Head Gasket Failure
Coming Apart at the Seams
The head gasket is one of the most critical gaskets in an engine because it has to seal all of the combustion chambers as well as the coolant and oil passages between the head and block. The 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 miles or more on many of todayís vehicles.
When a head gasket fails to go the distance, thereís usually a reason why. The reasons can be lumped into three categories:
A few recent examples of this kind of problem 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 couldnít 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. Thatís why surface finish is so important on todayís 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 gasketís 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 wonít 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.
MLS head gaskets are very durable because their solid steel construction retains torque very well and doesnít take a compression set like composition gaskets. But the rigid nature of MLS gaskets also means they have very little conformability. Thatís 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 of these engines. One alternative here is to install a conventional gasket (graphite or composition) that doesnít 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.
"No gasket is going to seal well if somebody uses the wrong procedure to tighten the head bolts. There are a lot of people who keep on using the same old torque specs year after year because thatís they way theyíve always done it. But many tightening procedures and torque specifications have been revised, some several times. So donít use an old manual thatís been sitting on the shelf for the past 10 years. Look up the latest procedures and specifications to make sure youíre doing it right. Good sources for this information are a current gasket installation book published by a gasket manufacturer, or reference specifications available through AERA or ALLDATA."
McKnight says to make sure 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. Sending a wrench in to a tool supplier or lab to check its calibration may cost $25 to $35, but itís a wise investment considering what it will cost you if a head gasket fails because the head bolts werenít torqued accurately."
Improper surface finish is probably the second-most common reason for gasket failures, according to McKnight. 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. Our philosophy with Victor Reinz gaskets is if a gasket requires a sealer, we will include it with the gasket. If no sealer comes with the gasket, it doesnít need any, and no sealer should be used.
"And never, 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," said McKnight.
Marty Novil of Corteco warns technicians not to tumble head bolts during the cleaning process because this can nick the threads, causing an increase in friction when the head bolts are tightened that will reduce the clamp load on the gasket. "Youíll think the bolts have been properly torqued but they wonít provide enough clamp load on the gasket, which can lead to leaks and premature failure."
Some general suggestions for head bolts include:
On some applications with retorque-style head gaskets, it may be necessary to retorque the head a third time after a specified time or mileage interval due to the design of the engine. Follow the vehicle manufacturerís recommendations.
If a cylinder head is being resurfaced, the amount of metal thatís removed should be kept to a minimum and should not exceed the limit recommended by the vehicle manufacturer. Milling a head increases compression and the risk of detonation. It also affects cam timing on OHC engines. One way to restore compression back to normal if a head has been milled excessively is to use a head gasket shim when installing a new head gasket.
The risk of detonation can also be minimized by making sure the ignition timing is right, the fuel mixture is correct and the cooling system has been properly filled and bled.
The second most common cause of head gasket failure, said Palmquist, is overheating. "The cooling system on many late model vehicles is just adequate, so over time it may not take much to cause the engine to run hot and stress the head gasket.
"If a head gasket has failed because of overheating, check the radiator, thermostat, water pump, cooling fan, EGR system and so on to determine why the engine got too hot. In some cases, the original radiator may have been recently replaced. Some aftermarket radiators donít cool as efficiently as the original and may be overlooked as a possible cause of overheating," said Palmquist.
Hunter Betts of Enginetech also said that preignition/detonation and overheating are probably the most common causes of head gasket failure, but added poor surface finish as a contributing factor in many cases.
Aluminum cylinder heads that have exhaust valves next to each other such as the Chrysler 2.2L and Honda 1.3L and 1.5L, are vulnerable to localized overheating in the area between adjacent exhaust valves. This is typical of head designs that restrict or limit coolant flow and circulation in critical areas. Some engine blocks with siamesed cylinders also provide minimal cooling between the cylinder bores. Even engines like small block Chevy V8s that have adjacent exhaust valves in the two center cylinders can experience hot spots if the engine overheats or experiences preignition or detonation.
Some engines are prone to hot spots no matter what you do, so thereís no way to eliminate the hot spot. For these engines, an aftermarket gasket thatís been specially designed to solve the hot spot problem may be needed to prevent repeat failures.
For example, one aftermarket gasket manufacturer redesigned their Honda 1.3L and 1.5L gasket to eliminate hot spot failures by adding a special Y-shaped shim between the two center cylinders. The shim is pressed into the underlying gasket material to improve the gasketís resistance to crushing.
Diagnosing Gasket Failures
One of the causes of overheating, for example, is a dirty cooling system. As little as 1/8" of calcium deposits in a radiator may reduce cooling efficiency by 40%. Itís also important to use the correct type of coolant and the proper mixture (50/50 is usually recommended for year-round driving).
Owens said ROLís High Temperature (HT series) gaskets are a good solution for applications where a head gasket has failed due to detonation. The HT gaskets provide additional sealing strength through the use of stainless steel fire rings, "Pozi-Seal" beading and a graphite facing. The gaskets also will lower compression ratio slightly because they are slightly thicker than a stock gasket.
If you would like to obtain a copy of the wall chart, call 800-810-4067.
Figuring out why a head gasket failed is the first step to preventing repeat failures. Sometimes the cause is obvious and sometimes it isnít.
A head gasket that failed because of overheating or a hot spot will be crushed and measurably thinner in the damaged area when checked with a micrometer. By comparison, a gasket that has failed due to detonation or pre-ignition will usually have cracked armor around the combustion chamber, which leads to burn-through.
The corresponding surface areas on both the head and engine deck where the gasket failed should be inspected for damage (erosion, pitting or cracks) as well as flatness. If either surface is damaged or is not flat, the head and/or engine block must be resurfaced, otherwise the new head gasket may not seal properly.
While the head is off the engine, check the flatness of both the head and block. Use a straight edge and feeler gauges to check all critical areas - especially those between the cylinders. Flatness specifications vary depending on the application, but on most pushrod engines with cast iron heads, up to .003". (0.076mm) out-of-flat lengthwise in V6 heads, .004" (0.102mm) in four cylinder or V8 heads, and .006" (0.152mm) in straight six cylinder heads is considered acceptable. Aluminum heads, on the other hand, should have no more than .002" (.05mm) out-of-flat in any direction.
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Larry Carley, Underhood Service, August 2002
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