How to check the thermal clearance of piston rings: expert advice

When overhauling the engine, there are often questions about choosing the right thermal gap. Piston rings with too much clearance in the lock and on the axis will not work correctly. But even worse, if the gap was taken too small. In this case, the engine will not work for a long time and after a few thousand kilometers it will again be asked for a bulkhead. Let's talk about how to properly choose the thermal gap of piston rings and what it, in fact, should ideally be.

General information and concepts

The piston consists of three rings, each of which performs its specific function. The two upper ones serve to maintain the compression in the engine. If they lie or reach a critical mark on wear, the dynamic characteristics of the internal combustion engine drop, the oil consumption increases and other problems arise. The lower ring is called "oil-removable". From the name it is quite clear its purpose in the process of the power unit.

When the internal combustion engine is running, the metal parts expand. For this simple reason, the thermal clearance of the piston rings is reduced. If it was originally selected incorrectly, that is less than permissible, then when the engine reaches its operating temperature, the piston will scratch the cylinder walls.

What is the thermal clearance of the VAZ piston rings?

Before going directly to the figures, I would like to note that the gap measurement should be performed on the unheated part. It is necessary to place the ring in the cylinder. Special probes or bars are used to define the parameter. In most cases, the allowable gap value ranges from 0.25 to 0.5 mm. But depending on the type of motor and its modification, this data may differ, therefore it is recommended to consult the ICE documentation beforehand.

What thermal gap of piston rings should be on VAZ cars? For compression rings, it should be in the range of 0.25-0.4 mm, and for the oil ring - 0.25-0.5 mm. Again, these indicators may differ slightly, so you need to focus on a specific engine model.

Measurement technology

The first thing to do is bring the ring to the side of the cylinder. Then place its outer part in the retracted groove. Next, you need a set of slats, with which you determine the gap. If the ring is located directly inside the cylinder, the measurements must be carried out separately, since the distance between the ends can vary.

The first thing to do is put the piston ring in the engine oil. The quality of the last special value here has no, because in the course of operation everything will burn completely. Then the ring moves along the wall of the cylinder. And it must be done only in the cylinder where this ring will be used in the future. It is especially convenient to perform such a calculation if the piston is already dismantled. In the case of a recent blockage, it is sufficient to move the ring 3-5 mm, approximately to the place where it will be in the process of operating the ICE. After that, we use a set of probes (bars) and get the necessary values. Use the "manual" when picking up the clearance of the piston rings. Diesel "Ford Escort" 1,6, for example, should have a clearance of 0.3-0.5 mm for the upper compression rings and 0.2-0.45 mm for oil-removal.

Complex repair of TsPG

The so-called "cap" of the engine implies a complete defect in the cylinder-piston group, in fact it is subject to the greatest thermal loads during operation. Consequently, the pistons, compression and oil-removal rings are also subject to increased wear. In some cases, when the rings have not yet reached critical wear, they are left. On most modern ICEs, a gap of 1 mm is considered critical, in which case replacement of the rings is necessary. So, with overhauling the engine is recommended to change all the rings on the pistons, even if they are also suitable. This is necessary in order to again not disassemble the engine after 50,000 kilometers.

It should also be understood that the gap for the first and second compression rings is not always the same. A vivid example of this is domestic heavy equipment. For example, the thermal clearance of the piston rings "KamAZ" should look like this:

• The first compression ring - 0.20-0.40 mm (new);
• The second - 0,30-0,50 (new);
• Oil-removal 0,25-0,50 (new).

The permissible wear for all types of rings should not exceed 1 mm. Even 0.9 mm can already be considered critical. Although it is not uncommon for the GPG to disassemble, it is clear that the rings are asking for a replacement.

Increased oil consumption

Most modern car manufacturers increase the clearances of the piston rings in their engines. If for some types of internal combustion engine is critical 1 mm, but on a new engine from the "BVM" or "Audi" 1-2 mm - this is a gap of new engine rings without mileage. It is necessary to deal with one important point.

The fact is that when a fuel-air mixture burns, gases are formed that fall into the piston groove. Accordingly, they begin to create pressure from the inside of the ring, ensuring that it is pressed against the wall of the cylinder.

That is why when the engine is operating at idle and low speed, the clamping force is not as great as under high loads. The reason is that the amount of gases in the combustion chamber is significantly different. The second compression ring partially performs the task of oil-removal, removing the film from the cylinder. If it is worn, the lubricant consumption is increased many times, especially at idle and low engine speeds.

Thermal clearance of piston rings VAZ-21083

The domestic car 2108, better known as the "eight" or "chisel", can not boast of an extremely reliable engine. Although it is with proper maintenance and runs a lot. Nevertheless, during major repairs, special attention should be given to the thermal gap. Many do not attach importance to this and leave the old rings. But such a major overhaul will not bring the expected results, especially if the engine is re-assembled because of the increased oil consumption.

There are several characteristic features. In particular, the maximum permissible clearance in all rings is only 0.15 mm. In this case, the nominal compression for the first compression is 0.04-0.075 mm, for the second compression - 0.03-0.065 mm, and for the oil-compression - 0.02-0.055 mm. Here it is necessary to achieve very high accuracy. The most optimal option is to measure the grooves on the piston and the rings. To do this, you can use a micrometer and a set of probes. Measurements on the piston must be carried out in several places along the circumference.

A few important nuances

From how correctly the thermal clearance of piston rings of a diesel engine or gasoline will be chosen, depends the resource of operation of the cylinder-piston group and, in fact, the rings. The appearance of scuffing due to friction of the rings on the cylinder leads to loss of not only compression, but also geometry. Pleasant in this little, because to restore the working state will need grinding, and in the most neglected cases - the boring of the cylinder block.

If there is a radial wear of the rings, the sealing in the combustion chamber deteriorates substantially. This leads to a deterioration in engine dynamics and an increased consumption of engine oil.

Than a small gap is bad

It would seem that to achieve good compression you need a gap as small as possible. In this case, the oil will not burn. All this is only partly true. The point is that if the gap becomes less than the allowable during expansion, heat transfer will be violated. This is partly due to increased friction of the rings on the cylinder. As a result, there are badasses, the wear of the cylinder and the ring is accelerated. In the end, the compression rings do not create the proper pressure, and the oil-retaining rings leave the oil on the cylinders. After a while, the engine's power is lost, the lubricant consumption is increased, and the stability of the engine is deteriorated.

Let's sum up the results

As you can see, it is extremely important to know what piston ring clearance is required for a certain type of engine. Each manufacturer indicates nominal and allowable values. They are recommended to adhere to. If during the overhaul the ring is worn out, say 50%, then it is better to replace it with a new one.

The enlarged gap is not so terrible, rather than the reduced one. Because of the latter, the temperature in the combustion chamber rises, mechanical damage is caused to the cylinder. All this later translates into serious costs. In order that nothing of the kind could happen, it is necessary to correctly select the piston ring gap and be guided only by proven sources. The instructions for operation and repair of each car indicate the data you need, and they should be used in practice.