Another trend that has emerged recently, at least as far as aftermarket gasket designs are concerned, is the development of special reinforcements to deal with hot spots that can crush conventional composition gaskets.
Honda 1.3L and 1.5L engines in 1984-’87 Honda Civics often suffer head gasket failures because of engine overheating. On these engines, each cylinder has a precombustion chamber. The precombustion chambers for the two center cylinders are located back-to-back, creating a localized hot spot in the head between the two adjacent exhaust valves in cylinders #2 and #3.
Coolant flow is also limited in this area. Consequently, if anything happens to cause the engine to overheat, thermal expansion will crush the head gasket in the area between the center cylinders causing the gasket to leak or burn through. In the past, replacing the head gasket only temporarily solved the problem because the hot spot often caused the replacement gasket to also eventually fail for the same reasons.
Both Federal-Mogul and Victor Reinz have introduced specially designed replacement head gaskets for the Honda 1.3L and 1.5L engines. Federal-Mogul’s Fel-Pro composition gasket features a large "Y-shaped" aluminum shim in the area between the center cylinders. The shim improves the gasket’s resistance to crushing and extrusion caused by the hot spot in the head.
The gasket material is also "pre-crushed" in the critical area and the armor around the combustion chambers has been redesigned for added strength and protection. The Victor Reinz solution is a graphite gasket with thicker stainless steel combustion armor. With this design, the graphite helps pull heat away from the hot spot while the added armor helps prevent burn-through.
Intake manifold gaskets
OEMs have used various approaches over the years to seal intake manifolds to the heads. Embossed steel and fiber gaskets work well on many applications. For long-term durability, many newer engines have special O-rings or plastic/rubber or rubber/metal gaskets. The plastic or metal carrier limits compressibility to prevent the rubber from extruding if the bolts are overtightened.
But as we’ve seen with head gaskets, some OEM gasket designs have their problems. The intake manifold gaskets on Mitsubishi 3.0L V6 engines, as well as GM 3.1L V6 engines, sometimes leak because of the low clamping load on the gasket at the manifold flange, and due to movement between the heads and manifold.
On an older engine, the sealing surfaces may be rough, scratched or pitted, so it may be difficult to get a leak-free seal if the gasket is replaced with one that uses the same OEM rubber-coated embossed steel design. The OEM gaskets also do not seal well against a rough or pitted surface. Applying a sealer to the gasket’s surface may improve initial sealing, but there’s a risk of the sealer reacting with the rubber coating which may cause the gasket to leak later on.
Several aftermarket replacement gaskets for these applications are made of expanded graphite bonded to a perforated steel core. The graphite facing material seals better than the OEM steel gasket, and is coated with molyteflon to help the gasket accommodate shearing motions and to seal small surface scratches and pits which are common on older engines.
Another OEM manufacturing and design trend is to eliminate the gasket altogether. On some engines, O-rings are used to seal the ports on the intake manifold to the head. On the Dodge Neon, the O-rings are molded into the intake manifold. One gasket supplier said if one or more of these O-rings are leaking, they must be replaced with ones that are compatible with the application.
O-rings come in a variety of materials which compress at different rates. If the wrong type of O-rings are used, the manifold may not seal properly and leak.
Valve cover and pan gaskets
Just as cork valve cover and pan gaskets have given way to molded silicone rubber gaskets in recent years, now molded rubber gaskets are being replaced by rubber/plastic and rubber/steel gaskets. Valve cover and pan gaskets with plastic carriers are popular with the OEMs these days because they’re easy to install on the assembly line. They also provide good crush control and added strength and durability. And, unlike cork, they won’t take a compression set or wick oil.
Most of these "high tech" gaskets can’t be replaced with a simple cut cork gasket without creating a problem, warn most of the gasket suppliers we interviewed. Their advice? Replace same with same. If the OEM gasket is molded rubber, use a molded rubber replacement gasket. If it is a rubber/plastic or rubber/metal carrier style of gasket, keep the replacement consistent to the original.
Cork gaskets are much less expensive (thus the incentive to substitute them for the more expensive molded and carrier style gaskets). But cork compresses and may not fill the void between the pan and casting as well as a molded gasket or carrier style of gasket.