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Engine Sealing: High Performance Head Gaskets
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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. As a rule, every .010" change in the thickness of the head gasket will change the combustion chamber volume about 2.5 cc.
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 .010" above the surface of the deck, and the thickness of the gasket should be about four times the protrusion of the wires in their grooves, or about .040".
According to one supplier of copper head gaskets, engines that produce over three horsepower per cubic inch 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 percent of the O-ring protrusion and the width of the grooves should be 1.5 times that of the wire.
Though most stock head gaskets do not require retorquing after they have been installed, racing creates an entirely different set of operating conditions. Consequently, many gasket manufacturers recommend retorquing their performance head gaskets after the engine has been warmed up and allowed to cool back down to room temperature. This includes copper head gaskets as well.
The recommended procedure is to start the engine and allow it to reach normal operating temperature. Then shut it off and allow it to cool until it is back down to ambient temperature. If the head gasket is copper, some recommend running the heat cycle with no coolant in the engine (to minimize the risk of a coolant leak). Each head bolt is then loosened about 1/4 to 1/2 turn, and retorqued to specifications in the same sequence they were installed.
Most gasket manufacturers recommend using the OEM torque specifications and tightening sequences, but some racers feel they get a better seal with copper head gaskets if they increase the torque load on the fasteners.
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 MLS 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 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 or less, 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 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. A steel wire ring composition head gasket, by comparison, might only cost $45, and a stock head gasket only $15.
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), so some customers might balk at the cost. Even so, 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.
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