When overhauling the engine, very often questions arise about choosing the right thermal clearance. Piston rings with too much clearance in the lock and along the axis will not work properly. 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 choose the correct thermal gap of the piston rings and what it should ideally be.
General information and concepts
The piston consists of three rings, each of which performs its specific function. The top two serve to maintain compression in the engine. If they lie or reach a critical level of wear, then the dynamic characteristics of the internal combustion engine fall, oil consumption increases and other problems arise. The bottom ring is called the "oil scraper". From the name, its purpose in the operation of the power unit is quite clear.
When an internal combustion engine is operating, metal parts expand. For this simple reason, the thermal clearance of the piston rings is reduced. If it was initially improperly selected, that is, less than acceptable, then when the engine reaches operating temperature, the piston will scratch the cylinder walls.
What is the thermal clearance of the VAZ piston rings?
Before proceeding directly to the numbers, I would like to note that the clearance should be measured on an unheated part. In this case, be sure to put the ring in the cylinder. To determine the parameter, special probes or bars are used. 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, these data may differ, so it is recommended that you check with the ICE documentation first.
What thermal clearance 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 scraper 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 into the allotted groove. Next, you need a set of slats with which to determine the gap. If the ring is located directly inside the cylinder, then 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 latter does not matter much here, since absolutely everything will burn during operation. Further, the ring moves along the cylinder wall. Moreover, this 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 block groove, it is enough to shift the ring by 3-5 mm, approximately to the place where it will be in the process of ICE operation. After that we use a set of probes (strips) and get the necessary values. Use the "manual" when picking up the clearance of the piston rings. A Ford Escort 1.6 diesel engine, for example, should have a clearance of 0.3-0.5 mm for the upper compression rings and 0.2-0.45 mm for the oil scraper.
Comprehensive repair of CPG
The so-called "kapitalka" of the engine implies a complete troubleshooting of the cylinder-piston group, because it is it that is subject to the greatest thermal loads during operation. Consequently, pistons, compression and oil scraper 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 already considered critical, and in this case, replacement of the rings is necessary. So, when overhauling the engine, it is recommended to change all the rings on the pistons, even if they are also suitable. This is necessary in order not to disassemble the motor again after 50,000 kilometers.
It should also be understood that the gap is not always the same for the first and second compression rings. A vivid example of this is domestic heavy equipment. For example, the thermal clearance of the KamAZ piston rings should look like this:
- first compression ring - 0.20-0.40 mm (new);
- the second - 0.30-0.50 (new);
- oil scattering 0.25-0.50 (new).
Moreover, the allowable wear for all types of rings should not exceed 1 mm. Even 0.9 mm can already be considered critical. Although often without disassembling the CPG it is clear that the rings are asking for a replacement.
Increased oil consumption
Most modern car manufacturers increase the piston ring clearances in their engines. If for some types of internal combustion engines 1 mm is critical, but on a new engine from BVM or Audi, 1-2 mm is the gap of the new engine rings without mileage. Here it is necessary to deal with one important point.
The fact is that when the fuel-air mixture is burned, gases are formed that enter the piston groove. Accordingly, they begin to create pressure from the inside of the ring, providing it with pressure against the cylinder wall.
That is why when the engine is idling and at low speeds, the pressing force is not as large as at high loads. This is due to the fact that the amount of gas in the combustion chamber is significantly different. The second compression ring partly performs the oil scraper, removing the film from the cylinder. If it is worn, then the lubricant consumption increases significantly, 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", cannot boast of an extremely reliable engine. Although it is with proper maintenance and runs quite a lot. Nevertheless, during major repairs, special attention should be paid to the thermal gap. Many do not attach any importance to this and leave the old rings. But such a major overhaul will not bring the expected results, especially if the engine was rebuilt due to increased oil consumption.
There are several salient features. In particular, the maximum allowable clearance in all rings is only 0.15 mm. At the same time, the nominal value for the 1st compression is 0.04-0.075 mm, for the second - 0.03-0.065 mm, and for oil scraper - 0.02-0.055 mm. Here it is necessary to achieve very high accuracy. The best option is to measure the grooves on the piston and 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 around the circumference.
Some important nuances
The life of the cylinder-piston group and, in fact, the rings depends on how correctly the thermal clearance of the piston rings of the diesel engine or gasoline is selected. The appearance of scoring due to the friction of the rings on the cylinder leads to the loss of not only compression, but also geometry. There is little pleasant in this, since to restore the working condition, grinding will be needed, and in the most advanced cases, boring of the cylinder block.
If there is radial wear of the rings, then the sealing in the combustion chamber is significantly impaired. This leads to a deterioration in engine dynamics and increased consumption of engine oil.
Than the small gap is bad
It would seem that to achieve good compression, a clearance of as little as possible is necessary. In this case, the oil will not burn. All this is only partially true. The fact is that if the expansion gap becomes less than acceptable, then there will be a violation of heat transfer. This is partly due to increased friction of the rings on the cylinder. As a result, scuffs appear, accelerated wear of the cylinder and ring. In the end, the compression rings do not create the proper pressure, and the oil scraper leaves oil on the cylinders. After some time, engine power is lost, the consumption of lubricant increases and the stability of the internal combustion engine deteriorates.
To summarize
As you can see, it is extremely important to know what kind of piston ring clearance is necessary for a particular type of engine. Each manufacturer indicates the nominal and permissible values. They are recommended to adhere to. If during the overhaul the ring is worn out, say by 50%, then it is better to replace it with a new one.
The enlarged rather than reduced clearance is not so terrible. Because of the latter, the temperature in the combustion chamber rises, mechanical damage is caused to the cylinder. All this subsequently translates into serious costs. To prevent this from happening, it is necessary to choose the correct clearance of the piston rings and be guided only by trusted sources. The instructions for the operation and repair of each car indicate the data you need, and they should be put into practice.