The head gasket seals the engine's combustion chambers as well as the coolant and oil passages between the head and block. The head gasket has to provide a leak-free seal from the moment it is first installed, and maintain that seal for the life of the engine, which might well be 150,000 chilometri or more on many of today's engines. When a head gasket fails, there is usually a reason why. The most common cause of head gasket failure is usually overheating, often as a result of a coolant leak, a bad thermostat or cooling fan failure. Even so, the causes for head gasket failure can usually be lumped into three categories: Design issues with the engine or gasket (hard to seal engine, thermal stress created by bimetal engine or weak gasket design). Installation errors (head or block not clean, smooth or flat, wrong surface finish, using the wrong bolt sequence, procedure or torque specifications when tightening the head bolts, reusing stretched, damaged or dirty bolts, using a sealer on a gasket that does not require a sealer, etc.). Operating conditions that overstress the gasket and cause it to fail (preignition, detonation and overheating). Bimetal Stress One factor that makes many engines hard to seal today is the use of aluminum cylinder heads on cast iron blocks. Aluminum heads save weight but expand 1.7 times faster than cast iron when they get hot. The difference in expansion rates creates a lot of motion and scrubbing between the head and block. If the head gasket cannot handle this motion and is not strong enough to withstand the shearing forces that occur every time the engine is started, run and allowed to cool back down, it will eventually leak. Examples bimetal stress causing a head gasket failure include the OEM head gaskets in the Dodge Neon 2.2L, Ford 3.8L V6 and Toyota 3.0L and 3.4L V6 engines. Going further back, we all remember the problems GM had with the 2.3L Quad Four head gaskets. All of these engines have experienced a high rate of premature failure because the OEM head gasket could not keep a seal. Fortunately, the aftermarket developed replacement gaskets for all of these applications that solved the original problem. If the surface finish on a bimetal engine head and block is too rough and digs into the gasket too deeply, it can literally tear the gasket apart over time. That is why surface finish is so important on today's engines. For bimetal engines with composition gaskets, the recommended surface finish is 20 to 50 RA. This compares to 60 to 120 RA for cast iron engines with the same type of gaskets. Equally important is the design of the head gasket itself. One way gasket manufacturers deal with the scrubbing problem in bimetal engines is to add a nonstick coating to the gasket. Teflon, molybdenum and similar low friction coatings prevent gaskets from sticking to either surface. This allows the head to expand and contract without ripping the gasket apart. This is the opposite approach to what is done with many head gaskets for cast iron engines. On these applications, the rate of thermal expansion for the head and block is the same so there is much less movement and scrubbing at the gasket surface. Raised silicone, Viton or fluoroelastomer sealing beads are often applied to the face of the gasket to increase the clamping pressure in critical areas. This improves the gasket's ability to form a good cold seal when it is first installed, and helps it maintain that seal by holding the gasket firmly in place. On many late model engines, graphite head gaskets are used because graphite has natural lubricity that can handle the differences in expansion between aluminum heads and cast iron blocks. Graphite is a relatively soft material that provides good conformability for cold sealing the engine, and it can withstand high temperatures and draw heat away from hot spots to reduce thermal stress and loading. Hot spots tend to form in areas where exhaust ports are located next to each other. Heat buildup in these areas makes the head swell. This can crush the gasket between the cylinder bores causing the gasket to leak and fail. To prevent this from happening, gasket manufacturers may use a gasket material such as graphite to dissipate heat away from the hot spot and/or reinforce the most highly stressed areas of the gasket with a shim so it will not crush. Head gasket durability can also be improved by using stronger or reinforced combustion armor, tougher, high temperature fibers such as aramid and Kevlar in composition gaskets, and adding extra reinforcements to critical oil hole grommets. Multi-Layer Steel Head GasketsAn increasing number of OEMs have turned to "Multi-Layer Steel" (MLS) head gaskets to provide improved durability. MLS gaskets, which are found on Ford, Chrysler and many Japanese engines, typically have three to seven layers of steel. The outer layers are usually stainless spring steel and embossed. The inner layers provide added support and thickness. The embossed multi-layer construction reduces the load on the head bolts, which in turn reduces bore distortion for less blowby and lower emissions. The outer layers of steel are coated with a thin layer (.001˝ to .0015˝) of nitrile rubber or Viton to improve cold sealing. MLS head gaskets are very durable because their solid steel construction retains torque very well and does not take a compression set like composition gaskets. But the rigid nature of MLS gaskets also means they have very little conformability. That is why MLS gaskets require an extremely smooth, flat surface finish on both mating surfaces (typically 20 to 30 RA or less). Reproducing the kind of high quality surface finish needed to seal MLS gaskets requires up-to-date milling equipment and precision resurfacing techniques. Even then, it may be difficult to get a good cold seal on some engines. One alternative here is to install a conventional gasket (graphite or composition) that does not require such a smooth finish (if such an alternative gasket is available for the application). Several gasket manufacturers have conventional replacement gaskets for the Ford 4.6L as well as other applications. Aftermarket gasket manufacturers are also using MLS technology to introduce more durable replacement gaskets for certain problem engines that came originally equipped with conventional head gaskets. These include the 2.0L Dodge Neon engine and the Toyota 5VZFE 3.4L V6 truck engine. The aftermarket MLS replacement gaskets for both of these applications have a thicker surface coating and can handle a more traditional surface finish of 60 to 70 RA. Head Gasket Installation Issues Fastener problems are probably the number one cause of installation-related gasket failures. No gasket is going to seal well if you use the wrong procedure to tighten the head bolts. Make sure the torque specifications you are using are the latest, up-to-date numbers. Make sure all of the head bolts are in good condition, free from nicks or corrosion and are not stretched. Torque-to-yield (TTY) head bolts should not be reused. Also, bolt threads should be clean and properly lubricated. Many people never check the accuracy of their torque wrenches. Beam and gauge type wrenches are less likely to get out of calibration than the adjustable dial type, but all torque wrenches should be checked for accuracy every year or two. Improper surface finish is probably the second most common reason for gasket failures. This includes surfaces that are too rough and surfaces that are not flat. Another installer error is using a sealer on a coated composition head gasket. Some sealers will react with the gasket coating and turn it into goo. If a head gasket requires sealer, it will usually come with the gasket. Never use a Scotchbrite abrasive pad to whiz off old gasket residue from a head or block. Abrasives can leave low spots on the surface that will prevent the head gasket from sealing. Some general suggestions for head bolts include: 1) Inspect all head bolts to make sure they are in perfect condition with clean, undamaged threads. Dirty or damaged threads can give false torque readings as well as decrease a bolt's clamping force by as much as 50%! Bolt threads should be wire brushed, then inspected. Replace any that are nicked, deformed or worn. 2) Dirty or deformed hole threads in the engine block can reduce clamping force the same as dirty or damaged threads on the bolts. Run a bottoming tap down each bolt hole in the block. The tops of the holes should be chamfered so the uppermost threads won't pull above the deck surface when the bolts are tightened. Clean all holes to remove any debris. 3) To be absolutely safe, do NOT reuse TTY head bolts. TTY bolts stretch slightly when installed, so there is a risk of breakage if the bolts are reused. Some TTY applications include Ford Escort 1.6L and 1.9L engines, Chrysler 2.2L and 2.5L, and GM 1.8L, 2.0L and 2.5L fours, 3.0L V6 and 381 V8 diesel engines, and the LS1 family of V8s. 4) Check bolt lengths. Make sure bolt lengths are correct for each hole location (some holes require longer or shorter bolts than others). Bolts should also be measured or compared to one another to check for stretch. Any bolt that is visibly longer than its companions should be replaced. 5) Use hardened steel washers under head bolts in aluminum heads to prevent galling and help distribute the load. Position the washers so their rounded or chamfered side faces up. 6) If the head has been resurfaced, check bolt lengths to make sure they do not bottom out in blind holes. A bolt that bottoms out will apply little or no clamping force on the head which may allow the gasket to leak. To compensate for resurfacing, you may have to install hardened steel washers under the bolts to raise them up, or use a head gasket shim to restore proper head height. 7) Lubricate bolt threads as well as the underside of the bolt head with 30 weight engine oil (not assembly lube or grease) when the engine i…

Fonte: AA1Car.com