Pistons (fig. 1) are made of aluminum alloy and plated with tin. The piston skirt has the shape of an elliptical cone, the large base of which is located at the lower edge of the skirt, and the largest axis of the ellipse lies in a plane perpendicular to the axis of the piston pin

The taper on the length of the skirt is 0.035-0.050 mm, and the difference between the largest and smallest diameters of the piston skirt is 0.52 mm.

Installing the piston and rings of the ZIL-131 engine

The final selection of the piston to the sleeve is carried out by checking the force required to pull the probe 0.08 mm thick and 10 mm wide between the cylinder wall and the piston from the side opposite the piston skirt cut. The force on the probe should be in the range of 3.5-4.5 kg.

Piston pins are floating, each pin is fixed in the piston by two retaining rings.

The pins are made with high precision and matched to the pistons and connecting rods, sorted into four groups according to the outer diameter (Table 1).

The designation of the group is applied with paint: on the piston - on the inner surface (on one of the bosses), on the connecting rod - on the outer cylindrical surface of the small head, on the finger - on the inner surface.

When assembling, the pin, piston and connecting rod are assembled from parts of the same group only.

This selection provides an interference between the pin and the piston in the range of 0.0025 - 0.0075 mm and a gap between the pin and the connecting rod in the range of 0.0045-0.0095 mm (at a temperature of 20 ° C).

In order to avoid scuffing on mating surfaces, assembly of the pin with the piston should be carried out only when the piston is heated to a temperature of 55 ° C.

Pistons should only be heated in liquid and clean oil.

Piston rings are installed four on each piston: three compression rings and one oil scraper.

Two upper compression rings are chrome-plated on the outer cylindrical surface.

The outer surface of the lower compression ring is made conical; the larger base of the cone faces down.

Compression rings are installed so that the undercut on the inner cylindrical surface of the rings faces upwards, as shown in fig. 1.

The oil scraper ring is composed of two flat steel rings and two expanders - axial and radial.

When installing the piston in the engine, flat ring discs 1 must be installed so that their locks are located at an angle of 180° relative to each other, and the locks of the axial 2 and radial 3 expanders are at an angle of 90° to the disc locks (each).

A special tool must be used to remove or install piston rings.

The dimensions of the piston rings and grooves in the piston, as well as the gaps between them and the gap in the lock are given in table 2.

To increase the service life, the piston rings in the free state have a complex shape, as a result of which, after the ring is installed in the cylinder, the most favorable distribution of the pressure of the ring on the liner wall is provided.

When installing the rings on the piston, their joints (locks) should be installed at an angle of 90 ° to each other.

connecting rods steel, I-section, Steel-aluminum thin-walled liners 2-0.16 mm thick are installed in the lower head of the connecting rod.

A coiled bronze bushing is pressed into the upper head of the connecting rod.

Inserts are made with great precision, are completely interchangeable and do not require scraping, sawing joints or installing gaskets during installation.

These operations are not allowed with thin-walled liners.

In the new engine, the diametrical clearance in the connection of the crankshaft crankpin - connecting rod with liners assembly is 0.032-0.076 mm.

When installing a piston assembly with a connecting rod to the engine, the arrow 8 on the piston crown must always point towards the front end of the crankshaft.

In the kit, the piston with the connecting rod assembly, designed for the left group of cylinders, the mark 11 on the rod of the connecting rod and the arrow 8 on the piston must face the same direction, and in the kit for the right group of cylinders, in different directions.

The nuts of the connecting rod bolts must be tightened with a torque wrench with a tightening torque of 7-8 kgm.

After tightening, the nuts should be carefully cottered.

If, at the specified tightening torque, the hole in the bolt and the slot in the nut do not match, additional rotation of the nut is allowed until the hole matches the nearest slot, while the tightening torque should not exceed 11.5 kgm.

Checking and tightening the nuts of the connecting rod bolts must be done each time the oil pan is removed.

The difference in weight between the piston and connecting rod assembly must not exceed 16 g.

Table 1


Finger diameter

in mm

Hole diameter in mm



in piston

and small end of the connecting rod













27.9900 - 27.9875





27.9875— 27.9850



Note. All measurements are made at a temperature of 20 ° C.

Table 2

Measurements in mm






Oil scraper

Pole heighttsa




Piston groove height




Height clearance

between ring

and piston (in groove)

0.050 -0.082

0.050 -0.082

0.025 -0.062

Gap in ring lock

0.35 - 0.7

0.3 - 0.65

0.9 - 1.5