THE STARTING GRID recommends THE FOLLOWING WELL-KNOWN Performance and aftermarket gasket BRANDS:
You don’t want to think of a failing head gasket, whether it is your race car, precious classic, tuned street or even your daily drive’s engine. Original head gaskets withstand well enough under normal loads. When compression ratios and combustion pressures rise, an original head gasket may not be able to provide a trouble free seal. Once hot combustion gasses start blowing past the gasket the combustion armor cracks or burns through, compression is lost, oil and coolant start finding new ways to circulate inside the engine, often with negative consequences.
A variety of technics has been applied to keep engines sealed such as; wire O-rings around combustion chambers, solid copper head gaskets. Also aftermarket gasket manufacturers developed many solutions for improved sealing such as; high temperature graphite and asbest free composition fiber gaskets with special coatings and strengthened combustion armour. Reinforced head gaskets with stainless steel, gas filled or copper rings inside the combustion chamber.
One of the best performing products that entered the market the last 25 years are Multi-Layer Steel (MLS) head gaskets. MLS gaskets first appeared on many Japanese engines in the early nineties, and were later adopted by all other manufacturers world wide.
MLS head gaskets are available for most popular engines, as a replacement or a performance up-grade. Consequently, MLS head gaskets have become the gasket of choice for many forms of racing because they have what it takes to handle the pressure without leaking, blowing out or burning through.
There are a lot of different style gaskets to choose from. Some use a traditional composition fiber material construction with a soft graphite or non-asbestos facing material on both sides of a solid steel or perforated steel core. The soft outer layer provides conformability for a good cold seal while the steel core provides rigidity and strength. Most have some type of outer coating such as silicone to improve cold sealing, or an anti-stick agent to improve lubricity and make removal easier. Many head gaskets also have raised elastomer sealing beads printed on the surface to concentrate loading in key areas.
Other performance composition head gaskets use a somewhat different approach and sandwich a layer of graphite between two layers of steel. The outer surface is coated with silicone or a similar material to improve cold sealing.
The facing materials used in performance head gaskets may be the same as those in a stock head gasket, or they may use a higher temperature version of a similar non-asbestos material, or graphite. Graphite can handle heat well and has natural lubricity, but it is a soft material and must be handled with care.
Another characteristic that also distinguishes performance head gaskets from ordinary head gaskets is stronger combustion chamber armor. Most use a high grade stainless steel armor which may be stronger and/or thicker than the armor on a stock head gasket. For higher horsepower applications, a stainless steel or copper wire ring may be placed around the combustion chambers inside the combustion armor. This provides added support for the armor, concentrates loading around the cylinders, and helps the gasket maintain a tight seal under high combustion pressures.
The point at which an upgrade to some type of high performance head gasket usually becomes necessary is when an engine’s power output approaches 125 HP/L.
For many years copper head gaskets considered to be the best for high horsepower engines. They do offer certain advantages over conventional head gaskets:
Copper head gaskets are great for compression ratios over 14:1, turbo- or super-charged engines that are running boost pressure over 1 Bar, or engines with nitrous oxide kits that add an extra 80 or more horsepower.
Copper conducts heat better than most other materials this helps spreading the heat and prevent hot spots that may cause detonation/preignition and warped heads and blocks. Engine tuning is easier and it reduces the risk of the gasket burning through.
Copper has a 25 percent elasticity-coefficient, which allows it to stretch before it fails. So if an engine goes into detonation because the fuel mixture leans out, there’s too much spark advance, or there’s too much compression and not enough fuel octane, copper provides an extra margin of safety. Copper is also strong and copper gaskets are reusable at least for a limited number of times. This is a plus in situations where the heads are on and off the block between races, or frequent tear downs are required.
One of the drawbacks of plain copper head gaskets, though, is that they do not seal oil and coolant very well. If the engine does not contain any coolant, coolant leaks are not an issue. But for engines that do run coolant, getting a copper head gasket to cold seal and maintain a seal can be a challenge. The gasket must be coated with some type of sealer, and both mating surfaces must be absolutely smooth, flat and clean.
Though many people think copper is a relatively soft metal, it does not provide much conformability. On one hand, this is a good thing because the gasket doesn’t crush when the head bolts are torqued down. Consequently, the thickness of the gasket remains the same and does not change. On the other hand, the gasket may not conform very well to small indentations and surface irregularities in the head or block. So some type of sealer coating must be applied to both sides of the gasket – unless it comes pre-coated with sealer or has raised elastomer sealing beads printed on its surface. Several companies make coated copper gaskets that reportedly seal as well as any other gasket, and maintain their seal. The gaskets are also reusable provided the sealer isn’t damaged when the gasket is removed.
Copper can be annealed (softened) by heating it in an oven or with a propane torch, and some racers do this to improve conformability. But heating also causes a certain amount of oxidation which may weaken the gasket and increase the risk of it cracking. Some gasket manufacturers do not recommend annealing copper head gaskets. Others say it is okay provided the gasket is not reheated more than three times.
The best way to anneal a copper head gasket is in a controlled environment such as an air-tight vacuum oven. The gasket should only be heated until it is a dark red, about 480° C, and no more. After the gasket has air cooled, the surface needs to be cleaned with a brush or abrasive pad to remove oxide from the surface. The gasket should then be cleaned with brake cleaner or a similar product and allowed to dry before it is recoated with sealer. Also, the sealer must be allowed to dry before the gasket is installed. This may range from 20 minutes to overnight depending on the type of sealer product used. Some aerosol sealers may require multiple coats for best results. Silicone also works, and may be applied around coolant openings in the gasket, block or head. But only a thin coating should be used, and it must be allowed to set before the gasket is installed. If a copper head gasket is accidentally bent during removal, it can be straightened and an-nealed. But if the gasket has kinked, it should be replaced because a kink concentrates stress and work hardens the metal. This increases the risk of cracking and failure. For the same reason, copper head gaskets should not be cleaned by bead blasting because it work hardens the metal. The same goes for hammering the metal.
Copper gaskets are popular with top fuel drag racers because the gaskets are available in a wide range of thicknesses, which can be changed to alter the compression ratio and "tune" the engine to changing track conditions. Different thicknesses can also be used to accommodate different piston and deck clearances.
The hot setup today is to minimize quench clearances between the pistons and cylinder heads, and to maximize compression ratios with small combustion chambers. Domed pistons may interfere with the propagation of the flame front, so engine builders may lower the deck surface and/or raise the height of the pistons so they can run flat top pistons. The thickness of the head gasket then becomes critical in controlling clearances and the compression ratio.
On most high horsepower engines, copper head gaskets are used with annealed (softened) stainless steel or copper wire o-rings installed on the gasket, or in grooves machined into the block or head. The wire rings help concentrate loading around the cylinders to prevent combustion pressure from blowing past the gasket. The wire rings are typically .041" in diameter, and are placed in a .039" wide x .030" deep groove. The wires should protrude only about about 0,25 mm. above the deck, and the thickness of the gasket should be about four times the protrusion of the wires in their grooves, or about 1.00 mm. According to one supplier of copper head gaskets, engines that produce over 180 HP/L should also have a corresponding receiver groove machined into the head opposite the O-rings in the block for optimum sealing. The depth of the receiver grooves should be 75% of the O-ring protrusion and the width of the grooves should be 1.5 times that of the wire.
The latest alternative to reinforced composite fiber head gaskets and copper head gaskets for performance engines are MLS gaskets. Unlike these other types of gaskets, MLS gaskets use a different strategy to seal the combustion chamber. They typically use 3 to 5 layers of stainless steel to create a spring-like effect that seals the gap between the head and block.
As compression ratio, rpm and combustion pressure go up, the cylinder head is lifting from the block every combustion cycle. The movement can't be seen, but can be measured and can be enough to break the seal between the head and block with conventional gasket designs. The amount of lift also depends on how much the head bolts stretch.
To maintain the seal when the head is lifted, the head gasket has to expand accordingly. This requires a certain elasticity that can be achieved with an MLS head gasket.
The multi-layer construction allows the inner layer(s) to act something like a valve spring. As the head lifts away from the block, the inner layer(s) of the gasket push the outer layers apart to maintain the seal. The spring steel expands and contracts without taking a permanent set or deforming under load, and the gasket maintains its seal. That’s why MLS gaskets have more "vertical recovery" than other types of gaskets and can handle high pressure applications.
In a performance engine, the maximum combustion pressures can reach 1.5 to 2.5 times higher rates than a standard engine reach, not to speak about the values reached if the engine goes into detonation. The higher the pressure, the greater the cylinder head separation from the block and the more the gasket has to expand and contract to maintain its seal.
Aftermarket MLS performance gaskets are engineered for racing and are not just copies of the OEM style MLS gaskets. They have strategically placed sealing beads (embossments) around the combustion chambers and coolant passages to concentrate clamping loads in the most critical areas. Some MLS gaskets have an additional stainless steel "stopper ring" to further increase sealing pressure around the combustion chambers.
One gasket supplier also has a line of MLS performance gaskets that incorporate a unique "gas-filled ring" around the combustion chambers. The pressure inside the ring is 600 to 700 psi, and increases as the engine heats up to increase the clamping load and combustion seal. Features like these have enabled MLS gaskets to become the gasket of choice for many forms of racing as well as street performance applications.
The all-steel construction of MLS gaskets makes them almost bullet-proof under even the most extreme operating conditions. The gaskets also have an exterior "Viton" or polymer coating that helps them cold seal on less than ideal surfaces. Most original equipment MLS require extremely smooth finishes (20 to 30 Ra) to seal. Most performance MLS gaskets require a surface of 50 Ra, and some have thicker coatings that can accommodate surface finishes as rough as 60 Ra.
As for reusability, MLS gasket suppliers say MLS gaskets should not be reused because the embossing may not fully recover once the gasket has been through a thermal cycle. But as long as the gasket appears to be in good condition when it is removed, many racers find they can reuse MLS gaskets with no problems. And if the surface coating has a damaged spot or two, it can often be repaired with a light coating sprayed on.
Many racers who used to run copper heads gaskets have switched to MLS because the gaskets hold up just as well and don’t have the sealing or installation issues associated with copper gaskets. The only drawback with MLS gaskets are their price.
The multi-layer construction of MLS head gaskets requires expensive precision tooling, and 3 to 5 layers of stainless steel. Consequently, the price for a performance MLS head gasket is 5 times higher than a composition head gasket.
So some customers think they pay to much. Considering the durability of these gaskets and it’s capabilities, paying more for a gasket that won’t blow out or fail is more than worth the money. And unlike composition gaskets that cannot be reused, MLS gaskets don’t have to be replaced every time the head comes off the engine, for a short inspection or ring change.