GASKET TECHNOLOGY: DEMANDING ENGINE ENVIRONMENTS REQUIRE APPLICATION-SPECIFIC SEALING SOLUTIONS, Larry Carley, ImportCar, October 2000

As today’s sealing requirements become more and more demanding, gasket technology continues to evolve to meet the challenges. The original equipment vehicle manufacturers want 100,000-mile durability, and the aftermarket wants gaskets that are affordable and don’t require any special tools or tricks to install.

One of the most demanding applications has turned out to be bimetal engines. Aluminum cylinder heads can save a considerable amount of weight, but aluminum has a much higher coefficient of thermal expansion than cast iron. This creates a scrubbing or shearing action across the face of the head gasket, which has turned out to be the Achilles’ heel for some OEM head gaskets.

Some engines, such as the Toyota 5VZFE 3.4L V6 truck engine, have had some problems with head gasket failures. The original Toyota head gasket is a graphite design that tends to fail prematurely because of motion between the castings in the engine. The aftermarket 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 70 roughness average (RA).

SEALING SOLUTIONS
One way gasket manufacturers are combating the scrubbing problem is to apply nonstick coatings to their head gaskets. Teflon, molybdenum and similar substances can prevent gaskets from sticking to either surface, allowing the head to expand and contract without ripping the gasket apart. This is the opposite approach to what was and still is used on many head gaskets for cast-iron engines. On these applications, raised silicone sealing beads are often used to provide a good cold seal. The added thickness of the bead increases the clamping pressure in critical areas of the gasket, but it also grips the head and block. That approach works fine if the head and block expand at the same rate, but it can create shearing forces in the gasket if the engine has an aluminum cylinder head. So slick is in, at least for bimetal engines, and stick is out except for all-iron engines.

SILVER GASKETS
On many late-model engines, graphite head gaskets are used because graphite has a lot of desirable characteristics as a gasket material. It has natural lubricity to handle the differences in expansion between aluminum heads and cast-iron blocks and, because it is a relatively soft material, it has excellent cold sealing properties. 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. That’s why graphite head gaskets have become so popular both with the OEMs and as a "premium" aftermarket material. Unfortunately, graphite costs more than traditional nonasbestos materials. This has limited its use in many applications. "Graphite is a good gasket material," said one aftermarket gasket engineer, "but it isn’t necessarily the best material for every application. You have to choose whatever material works best in a given situation."

One of the drawbacks of graphite 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. That’s why some aftermarket replacement gaskets for engines that came originally equipped with graphite are nonasbestos composite materials on solid or perforated steel cores.

Graphite doesn’t act like a standard composition material when it is made into a gasket. So a lot of research is being done on controlling the density of the graphite facing material during its manufacture. By carefully controlling the density of the graphite when it is pressed onto a perforated steel core, durability can be significantly improved.

New coatings have also been developed for graphite gaskets that improve long-term durability. Some of the latest Teflon and moly-based coatings allow much more movement between the head and gasket than previous coatings, and eliminate the grabbing and pulling that can lead to premature gasket failure.

MULTI-LAYER STEEL
Another approach the OEMs have used to handle the shearing forces created by an aluminum head on a cast-iron block is to use a multi-layer steel (MLS) head gasket. Such gaskets typically have three to seven layers of steel. The outer layers are usually stainless spring steel and embossed. 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 blowby and lower emissions. The outer layers of steel are also coated with a thin layer (.001 to .0015 in.) of nitrile rubber or Viton to provide extra cold sealing.

MLS gaskets are found on a variety of Japanese applications including 1990 & up Honda Accord 1.8L, 1990 & up Honda 1.5L, 2.2L & 2.3L, 1988 to ’91 Mazda 3.0L V6, 1990 & up Mazda SOHC & DOHC 1.8L, and 1992 & up Mazda 1.8L. MLS head gaskets are very durable because they retain torque well and resist burn-through. They don’t take a compression set nor lose their ability to seal over time like composition gaskets do. Durability can be as high as 150,000 miles or more. Solid core, all-steel construction also provides added strength and reinforcement to resist blow-outs. But the tooling to manufacture MLS gaskets is very expensive (up to half a million dollars for one gasket!) and the multi-layer construction increases manufacturing costs. MLS gaskets also require extremely smooth surface finishes to seal properly (20 to 30 RA or less), which poses a real challenge for the aftermarket. Many shops do not have the proper milling equipment to reproduce such a smooth surface finish. For this reason, some say it’s better not to attempt to resurface a head on an engine with an MLS gasket unless absolutely necessary. Even so, many of the heads on these engines do need to be resurfaced by the time our readers see them. That’s why the gasket manufacturers are trying to develop more conventional replacement gaskets for some engines that were originally equipped with MLS head gaskets.

Aftermarket MLS gaskets have also been introduced as replacements for some engine applications that were not originally equipped with an MLS head gasket, such as the one for the Toyota 5VZFE 3.4L V6 truck engine. The MLS replacement gasket provides a much more durable sealing solution and does not require any special surface finish. By using a thicker surface coating on these MLS gaskets, the need for an ultra-smooth finish is eliminated. The new MLS replacement gaskets can reportedly handle a surface finish from 60 to 70 RA, which is on par with conventional gaskets.

Does that mean MLS gaskets will soon be available for a growing variety of older engines? It’s unlikely because of the high tooling costs. One gasket manufacturer said the investment required to develop and manufacture a MLS gasket for a given engine application may be justified if the potential replacement market is there. That might include MLS gaskets for high-performance engine applications as well as some other engines. Another gasket manufacturer said they are currently developing MLS replacement gaskets for a number of aftermarket applications.

SURFACE FINISH & FLATNESS
For many years, most aftermarket gasket manufacturers have said a surface finish of 55 to 110 microinches RA or 60 to 125 RMS (root mean square) is acceptable for conventional gaskets. The preferred range has traditionally been 80 to 100 RA.

More recently, though, some gasket manufacturers have changed their recommendations because today’s engines are lighter and castings are thinner and less rigid. The latest recommendations are for a surface finish of 30 to 110 RA for cast-iron head and block combinations, with a preferred range of 60 to 100 RA, and 30 to 60 RA for aluminum heads on cast-iron blocks, with a preferred range of 50 to 60 RA.

Flatness is important, too. On most pushrod engines with cast-iron heads, up to .003 in. (0.076 mm) out-of-flat lengthwise in V6 heads, .004 in. (0.102 mm) in four-cylinder or V8 heads, and .006 in. (0.152 mm) in straight six-cylinder heads is acceptable. The maximum allowable limit for out-of-flat sideways in any head is .002 in. (.05 mm) — with no sudden irregularities that exceed .001 inch in any direction.

Aluminum heads, on the other hand, should have no more than .002 in. (.05 mm) out-of-flat in any direction. If the clearance between the straight edge and surface exceeds the maximum limits, the head or block should be resurfaced.

Aluminum OHC heads should be checked for flatness in two places: across the face of the head with a straight edge, and down the OHC cam bores with a straightedge or bar. In most instances, both will be off if the head is warped. If the cam bores are still straight and only the face of the head is out-of-flat (a rare situation), resurfacing should be all that’s needed to make the head flat. But if the cam bores are out of alignment (much more common), the head will have to be straightened and/or align-bored or honed — and then resurfaced as needed to make it flat.

REINFORCED HEAD GASKETS
Another trend that has emerged recently 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 1.3 & 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 crushes 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 eventually fail, too. Several aftermarket gasket suppliers have introduced specially designed replacement head gaskets for the Honda 1.3 & 1.5 engines. One 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 also been redesigned for added strength and protection.

The other design is a graphite gasket with thicker stainless steel combustion armor. The graphite helps pull heat away from the hot spot, while the added armor helps prevent burn-through.

FATTER GASKETS
Something else you’ll find in some aftermarket gaskets is increased thickness (.020 to .030 in.) to compensate for head resurfacing. A thicker gasket can also be installed to lower compression slightly and reduce the risk of detonation causing a repeat failure.

Head gasket shims are another product that can help save overhead cam heads that have been overmilled. Installing a .020-inch thick shim under the head gasket can raise the head enough to restore proper valve timing and compression. Both copper and steel shims are available for a wide range of applications. Copper provides better conformability, while steel is more durable over the long haul and retains torque better than copper. Most shims require a brush-on or spray-on tacky sealer on the underside that faces the block, but no sealer should be used on the top side that faces the gasket.

ADJUSTABLE HEAD GASKETS
The latest innovation in aftermarket sealing solutions is an adjustable thickness head gasket for 1992-’95 Honda 1.6 VTEC engines. The multi-layer "stackable" head gasket allows the use of one to three inner layers to vary the engine’s compression ratio. Increasing the overall thickness of the gasket by adding layers can compensate for head or block resurfacing, and reduce compression for turbocharged street performance engines. Similar gaskets are in the works for other engine applications where compression and head resurfacing present similar challenges.

PAN & COVER GASKETS
Many replacement gaskets today for pan and cover applications have been improved by the use of grommets that prevent overtightening. If a new gasket is overtightened when it is installed, it can extrude and slip out of place. Or it can be squeezed to the point where something has to give and the gasket fails. The use of grommets prevents this by creating a positive stop for the cover or pan when it is tightened down.

Some cork/rubber pan and cover gaskets also have a steel carrier sandwiched in the middle to reinforce the gasket and make installation easier. Others are made entirely of molded rubber or silicone, or have a steel or plastic carrier with a rubber sealing bead. These high- tech gaskets are used on many late-model engines, but are now available for a growing number of older engines as a premium replacement alternative to traditional cork/rubber gaskets. The high-tech, carrier-style pan and cover gaskets would be a good choice for applications where long-term durability is more important than the initial cost of the gasket itself.

It’s important to note that most of these high-tech pan and cover gaskets in late-model engines cannot be replaced with a traditional cut cork/rubber gasket. The reason? Cork compresses and may not fill the void between the pan and casting as well as a molded gasket or carrier style of gasket. The best advice is to use a replacement gasket that is the same (or better) than the OEM gasket.

On some applications where RTV is used to seal a flat pan or cover flange to the cylinder head or block, conventional cut cork/rubber gaskets are available as a replacement alternative. RTV is a good sealant but must be applied correctly for it to work properly. Both surfaces must be clean and oil-free, and the RTV must be allowed to cure for 30 minutes or more before it is exposed to oil or other liquids. For this reason, many technicians would rather install a conventional gasket than use RTV.

As for molded silicone valve cover and pan gaskets on late- model engines, they should not be reused. Oil makes the rubber swell. This keeps the gasket from leaking as long as it is in service, but is also causes the gasket to distort if it is removed. Reinstalling the gasket may be difficult and create a potential for oil leakage, so always use a new gasket for such applications.

INTAKE MANIFOLD GASKETS
The OEMs have used various approaches over the years to seal intake manifolds to cylinder heads. Embossed steel and fiber gaskets work well on many applications. But 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 sometimes leak because of the low clamping load on the gasket at the manifold flange and 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 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 that are common on older engines.

Another trend is the elimination of the gasket altogether. On some engines, O-rings are used to seal the ports on the intake manifold to the head. 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 that compress at different rates. If the wrong type of O-rings are used, the manifold may not seal properly and leak.