Even the pan on the stove heats unevenly - what can we say about the engine, in which there are dozens of parts. In addition, their materials are very different in physical properties. For example: for VAZ engines, the cylinder head and pistons are made of aluminum alloys, the camshaft is cast-iron, the valves are steel, while the temperature of the latter (if we talk about exhaust) in the plate area reaches 500–600 ° C, and for surrounding parts it is three - four times lower. Again, this picture is simplified, since the parts themselves are heated differently at different points. How to cope with such a mess? The most justified approach is known a long time ago: for the machine to work, thermal gaps are left between the parts. This also applies to the gas distribution mechanism. The hot gaps are minimal, otherwise the valves, “not opening”, limit the power, and the noise during operation increases. It is impossible to remove the gaps to zero - the valves will no longer fit tightly in the saddles and, when overheated, will begin to burn. The range of acceptable values is very narrow, but they depend on the initial adjustment. Here it is - the headache of the car owner: what gaps need to be set in the cold state?
It is obvious that, by cooling, the stem of the exhaust valve will be the most shortened in length. The aluminum head, although it has a larger thermal expansion coefficient, “shrinks” less: after all, on average it was four hundred degrees colder than the valve! As a result (see Fig. 1), a considerable gap will appear between the lever 2 and the camshaft cam 3. On the Zhiguli engines, the descendants of the FIAT-124, this gap is about 0.14–0.17 mm at a temperature of 20 ° C. Usually, in services, 0.15 mm is considered the norm. How fair is this?
We have seen from our own experience that such a gap is not enough for the exhaust valve - it is more correct to navigate the upper value of the Italian tolerance. Otherwise, the cams are worn from the back, which means that in some modes the gap disappeared (photo 1). Even slight scuffs indicate an underestimated clearance: the fact is that after starting the engine, the valves heat up very quickly, the cylinder head is much slower, and at some point the gap disappears. Here is another reason why it is not recommended to “pedal” a cold motor. And the strongly erased neck of the cams is a sign that the gap has disappeared for a long time, the valves are not tightly closed and in power modes. This leads to trouble: an overheated plate does not give off heat to the saddle and begins to collapse. Especially at risk are "yard racers." The combination of heavy loads with inept regulation of power, ignition and gas distribution systems is a sentence to valves (photo 2).
On the 21213 engine, VAZ still increased the clearance in the exhaust valve actuator. We on the “classic” motors themselves came to this, adding 0.015-0.02 mm for graduation. Take a look at the pic. 1. The indicator readings at the end of the lever are increased from the usual 0.52-0.53 mm to 0.57-0.6 mm. This eliminates the risk of valve overheating, but does not cause additional noise. By the way, it's not just about comfort! Noise in the drive is a sign of parts working with vibrations leading to quick and incorrect wear.
Camshafts sometimes come across in which the geometrical axis of the radius (rear) surface of the cam does not coincide with the axis of rotation of the camshaft. Here adjustment is a troublesome business. We check the clearance in two positions of the cam relative to the lever - before and after lifting. The maximum value of the two is about 0.65 mm in the indicator, otherwise the noise will start.
I want to protect those who first decided to adjust the gaps on their own from mistakes. The approach "twisted … started … cheers, it works!" - is not good. The "clamped" drive rustles more pleasantly than others! You need to measure the gap accurately - it is better to use an indicator rather than a probe. A very common mistake is to adjust the clearance “to 0.15” on a motor that has not cooled sufficiently. In order for the motor to cool evenly to + 20 ° C, it takes several hours even in winter. How to be? We use the nomogram shown in Fig. 2. We measure the temperature of the cylinder head with a thermometer - we put it in a technological hole filled with oil.