A compression gasket is a preformed piece of material designed to fit between two objects and prevent leakage while under pressure. Commonly located between two surfaces that are bolted together, a compression gasket fills in irregularities between the mating surfaces. Typically, it also prevents gases or fluid from escaping through tiny gaps or openings where the surfaces aren't perfectly smooth and seamless. Compression gaskets are commonly manufactured from materials like rubber, neoprene, graphite, silicon, felt, metal, or plastic.
At one time, compression gaskets were often made from materials that contained asbestos. Since asbestos has been shown to be a contributor to the deadly lung disease asbestosis, materials in the United States (US) that contain asbestos are no longer used in the manufacture of gaskets of any type. Asbestos materials are still used in the manufacture of gaskets, however, in other parts of the world.
A properly functioning compression gasket typically should not be made from material that is too thick. Generally, a thicker gasket is more likely to allow leakage through the pores of the gasket itself. The best gasket thickness is just thick enough to compensate for any surface irregularities and still be compressible.
While some types of gasket materials cannot be overtightened — primarily elastomer- or rubber-based materials — other gasket materials, such as metal or Teflon®, can be overtightened, which can ruin their sealing qualities. Metal compression gaskets typically need to have the ability to recover, or expand, to work properly. If tightened too much, a metal compression gasket will lose this recoverability and won't be able to provide a seal.
A perfect example of the need for proper tightening pressure of a metal gasket is the head gasket installed on the engine in virtually all vehicles. Designed to provide a seal between the engine block and cylinder head, the head gasket is commonly made from metal. Automobile manufacturers usually publish the proper torque pressure a mechanic should apply to the bolts holding the engine together. This helps mechanics to replace head gaskets on an engine while not over-torquing the bolts, which can ruin the gasket.