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Pic. 8. Crank mechanism: 1. Connecting rod cap; 2. A bolt of fastening of a cover of a rod; 3. Connecting rod; 4. Piston; 5. Piston thermostatic plate: 6. Oil scraper ring; 7 Lower compression ring; 8. Top compression ring; 9. Expanding spring; 10. Piston pin; 11. Insert connecting rod bearing; 12. Thrust half rings of the middle main bearing; 13 Main bearing shells; 14 Channels for supplying oil from the main bearing to the connecting rod; 15 Crankshaft rear oil seal holder; 16. Rear crankshaft oil seal; 17. Pin for TDC sensor; 18. Label (hole) TDC pistons of the 1st and 4th cylinders; 19. Scale in the hatch of the clutch housing, 20. TDC mark of the pistons of the 1st and 4th cylinders on the flywheel rim; 21. Washer of bolts of fastening of a flywheel; 22. Clutch locating pin; 23. Flywheel toothed rim; 24. Flywheel; 25. Crankshaft; 26. Plug of oil channels of the crankshaft; 27. Front crankshaft oil seal (pressed into the oil pump cover); 28. Camshaft drive pulley; 29. Generator drive pulley; A. Marking the category of the piston on the hole for the piston pin; B. Piston class marking by outer diameter; C. Piston oversize marking; D. Installation mark; I. Tags for setting the ignition timing. II. Marking of covers of main bearings of a cranked shaft (bearings are counted from the front of the engine).
The crank mechanism is used to convert the translational movement of the piston under the action of the expansion energy of the combustion products of fuel into the rotational movement of the crankshaft. The mechanism consists of a piston with piston rings and a pin, a connecting rod, a crankshaft and a flywheel.
Piston 4 is cast from a high-strength aluminum alloy. Since aluminum has a high temperature coefficient of linear expansion, in order to eliminate the risk of piston jamming in the cylinder, a temperature-controlled steel plate 5 is filled in the piston head above the hole for the piston pin.
Pistons, as well as cylinders, are sorted into five classes according to the outer diameter:
| Class | Piston diameter of engines 2108 and 21081 | Engine piston diameter 21083 |
| A | 75,965-75,975 | 81,965-81,975 |
| IN | 75,975-75,985 | 81,975-81,985 |
| WITH | 75,985-75,995 | 81,985-81.995 |
| D | 75,995-76,005 | 81,995-82,005 |
| E | 76,005-76,015 | 82,005-82,015 |
It is possible to measure the piston diameter to determine its class in only one place: in a plane perpendicular to the piston pin at a distance of 51.5 mm from the piston crown. In other places, the piston diameter differs from the nominal one, because the outer surface of the piston has a complex shape. It is oval in cross section and conical in height. This shape makes it possible to compensate for the uneven expansion of the piston due to the uneven distribution of the metal mass inside the piston.
On the outer surface of the piston, annular microgrooves up to 14 microns deep are applied. This surface contributes to a better running-in of the piston, as oil is retained in the micro-grooves. In the lower part of the bosses under the piston pin there are holes for the passage of oil to the piston pin. To improve lubrication conditions, two longitudinal grooves 3 mm wide and 0.7 mm deep are made in the upper part of the finger holes, in which oil accumulates.
The axis of the hole for the piston pin is shifted by 1.2 mm from the diametral plane of the piston towards the location of the engine valves. Due to this, the piston is always pressed against one cylinder wall, and piston knocks on the cylinder walls are eliminated when it passes TDC. However, this requires the piston to be installed in the cylinder in a strictly defined position. When assembling the engine, the pistons are installed so that the arrow on the piston crown points towards the front of the engine.
Pistons are sorted by weight into three groups: normal, increased by 5 g and reduced by 5 g. These groups correspond to the marking on the piston bottom; "G", "+" and On an engine, all pistons should be of the same mass group to reduce vibration due to uneven masses of the reciprocating parts.
Spare parts are supplied with pistons of nominal size only in three classes: A, C and E. This is enough to match the piston to any cylinder during engine repair, since pistons and cylinders are divided into classes with some overlap. For example, a class C piston can fit class B and D cylinders. The main thing when choosing a piston is to provide the necessary mounting clearance between the piston and the cylinder - 0.025-0.045 mm.
In addition to pistons of nominal size, repair pistons with an outer diameter increased by 0.4 and 0.8 mm are also supplied as spare parts. On the bottoms of repair pistons, a marking in the form of a square or a triangle is placed. A triangle corresponds to an increase in the outer diameter of 0.4 mm, and a square corresponds to 0.8 mm.
The piston pin 10 is steel, tubular section, pressed into the upper head of the connecting rod and freely rotates in the piston bosses. According to the outer diameter, the fingers are sorted into three categories through 0.004 mm, respectively, to the categories of pistons. The ends of the fingers are painted in the appropriate color; blue is the first category, green is the second and red is the third.
Piston rings provide the necessary sealing of the cylinder and remove heat from the piston to its walls. The rings are pressed against the walls of the cylinder under the action of their own elasticity and gas pressure. Three cast iron rings are installed on the piston - two compression rings 7, 8 (sealing) and one (bottom) oil scraper 6, which prevents oil from entering the combustion chamber
The upper compression ring 8 operates under conditions of high temperature, aggressive effects of combustion products and insufficient lubrication, therefore, to increase wear resistance, the outer surface is chrome-plated and has a barrel-shaped generatrix to improve running-in.
The lower compression ring 7 has a groove at the bottom for collecting oil during the downward stroke of the piston, while performing the additional function of an oil drop ring. The surface of the ring is phosphated to increase wear resistance and reduce friction against the cylinder walls.
The oil scraper ring has chrome-plated working edges and a groove on the outer surface into which the oil removed from the cylinder walls is collected. A coiled steel spring is installed inside the ring, which unclenches the ring from the inside and presses it against the cylinder walls. Repair rings are made (just like pistons) with an outer diameter increased by 0.4 and 0.8 mm.
The connecting rod is steel, machined together with the cover, and therefore they are not interchangeable individually. In order not to confuse the covers and connecting rods during assembly, they are stamped with the number of the cylinder in which they are installed. When assembling, the numbers on the connecting rod and cap must be on the same side.
The crankshaft 25 is cast from high-strength special cast iron and consists of connecting rod and main journals. To reduce deformations during engine operation, the shaft is made five-bearing and with a large overlap of the main and connecting rod journals. Channels 14 are drilled in the body of the shaft to supply oil from the main journals to the connecting rods. Technological outlets of the channels are closed with cap plugs 26.
To reduce engine vibrations, the shaft is equipped with counterweights cast integrally with the shaft. They balance the centrifugal forces of the crankpin, connecting rod and piston that occur when the engine is running. In addition, to reduce vibrations, the crankshaft is also dynamically balanced by drilling metal in counterweights.