LO 9-9 Identify and describe the purpose, construction, and applications of cylinder head gaskets

1. Cylinder Head Gaskets

Cylinder Head Gaskets

The cylinder head gasket is arguably the most important seal in the diesel engine. The head gasket must maintain the seal around the combustion chamber at peak operating temperatures and pressures as well as seal compression air, coolants, and engine oil at their respective peak temperatures and pressures. The design and materials used must be thermally and chemically resistant to the products of combustion and to the coolants and oils used in the engine.

The most widely used materials include:

• Steel
• Stainless steel
• Fiber-based composition materials
• Graphite
• Fluoropolymers, nitriles, neoprenes, polymeric resins, polytetrafluoroethylene (PTFE or Teflon ® ), and others (i.e., chemical formulations containing silicone)

Metal and asbestos gaskets replaced traditional copper and brass during the 1950s. Composite metal fiber or graphite composite gaskets superseded these gaskets in the 1980s. Since the early 1990s, those gasket systems have largely been replaced by the development of the multilayer steel (MLS) gasket. An MLS gasket is the preferred method of sealing between the cylinder head and the engine block. Today an estimated 80% of new engines are designed with MLS gaskets as standard equipment. In the MLS system, multiple thin layers of cold-rolled, spring-grade stainless steel are coated with elastomeric (rubber) material. The resilient elastomer is necessary to the gasket structure to provide micro-sealing of metal surface imperfections while making the gasket chemically resistant to aggressive combustion gases, oils, and coolants Figure 67.

Advantages of an MLS gasket include the following: Uniform loading distribution on sealing beads Constant operation thickness High crush strength Low relaxation coefficient Reduced clamp load requirement, resulting in diminished cylinder bore distortion High strength for increased durability Do not require retorquing after engine run-in

FIGURE 09-67: This MLS gasket has elastomeric sealing material between layers of laminated steel. The elastomer seals liquids, such as coolant and oil.

Cylinder bores are commonly sealed by a ring of stainless steel positioned over a wire around the perimeter of the combustion chamber. This is known as the fire ring because it is directly exposed to combustion. This ring is critical to sealing the combustion chamber during high-pressure conditions. Graphite or rubber facings are applied to a head gasket for sealing liquids. Graphite lubricant is especially useful on aluminum-headed engines because it allows the head to slide or creep easily. This is especially important when an aluminum head is used over a cast iron block because the materials have different expansion coefficients. Surface finish of the block deck and cylinder head is a critical factor when installing gaskets. If the surface is too smooth, the gasket will be unable to properly grip the cylinder head and block deck surfaces and the gasket will tend to slip, resulting in combustion leaks. If the surface is too rough, the gasket may be unable to fill in the voids between the metal peaks and valleys. These voids can form leak paths for fluids and combustion gases.