The parts of the piston-connecting rod assembly are divided into categories and matched individually to one another. The tolerance group, indicated by a letter and stamped on the piston crown, must match the group marked on the cylinder liner.
Size groups of pins, pistons and connecting rods
| Diameter, mm | Marking | |||
|---|---|---|---|---|
| Of the pin | Bore in the piston boss | Bore in the connecting rod bush | of the pin and rod | of the piston |
| 25.0000–24.9975 | 25.0000–24.9975 | 25.0000–24.9975 | white | I |
| 24.9975–24.9950 | 24.9975–24.9950 | 25.0045–25.0020 | green | II |
| 24.9950–24.9925 | 24.9950–24.9925 | 25.0020–24.9995 | yellow | III |
| 24.9925–24.9900 | 24.9925–24.9900 | 24.9995–24.9970 | red | IV |
The values of the bore diameters in the piston bosses, the connecting rod head and the outside diameters of the piston pin are divided into groups and marked with paint. On the piston pin, the group is indicated by paint applied to its end face or inner surface. It must match the group marked on the piston boss.
Materials and coatings
Pistons for ZMZ engines are made from a high-silicon aluminium alloy. To improve the wear resistance of the ring grooves, an insert of Niresist (austenitic cast iron) is often used. Piston pins are made of steel, carburised or nitrided for surface hardness. Cylinder liners are cast iron, bored to size. The sealing washers of the liners are copper. Piston rings are cast iron, with a chrome or molybdenum coating on the working surfaces.
On the connecting rod, the group of the bore for the piston pin is also marked with paint. It must either match or be adjacent to the group of the pin. We check the correct selection of the connecting rod and piston pin as follows: the pin, lubricated with engine oil, must move in the connecting rod head under the pressure of the thumb but must not fall out of the bush.
The serial number of the cylinder in which it was installed is stamped on the side surface of the lower connecting rod head and on the cap. The numbers on the cap and on the connecting rod must match and be on the same side.
Connecting rods supplied as spare parts do not carry this marking, so before dismantling, mark the rods and caps as at the factory so that the caps are not inverted or mixed up during assembly.
Heat the piston to 60–80 °C. It is permissible to heat the piston in hot water. Insert the connecting rod head between the piston bosses and drive in the piston pin, lubricated with engine oil, with a hammer through a mandrel or with a suitable tool.
Fit the circlips on both sides. The lug on the connecting rod cap must be on the same side as the “Front” mark on the piston.
Carefully clean the liner seats from scale and corrosion. Replace the copper sealing washers of the cylinder liners with new ones. Drive in the liners with light hammer blows through a wooden block.
Using a feeler gauge, check that the liner protrusion above the block face must be 0.02–0.10 mm.
Selecting the piston rings
We select the piston rings for the cylinders. We install the rings one by one in the cylinder to a depth of 20–30 mm and measure the gaps with a feeler gauge. The compression rings must have an end gap of 0.3–0.6 mm, the oil control rings — 0.3–1.0 mm.
If piston replacement is not planned, check the groove width using new piston rings. Check the gap at several points around the piston circumference. The side clearance for compression rings must be 0.050–0.082 mm, for the composite oil control ring 0.135–0.335 mm. In worn cylinders, rings of the nearest repair size can be fitted and, if necessary, the ends filed to obtain a gap of 0.3 mm.
We fit the rings onto the piston, starting with the oil control ring. Having opened the expander joint of the oil control ring, we install it in the lower groove of the ring, then bring the ends of the expander together. We fit the oil control ring onto the expander with the marking towards the piston crown. The angle between the joints of the expander and the ring is 45°.
We install the lower compression ring with the marking and the chamfer on the inner side of the ring towards the piston crown.
We install the upper compression ring.
