Engine Gaskets 101: The Sealing Science, Larry Carley, Counterman, October 2001

Ever wondered why engines need so many different gaskets? It's because engines have a lot of different sealing needs - thus translating into different sales situations that every counterperson should know.

SURFACE MATING: CUT GASKETS
When two surfaces are mated together, it's very difficult to get a leak-free seal unless something is used to fill in the gap. So various types of gaskets are used to essentially fill the gap between the parts.

The simplest and most basic type of gaskets are "cut" gaskets (also called "flat" gaskets). These are made by die-cutting sheets of fiber-reinforced paper-like material or ground cork/rubber to create the desired shape. Cut gaskets are typically used to seal timing chain covers, valve covers, oil pans, intake manifolds, carburetors, throttle bodies, water pumps, thermostat housings, transmission pans and differential covers.

Cut gaskets are usually the least expensive type because they are relatively easy to manufacture. They also allow a vehicle manufacturer to use less-expensive stamped steel valve covers, timing covers and oil pans on engines instead of more costly diecast aluminum covers.

But cork/rubber cut gaskets don't last forever. Over time, exposure to heat causes the gasket material to lose its elasticity and become hard and brittle. This decreases the gasket's ability to maintain a leak-free seal and increases the risk of gasket breakage and failure.

MOLDED RUBBER
On most newer engines, many cut gaskets have been replaced by more durable (and more expensive), "molded rubber" gaskets. These are made by injection molding synthetic rubber (nitrile, neoprene, silicone, etc.) to create a one-piece gasket. Molded gaskets are most often used to seal diecast valve covers and oil pan covers.

Molded gaskets have several advantages over cut gaskets. One is that molded rubber gaskets retain their flexibility much better than cork/rubber cut gaskets. That makes them more durable and less likely to leak as the miles add up. This type of gasket can often be reused as long as it is in good condition.

But oil makes molded gaskets swell. This may prevent the gasket from fitting or sealing properly when an attempt is made to reinstall it. That's why a replacement is often recommended. (Warning: sealers or adhesives should never be used with molded rubber gaskets).

Another advantage of molded gaskets is easier installation when they're new. A one-piece molded rubber gasket for an application like the oil pan on a smallblock Chevy V8 eliminates the seams and joints that are required with a traditional multi-piece cut gasket set. This eliminates potential leak paths and makes installation easier, too.

Some molded gaskets also incorporate a steel "carrier" to provide added stiffness and reinforcement. This type of gasket is much easier to handle and install than a floppy rubber gasket that lacks any reinforcement. The gasket may also have built-in grommets to limit the amount of crush when the cover is tightened down.

Steel carriers and grommets are also used in some cork/rubber cut gaskets for the same purpose. What's more, several aftermarket gasket suppliers make less expensive cork/rubber cut gaskets that can be used to replace some molded rubber gaskets or "gasketless" applications that were originally sealed with RTV silicone sealer. Likewise, they also make special molded rubber gaskets to replace cork/rubber cut gaskets.

On some late-model engines, molded "composite" gaskets are used to seal the intake manifold to the cylinder head. With this type of gasket, a metal or plastic carrier is used to support O-rings or molded sealing beads around the port openings. Though these types of gaskets are very durable, they are also very expensive to manufacture and replace. In a few instances, a less expensive cut-style replacement gasket may be available - but usually not because of the design requirements of the engine. This same type of high-tech composite gasket construction is also used for some performance valve covers and thermostat housing gaskets.

HEAD GASKETS
One of the most demanding sealing applications on any engine is the head gasket. The head gasket's job is to seal the gap between the cylinder head and block. The head gasket has to withstand the effects of oil, coolant and gasoline as well as extremes of temperature and pressure.

Combustion temperatures can soar to over 4,000 degrees F under heavy load and average around 2,000 degrees F during normal driving. At the same time, combustion pressures change during the intake stroke to 400 to 600 pounds per square inch during the power stroke over 50 times a second at highway speeds!

To make matters worse, many head gaskets find themselves sandwiched between dissimilar metals that have different rates of thermal expansion. Most late-model engines have aluminum cylinder heads and cast iron blocks. Aluminum expands and contracts at over twice the rate of cast iron. This creates a lot of back-and-forth scrubbing on both surfaces of the head gasket that can literally tear a gasket apart over time if it is not designed to handle this kind of movement.

One way gasket manufacturers combat the scrubbing problem in bimetal engines is to apply nonstick coatings to their head gaskets. Most head gaskets today are a composite design with a soft nonasbestos facing material or graphite on a solid or perforated steel core. Teflon, molybdenum and similar coatings are used to prevent the gasket 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 improve cold sealing. The added thickness of the bead increases the clamping pressure in critical areas of the gasket, but also increases the grip against the head and block. This 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.

On many late-model engines, the OEM head gaskets are graphite because graphite has natural lubricity to handle the differences in expansion between aluminum heads and cast iron blocks. Graphite is also a relatively soft material that has excellent cold sealing properties, and it can withstand high temperatures. It is an "anisotropic" material that can draw heat away from hot spots to reduce thermal stress and loading. But graphite is also expensive and more difficult to manufacture.

"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. For some engines that would be a nonasbestos material rather than graphite."

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 made of less expensive nonasbestos composite materials.

MULTI-LAYER STEEL (MLS)
The ultimate head gasket design today is multi-layer steel (MLS), which is used on Ford 4.6L V8s, other Ford engines, as well as Chrysler, Honda, Mazda and others makes. This design typically has three to seven layers of steel. The outer layers are usually stainless spring steel, embossed and coated with a thin layer of nitrile rubber or Viton to provide extra cold sealing. The inner layers provide added support and thickness.

MLS gaskets are extremely durable compared to other types of head gaskets because of their all-steel construction. They won't burn through, and they won't relax and take a compression set that can lead to leaks and sealing problems. MLS gaskets also reduce blowby and improve compression by reducing bore distortion in the cylinders (less load is needed to seal this type of gasket.) But MLS gaskets require a special type of head bolt that stretches when it is tightened ("torque-to-yield" or TTY head bolts, which should not be reused). They also require extremely smooth surface finishes on both the cylinder head and engine block to seal properly.

Aftermarket MLS gaskets have also been introduced as replacements for some problem engine applications that need a more durable head gasket. These include the Toyota 5VZFE 3.4L V6 truck engine and the 2.0L Dodge Neon. On both applications, the MLS replacement gasket works better than the OEM gasket - and requires no special surface finish because an extra thick coating provides the proper cold seal.

PREVENTING REPEAT FAILURES
One of the leading reasons why head gaskets fail is because of overheating.

If the engine gets too hot, thermal expansion in the head can crush portions of the gasket causing it to leak. Too much heat can also permanently warp the sealing surface on the head and block, making it difficult for a replacement gasket to seal properly. So one thing that should always be checked before a new head gasket is installed is to check the flatness of both surfaces. This can be easily done with a straight edge and feeler gauge.

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.

Surface finish is also very important too. For many years, most aftermarket gasket manufacturers said a surface finish of 55 to 110 microinches RA (roughness average), 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 for nonasbestos and graphite gaskets is 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. For MLS gaskets, a surface finish of 30 RA or smoother is usually required.