Replacing a head gasket is a significant investment for the customer. It also represents significant risk for a shop if the replacement gasket fails. To do the job right, you have to ask why the gasket failed in the first place and what it will take to prevent it from happening again.
Can an overheating motor cause a head gasket failure?
One reason head gaskets fail is because of engine overheating. If the engine gets too hot, the cylinder head can swell to the point where it crushes the head gasket (usually between the cylinders because this is the thinnest point). The extruded material and/or cracked combustion armor then provides a leak path for coolant and/or combustion gases.
Can the wrong coolant cause head gasket problems?
If the owner has been fighting a leak by topping off the coolant with the water or the wrong coolant, it can degrade the head gasket and other gaskets that come in contact with the coolant. Coolant with the wrong base chemistry can damage the engine. If a cooling system requires Organic Acid Technology (OAT) or Hybrid Organic Acid Technology (HOAT) and conventional green coolant is used, it can also degrade the head gasket. Beyond that, some import nameplates specify levels of silicates and phosphates. If the levels of these additives are not suited to the engine, even more damage can occur.
Topping off the system with water might temporarily solve the overheating problem. But, as the customer dilutes the coolant, the buffers and additives that prevent corrosion are further depleted. This can quickly cause a head gasket to fail.
Can an engine misfire cause a head gasket to fail?
Another reason a head gasket can fail is because of an ill-timed combustion attempt. Detonation, pre-ignition or knock all cause a sharp spike in combustion chamber pressure, which, over time, can overload and crack the gasket that surrounds the cylinder. This leads to burn-through and loss of compression.
Detonation can be caused by a variety of problems. One is an accumulation of carbon in the combustion chamber that increases compression. Many late-model engines run fairly high compression ratios, while some require premium octane fuel. If compression reaches a point where the fuel ignites spontaneously before the spark can set it off, the engine will knock and ping under load.
Why are some vehicles more prone to head gasket failures?
Sealing problems are inherent in bi-metallic engines because aluminum cylinder heads expand faster than cast-iron cylinder blocks during the warm-up cycle. The difference in expansion rates is further aggravated because the cylinder head tends to heat up much faster than the cylinder block.
The difference in expansion rates between aluminum and cast iron creates a “scrubbing” effect that eventually wears out the stainless steel “fire ring” that keeps combustion gases from entering the cooling system. Because the cumulative effect of these repeated thermal events results in failed cylinder head gaskets, most aftermarket gasket manufacturers have designed head gaskets using space-age materials that resist scuffing wear in bi-metallic applications.
When you are replacing the head gasket, the kit usually comes with gaskets to reseal the valve cover(s), intake manifold and exhaust manifold. You could be dealing with more than five different gasket technologies in order to complete the job. So how do you match the gasket with the correct chemicals and surface finish? See the below guide for some helpful tips.
Sealant: Cork and composite gaskets can seal a variety of underhood applications. The type of sealant used with these gaskets depends on a variety of factors. First, you must consider if the gasket needs to remain flexible. Second, you need to consider the temperatures the sealant will have to endure. Third, what types of chemicals and fluids will come in contact with the sealant? Also pay attention to the washers and spreaders that distribute the clamping load of the fasteners.
Sealant: In most cases, no sealant is required. Most installation instructions advise that O-rings are to be coated with coolant or oil to lubricate the seal and allow the two components to expand and contract independently.
Surface Finish: Smooth and free from debris.
Sealant: None. Only small amounts of high-tack liquid sealant should be used in strategic areas to hold the gasket in place. Sealants can add thickness, which creates uneven clamping force since the gasket will not be able to seat properly. Sealant debris can extrude out when the gasket is compressed, and it could end up in an oil or coolant passage and restrict or block flow.
Surface Finish: The sealing surface should be clean and dry.
Metal/Plastic Carrier Gaskets
Sealant: Dry. The sealer can chemically attack the sealing beads and carrier material, causing them to degrade. Sealants can act as a lubricant and cause the rubber gasket to slip out of place and split. Also, RTV can extrude out when the gasket is compressed, and it could end up in an oil or coolant passage and restrict or block flow. If a carrier-style gasket requires a sealant, some manufacturers will include a portion in the kit. Most replacement gasket manufacturers will include instructions for installation that will outline whether sealants need to be applied.
Surface Finish: The sealing surfaces should be dry and clean. If the surface surrounding a port is rough and pitted, the new gasket may not seal any better than the old one. Resurfacing or replacing a component may be required to restore a smooth, flat surface.
Sealant: Some components like water outlets or front covers may need a bead of sealant to seal the component. You need to consider the temperatures the sealant will have to endure, and what types of chemicals and fluids with which the sealant will come in contact.
Lastly, consider if there is a possibility that the engine could ingest some of the sealant and possibly contaminate the oxygen sensor. If so, use a sealant that will not harm the sensor.
Can a leak occur only during warm-up?
Yes. Temperature-related failure patterns are the most common. As always, the primary symptom of a leaking head gasket is increased consumption of coolant with no apparently visible external coolant leakage.
If cylinder leakage occurs during cold-engine operation, combustion gases accumulate under the engine’s thermostat during cold engine operation, forcing the coolant back through the radiator and resulting in an overflow condition at the coolant reservoir. If, on the other hand, leakage occurs only under full throttle, hot engine operation, the coolant may become aerated with combustion gases, which reduces cooling system efficiency and increases operating temperatures.
What determines the gasket surface of the head and block?
The level of surface finish is determined not only by the engine materials, but also the gasket type. In some cases, the block and cylinder head will not need to be resurfaced. If there is significant scuffing or damage, the cylinder head surface should be resurfaced to the cylinder head gasket manufacturer’s specifications.
After removing the old head gasket, carefully remove any residue from the head and block using a gasket removal chemical and scraper. Do not use an abrasive pad in a drill to whiz off or clean the head or block surfaces because doing so may also remove metal and create shallow depressions that can prevent a new head gasket from sealing.
What is Roughness Average (RA)?
Roughness average is only one of several perimeters that can be measured. A more accurate perimeter is “Rz” which is the average difference between the peak height (Rpk) and valley depth (Rvk). RA can have a wide variance across a given surface profile, so Rz gives a more accurate indication of the actual texture across the surface.
If you are using a profilometer to check surface finishes, use the Rz readings rather than the RA to see what’s really going on. A 20 RA surface finish for an OE MLS head gasket would be about 120 Rz. For an aftermarket performance MLS gasket, look for Rz readings in the 180 to 300 Rz range.
For composition gaskets, don’t go smoother than 240 Rz (40 RA) or rougher than 600 Rz (100 RA). A surface that is too rough can leave gaps between the gasket and metal while a surface that is too smooth for a composition gasket can increase the tendency for the gasket to flow and blow out.
What should be done to ensure a head gasket repair lasts?
Make sure any factors that may have caused the original head gasket to fail have been identified and corrected so the new head gasket won’t suffer the same fate. Make sure the cooling system is functioning normally and holds pressure. Also be sure that the engine doesn’t knock or ping under load. And ensure that the vehicle owner is using the correct grade of gasoline (premium if required). Also, perform a complete flush of the cooling system. This includes the heater core, radiator and hoses.