For most car owners, the phrase "cylinder scoring" sounds almost like a final verdict for the engine. The situation is especially alarming when it comes to an engine with a mileage of over 120 thousand kilometers — and such units are common today. Such fears are often well-founded.
The fact is that modern power plants, especially those made without sleeves and with aluminum blocks, are in most cases structurally poorly protected from damage to the cylinder walls. Such engines actually remain vulnerable to the formation of scoring.
In simple terms, scoring is a local destruction of the sleeve surface. Outwardly, it can manifest itself in different ways: from a barely noticeable matte area to a set of parallel scratches or even a deep groove. Regardless of the form, the result is the same — compression rings lose their ability to hermetically hold the fuel-air mixture during compression.
This leads to a decrease in the compression ratio, and part of the mixture begins to break into the crankcase. This picture is typical for carburetor engines and engines with distributed injection. In direct injection installations, the situation is somewhat different: an air-oil emulsion often gets there.
However, the main problem is not so much the loss of power of a single cylinder, but the more serious consequences. Hot gases penetrate into the chamber through a microscopic gap between the ring and the damaged area. These combustion products:
- burn out the lubricant on the rings, bringing it to the state of coking
- damage the jumpers and ring grooves on aluminum pistons
- sharply worsen the condition of the engine oil in the crankcase
The engine itself does not stop, but begins to work unstably — "trotting" appears. In addition, a significant amount of bluish smoke comes out through the breather and crankcase ventilation system. It is the appearance of burning and characteristic blue clouds from under the hood that serves as the first signal for urgent diagnostics.
Modern testing tools, including electronic testers and computer systems, allow you to determine the problem cylinder in a matter of minutes. However, there are simpler ways. For example, you can measure the compression in each cylinder, after adding a small amount of oil. Nevertheless, the final conclusion is usually possible only after disassembly — removing the cylinder head and removing the piston with the connecting rod.
In most cases, scoring can be seen visually on the cylinder mirror. It manifests itself in the form of a matte area, a system of risks or even a depression. An experienced mechanic is able to detect damage not only by eye, but also by touch.
The causes of scoring are diverse. Among the most common factors are:
- dust, sand and road abrasive entering the engine, for example, when driving behind a dusty truck or in the absence of an air filter
- the presence of small particles of metal or ceramics that have broken off from the intake manifold or entered the cylinders from a destroyed catalytic converter
There are also less typical, but very dangerous situations:
- destruction of the spark plug electrode with subsequent entry of fragments into the cylinder
- loosening and unscrewing of small parts, such as damper screws
- destruction of the piston pin retaining ring
- disintegration of the compression ring due to prolonged detonation
In some of these cases, the damage is so severe that it can lead to the destruction of the sleeve wall.
The owner's reaction to the appearance of scoring is often accompanied by confusion, but the choice of solution directly depends on the material of the block and the degree of damage. If we are talking about an aluminum block with dry cast-iron sleeves, then a major overhaul with cylinder boring is quite real. In the case of fully aluminum blocks, which are widely used in German engines, the possibilities are limited — often you have to either perform sleeving or completely replace the unit.
With minor damage, when the depth of the scoring is only a few tens of microns, the problem can be eliminated by honing with diamond paste. This process allows you to restore the geometry of the surface and restore the normal operation of the compression rings. However, this will require either the installation of repair pistons, or additional galvanic treatment with the application of a tin-lead layer.
If there are pronounced risks on the surface of the cylinder, it will be necessary to bore all the sleeves to the repair size, followed by processing. In the most severe cases, the sleeves are replaced: the old ones are bored and removed, after which new ones are installed. Despite the apparent complexity, this procedure is standard for many specialized workshops. However, not every service station has high-quality equipment for such work.
It is worth considering that the elimination of scoring actually accounts for about half of the work during a major engine overhaul. Therefore, for cars with high mileage, the appearance of such defects becomes a good reason not to postpone the restoration and to carry out a complete overhaul of the power unit in a timely manner.