WHY MLS?
One thing no engine builder wants to worry about is a head gasket failure, whether it is a NASCAR cup engine, a Pro Stock drag racing engine, a hot street engine, or even a stock engine. Stock head gaskets hold up well enough under normal loads, but as compression ratios and pressures go up, stock head gaskets begin to fail. And once hot combustion gasses start blowing past the gasket's combustion armor, bad things begin to happen. The combustion armor cracks or burns through, compression is lost, and the oil and coolant start finding new ways to circulate inside the engine, often with negative consequences.
Over the years, performance engine builders have used a variety of tricks to keep their engines sealed such as using wire O-rings around combustion chambers and running solid copper head gaskets. Various kinds of sealing solutions have also been developed by aftermarket gasket manufacturers for sealing high performance engines such as high temperature graphite and non-asbestos composition gaskets with special coatings and beefed up combustion armor, head gaskets with stainless steel or copper wire rings inside the combustion chamber armor for added reinforcement, and head gaskets with oversized wire rings and combustion chamber armor that function similar to O-rings.
One of the hottest products to emerge in recent years for high performance engines are Multi-Layer Steel (MLS) head gaskets. MLS gaskets first appeared on many Japanese engines in the late 1980s and early 1990s, and were later adopted by Ford, Chrysler, and General Motors.
The latest alternative to reinforced composite 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 engine compression, rpm and combustion pressure go up, the cylinder head is pushed away from the block every time the cylinders fire. The movement isn't enough to be seen with the naked eye, but it can be measured - and it can be enough to break the seal between the head and block with conventional gasket designs. The amount of lift depends on cylinder pressures and how much the head bolts stretch.
To maintain the seal when the head is pushed up and away from the block, the head gasket has to expand as the head lifts. This requires a certain amount of springiness or elasticity that can only be achieved with an MLS head gasket. The multi-layer construction of MLS head gaskets 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 stock engine, the maximum combustion pressures may only reach about 1,000 psi. But in a performance engine, they can reach 1,500 to 2,200 psi under race conditions, and soar as high as 3,500 psi 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 gaskets. They have strategically placed sealing beads 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 (such as in Chevy LS1/LS6 engines). One gasket supplier (Cometic) 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, while most performance MLS gaskets require a surface of 50 Ra or less. Some have thicker coatings that can accommodate surface finishes as rough as 60 Ra. (NOTE: The gaskets I'll be ordering require 50 Ra)
As for reusability, 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. If the surface coating has a damaged spot or two, it can often be repaired with a light coating of RTV silicone.
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 jobber price for a performance MLS head gasket for a small block Chevy V8 is around $70 to $75 (per side). What's more, on some applications the cost is even higher because of limited volume or availability. The jobber price for a MLS performance head gasket for a sport compact engine such as Mitsubishi 2.0L, for example, might be as high as $170. Of course, you only have to buy one for a four cylinder (instead of two for a V8). An inline six is higher yet, as they are longer and require more material. Some customers might balk at the cost, but considering the durability these gaskets are capable of providing, paying top dollar 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.
