Piston rings are installed three on each piston: two compression rings and one oil scraper.
Compression rings are cast from special cast iron.
The outer surface of the upper compression ring is porous chrome plated, while the surface of the second compression ring is either tin plated or dark phosphate coated.
Fig. 1. Installing the rings on the piston
On the inner cylindrical surfaces of both compression rings, grooves are provided (Fig. 1, a), due to which the rings turn out somewhat when the piston moves down, which contributes to a better removal of excess oil from the surface of the sleeves.
The rings must be installed on the piston with the grooves up, towards the piston bottom.
The UMZ-4218.10 engine can be equipped with two versions of compression rings (Fig. 1, b, c).
One version of the upper compression ring 2 (Fig. 1, b) has a groove on the inner cylindrical surface. The ring must be installed on the piston with the groove up.
Another version of the upper compression ring 2 (Fig. 1, c) has a barrel-shaped profile of the outer surface, there is no groove on the inner cylindrical surface of the ring.
The position of the ring when installed in the piston groove is indifferent.
Lower compression ring 3 (Fig. 1, b, c) - scraper type, on the lower end surface it has an annular groove, which, together with the conical outer surface, forms a sharp lower edge ("scraper") .
The ring is made in two versions - with a groove on the inner cylindrical surface of the ring (Fig. 1, b) and without a groove (Fig. 1, c).
The ring must be installed on the piston with a sharp edge "scraper" down.
The oil scraper ring is composite, has two annular discs, radial and axial expanders.
The outer surface of the oil scraper disc is hard chrome plated. The lock of the rings is straight.
Oversize Piston Rings (Tolerances of the main parts of the UAZ-3151 engine) differ from rings of nominal sizes only in outer diameter.
Oversize rings can be installed in worn cylinders with the next smaller oversize by sawing their joints to obtain a gap in the lock of 0.3–0.5 mm (0.3–0.65 mm for engines mod. 4218).
Fig. 2. Selection of piston rings according to the cylinder (checking the side clearance at the junction of the ring)
Check the side clearance at the joint of the ring, as shown in fig. 2.
For reground cylinders, adjust the rings along the upper part, and for worn ones, along the lower part of the cylinder (within the stroke of the piston rings).
When adjusting, install the ring in the cylinder in the working position, i.e. in a plane perpendicular to the axis of the cylinder, for which advance it in the cylinder using the piston head.
The joint planes in a compressed ring must be parallel.
Fig. 3. Removal and installation of piston rings
Remove and install rings on the piston using tool (fig. 3) model 55-1122.
Fig. 4. Checking the backlash between the piston ring and the piston groove
After fitting the rings to the cylinders, check the side clearance between the rings and grooves in the piston (Fig. 4), which should be:
- for the upper compression ring 0.050–0.082 mm,
- for the lower compression - 0.035–0.067 mm.
With large gaps, replacing only the piston rings will not eliminate the increased oil consumption due to the intensive pumping of it by the rings into the space above the piston. In this case, at the same time as replacing the rings, replace the pistons.
Simultaneous replacement of piston rings and pistons dramatically reduces oil consumption.
Fig. 5. Cleaning the grooves of the piston rings from carbon deposits
When replacing only piston rings without replacing pistons, remove carbon deposits from the piston crowns, from the annular grooves in the piston head and from the oil drain holes located in the grooves for the oil scraper rings.
Remove carbon deposits from the grooves carefully so as not to damage their side surfaces, using a tool (Fig. 5).
Remove carbon deposits from oil drain holes with a 3 mm drill.
When using new or oversized cylinder liners, the top compression ring must be chrome-plated and the other rings tin-plated or phosphated.
If the liner is not repaired, but only the piston rings are changed, then all of them must be tin-plated or phosphated, since the chrome ring is very poorly run-in to the worn liner.
Before installing the pistons into the cylinders, spread the joints of the piston rings at an angle of 120° to each other.
After changing the piston rings, do not exceed the vehicle speed of 45 - 50 km / h within 1000 km.