THG Logo no background o3xyf187z2q6dxghrb5capx50y151mi75b4ao8syxe - What causes a blown gasket?

What causes a blown gasket?

The requirements for gasket materials and their production technologies are very high and increase along with an increase in engine power.

The essence of all gasket defects encountered in practice is in the very scheme of sealing the junction of the block with the cylinder head. Indeed, the gasket must simultaneously provide gas seal (cylinder cavity), water seal (channels of the cooling system) and oil (channels of the oil system). The close arrangement of these channels and cavities to each other, obviously, can lead to internal leakage. But external leakage is also not a gift.

Head Gasket header e1550671933303 1024x622 - What causes a blown gasket?

The cylinder head gasket can lose its tightness for many reasons. But the main ones still remain violations of the rules of operation, technology of maintenance and repair of the car. It is equally important to note that the external manifestations of cylinder head gasket malfunctions are also very diverse. You can even say that one individual symptom rarely appears independently of the other. But, taken together, they allow you to diagnose and accurately determine the cause of the malfunction.

Of course, for the correct diagnosis, the experience and knowledge of the engine, including the design and working processes occurring inside it, are necessary. And then, according to detected traces of oil or coolant, discoloration of fluids or parts, deposits of carbon deposits, traces of compression, burning or erosion, the truth will be established on the surface of the parts.

Gasket defects after long-term operation of the car are most often associated with not noticed and not resolved malfunctions of the cooling system in time, less often – with violations of the combustion process (detonation, glow ignition). On the contrary, after engine repair, according to statistics, in more than 80% of cases, the gasket is damaged due to improper bolt tightening (including due to non-observance of the torque and tightening order).

Overheating is one of the main causes of damage to gaskets. And not only them: during overheating, the plane of the head, and sometimes even the cylinder block can be deformed.

But first of all, of course, the block head suffers. In addition to local overheating of individual sections of the combustion chambers, causing cracks, the general heating of the head leads to an increase in the compression force of the gasket, since the aluminum alloy of the head expands more than steel bolts. After cooling, the compressed gasket may no longer provide tightness where the specific pressure (force related to surface area) is too low. In fact, when overheating, a kind of “hardening” of the surface layers of the gasket occurs, as a result of which it loses its elasticity and can no longer provide a seal for the connection of the head with the cylinder block over the entire plane.

Detect the cause, i.e. to establish that the gasket “leaked” due to overheating of the engine, it is possible during its inspection. Usually in such cases, the surface of the gasket becomes hard, and in some places near the combustion chambers it becomes carbonized.

Repair in this case is rarely limited to just replacing the gasket. In addition to processing the head plane, it will not be out of place to find the cause of overheating in the cooling system – there may be a malfunction in the thermostat, fan, or simply leaking hoses.

But, let’s say the engine was very overheated, and the gasket seemed to resist. In such situations, there are two ways out: either try your luck, waiting for a leak to appear (and it will most likely be so), or replace the gasket right away. The second solution will be more successful: after all, whatever one may say, the planned repair is better than an unexpected breakdown on the road.

If the gasket is not crimped as it should, then it will definitely “leak”. This usually happens when the head bolts are not tightened properly. But such errors today are more an exception than a rule: now the necessary information is available at any service station. Another thing is when everything is tightened correctly, and the gasket is not crimped. What’s the matter?

The reasons usually lie in violations of basic repair technologies. For example, repair manuals write that the threaded part of the bolts must be lubricated with oil before tightening.

And if you do not lubricate? Then the tightening torque will be spent almost all on overcoming the friction forces in the thread, and not at all on pressing the head to the block.

Maybe this is one of the reasons that on some modern engines the threaded part of the bolts is coated? These bolts do not need to be lubricated. But seriously, the “solid” lubricant is much more effective and significantly improves the operation of the bolts (recall that with ordinary bolts only 20-25% of the torque is directly converted to a tightening force).

Even worse, when the bolts are doused too plentifully with oil. In this case, the well-known rule “you won’t spoil the porridge with oil (butter)” will not work: the oil is incompressible, filling the threaded hole, it simply will not let the bolt go further. And it’s good if the block does not crack through the threaded holes.

Modern engines often use yield strength bolts. After a single use, they are supposed to be replaced with new ones, since they can stretch beyond acceptable standards.

That the gasket has not been crimped properly is easy to detect when inspected. Often, there are practically no signs of compression on it, and the thickness remains the same as the new gasket. In such cases, a leak appears in the very first hours of engine operation, which indicates the true cause of the malfunction.

Prevention is simple: it is necessary to precisely follow all the manufacturers’ recommendations on the torque, tightening order and bolt replacement, as well as their lubrication before assembly.

It is known that, depending on the material and construction, the gaskets can “weaken” under the influence of temperature and vibration. And if after some time the engine does not retighten the bolts, the specific pressure at the junction of the head with the block may unacceptably decrease, after which the gasket will “leak”.

Not all engines and not all gaskets require retightening. But when such recommendations are made by the engine manufacturer, they must be strictly observed. If this is not done, a leak will appear after several thousand kilometers of the car’s run, which will directly indicate the cause of such a malfunction.

To violations of the combustion process in the engine include detonation and glow ignition. Without going into their nature (this is a topic for another discussion), we point out that these phenomena cause a significant increase in temperature in individual sections of the combustion chamber. Excessive pressure in the cylinder due to premature ignition of the fuel mixture dramatically increases the load on the head of the block, “stretching” the bolts and thereby reducing the compression force of the gasket. And the shock waves of detonation “knock” not only on the driver’s ears, but also on the edging of the cylinder head gasket.

The fact that it was a violation of the combustion process that caused damage to the gasket can often be established by its appearance. In such cases, it often burns out between the cylinders. Burnout along the edging is often accompanied by erosion of the surface of the block head and the edging itself near the damage. A change in the color of the gasket material near the edging can also indicate a high temperature in the combustion chamber.

It is not difficult to eliminate the causes of damage to the gasket. It is enough to set the correct ignition timing, put the spark plugs required for the given engine and pour gasoline with the corresponding octane rating into the tank. True, in some cases this may not be enough.

Mechanical problems. As you know, the cylinder head gasket is a very delicate part that is easily damaged by careless handling. If it has obvious defects, then putting it is either dangerous, or simply pointless – it will “flow” sooner or later.

The same applies to attempts to reuse an old gasket. Its material is already crimped and will never provide a reliable seal. It may be enough for several thousand kilometers, but this can only be regarded as a temporary way out. Sometimes burnout gaskets occur due to the ingress of foreign materials between it and the sealing surface. This can happen if you do not clean the plane of the head and block from the remnants of the old gasket before installing a new one.

By the way, the same thing will happen if the planes turn out to be deformed – in places of “failures” the specific pressure will be unacceptably low, and in the end the seal will break. Therefore, before installing the gasket, checking the planes of the head and block is mandatory.

It is clear that the deformed plane must be processed, i.e. align. In some workshops, the heads are milled, in others they are ground, and in others they are ground on a plate with abrasive paste. And which way is better?

Let’s figure it out together. If the surface is too rough, leakage of working fluids and gases is not excluded. On the contrary, if the surface is excessively smooth, it is possible for the gasket to slip between the parts to be sealed and ultimately a leak. Therefore, when processing the surfaces of the head and block, not all means are good. It is desirable to provide a certain optimum roughness, which mainly depends on the materials of the parts to be sealed. Of course, these are general recommendations. But they can be used when other data, for example, from the engine manufacturer, are not available.

Source (Russian): AG Детали Машин

Author: Alexander Hrulev, “АБС”