Cylinder Head Servicing To prevent warpage of cylinder heads, damaged bolts, threads and gasket leakage, even, consistent and adequate clamping of cylinder heads is necessary. An earlier technique for torquing heads required the use of a torque wrench and a pattern for torquing the head bolts down in correct sequence. This technique illustrated in the diagrams prevented pinching of the head gasket as well by ensuring an even spreading out of the gasket along the block deck surface. Bolt tightening sequence should take the cylinder head down in a minimum of three progressive steps of torque. Following this same pattern of After removing a cylinder head it should be checked for warpage. No more than usually 0.006 of warpage are allowed on a cylinder head. Manufacturers techniques and specification will vary generally. Diesel engines are allowed very little machining of the heads since machining will cause valve to piston interference. installation when removing a cylinder head will prevent possible warpage of the head. This practice should be followed especially in the case of aluminium cylinder heads. Other checks to a cylinder head should include inspection for cracking. Several techniques are available to the technician. Dye Penetrant Magnetic Flux Pressure Testing Dye penetrant involves cleaning and spraying a component with a dye that will penetrate cracks. When a developer is later applied the cracks will be conspicuous. Magnetic Flux involves applying fine iron particles to a ferrous metal part that is suspected of having a crack. When a powerful magnet is moved around the cylinder head or the head is placed in a magnetic field the metal particles will line-up on either side of the crack since poles of a magnet are created on opposing sides of the crack.
Pressure testing involves disassembly of a component like a cylinder head and sealing the intake/exhaust ports of the head. Air pressure can be applied to the head while the component is immersed in hot water. Cracks or porosity will show up as an air leak from the affected area.
Cylinder Head Gaskets The cylinder head gasket is arguably the most important seal in the automotive engine. The head gasket must maintain the seal around the combustion chamber at peak operating temperatures and pressures as well as seal 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 the coolants and oils used in the engine. and engine block, as yet unmatched by any other sealing system. Today an estimated 80 percent of new engines are designed with MLS gaskets as standard equipment, and further growth is projected. In the MLS system, multiple thin layers of coldrolled spring grade stainless steel are coated with 7-25 microns of elastomeric material. The resilient elastomer is essential to the structure by providing micro-sealing of metal surface imperfections while resisting aggressive combustion gases, oils and coolants at temperatures up to 250 C. the advantages then of a MLS steel gasket are the following: Uniform loading distribution on sealing beads. Constant operation thickness (reduced head deflection and internal stress). High crush strength. Low relaxation. Reduced clamp load potential - resultant reduced bore distortion. High radial strength - increased durability The most widely used materials are as follows: Steel, of many types and forms. Stainless steel, of a wide variety of grades. Fibre based composition materials of many formulation. Graphite in a wide range of densities and constructions. Numerous chemical formulations containing silicones, fluoropolymers, nitriles, neoprenes, polymeric resins, polytetrafluorothylene (PTFE) and others. Traditional copper and brass gave way to metal and asbestos in the 1950s, superseded in turn by composite metal and impregnated fibre or graphite composites by the 1980s. However, those systems were largely overtaken by the development of the Multi-Layer Steel (MLS) gasket during the early 1990s. MLS gaskets remain the automotive industry's preferred method of sealing between cylinder head When using an MLS gaskets retorquing the head after engine run-in is not usually necessary since the gasket does not relax. 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 since it is directly exposed to
combustion. This ring is critical to sealing the combustion chamber in high-pressure situations. Above: Back-up ring to fire ring Graphite or rubber facings are applied to a head gasket for sealing. Graphite is especially useful on aluminum headed engines since it allows the head to creep easily. Above: Dowel Pins Hold Head Gasket In Place Surface Finish is a Critical Factor in Sealing when installing gaskets, particularly cylinder head gasket, having the proper surface finish can be a critical factor in achieving a secure seal. If the surface is too smooth, the gasket will be unable to properly grip the surface. It will tend to slip, and combustion leaks may result. 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 gasses.