The piston pin is fitted with interference in the upper connecting rod eye and rotates freely in the piston bosses.
The engines are equipped with SMP pistons.
Remove the piston rings using special piston ring pliers.
- Mount the piston with the connecting rod on the prism (1) of the Mot574-22 tool (see Fig. 1).
- Align the piston pin with its exit hole in the prism (1).
- Using a press, push out the piston pin with the drift (2) of the Mot574-22 tool.
After dismantling the piston-connecting rod assembly, a series of measurements must be taken in order to reject worn parts.
Main wear points
- Piston rings: wear during operation and also wear in width in the piston groove — replace the rings as a set at every dismantling and assembly. Do not replace the rings more often than necessary: this accelerates wear of the cylinder in the block.
- Piston ring grooves: if the width of the grooves exceeds the permissible value, both the pistons and the rings must be replaced.
- Piston skirt (the part of the outer cylindrical surface of the piston below the piston pin axis): during operation the skirt surface lying in the plane perpendicular to the piston pin axis wears — the piston must be replaced.
- Piston pin hole in the piston: wears along the piston axis; with a large clearance — replace the piston.
- Bush in the upper connecting rod eye: if worn — replace the bush.
- Piston pin: the contact area with the connecting rod eye and the contact area with the piston bosses wear. Must be a tight (interference) fit in the upper connecting rod eye. If worn, when replacing pistons, connecting rods, upper connecting rod bushes — replace the piston pins.
- Connecting rods: during operation they are subjected alternately to compression and tension, so they generally lose their shape (bend, twist). Do not straighten such rods — replace them immediately. Rarely, but it happens that a rod breaks or stretches the lower eye (on the crankshaft journal) — replace damaged rods.
The piston pin must match in diameter both the upper connecting rod eye and the piston pin hole in the piston.
The piston diameter must match the cylinder diameter in the block.
When assembled, the connecting rod + piston + piston ring assemblies must not differ in mass from cylinder to cylinder; a difference of not more than 6 g is permissible. Otherwise the engine will run insufficiently evenly, which over time leads to fatigue failure of the parts.
The piston diameter must be measured in accordance with Figure 2. The piston diameter is measured at a distance X = 42,00±0,01 mm from the piston crown. The total height of the piston is 55 mm.
Piston size groups (Table 1)
| Mark | Piston diameter, mm |
|---|---|
| A | 79,475±0,005 |
| B | 79,485±0,005 |
| C | 79,495±0,005 |
The piston pin is pressed into the upper connecting rod eye and sits freely in the piston bosses.
Piston pin dimensions
- Length, mm: 61,7–62,0;
- Outer diameter, mm: 19,986–19,991;
- Inner diameter, mm: not more than 11,6.
The arrangement of the piston rings is shown in Figure 3.
Piston ring dimensions (Table 2)
| Ring | Thickness, mm | Gap in the joint, mm |
|---|---|---|
| Upper compression ring | 1,470–1,490 | 0,15–0,30 |
| Lower compression ring | 1,470–1,495 | 0,40–0,60 |
| Oil scraper ring expander | 1,470–1,530 | 0,20–0,90 |
| Oil scraper ring disc | 0,420–0,449 | 0,20–0,90 |
The wear of the ring around its perimeter is difficult to measure, so the gap in the ring joint is measured. This gap ensures normal expansion of the ring when it heats up together with the engine, so it is in no case equal to zero on a cold engine. As the ring wears around its perimeter, this gap increases, which is checked by measurement.
- Fit the piston ring (1) into the cylinder (see Fig. 4).
- Advance the piston ring (1) to the middle of the cylinder using the inverted piston (2).
- Measure the ring gap in the joint with the feeler gauge set (3).
Replace the piston rings if the gaps are outside the permissible limits. If the gaps remain outside the limits even with new rings (see Table 2), replace the cylinder block.
Checking the piston ring clearance in the piston grooves
Thoroughly clean both the rings and the piston grooves of resins, otherwise the measurement will be inaccurate.
Using flat feeler gauges, check the clearances between the piston grooves and the piston rings (see Fig. 5).
- The clearance for the upper compression ring must be within 0.04—0.08 mm.
- The clearance for the lower compression ring must be within 0.025—0.07 mm.
- The clearance for the oil scraper ring must be within 0.08—0.22 mm.
If the clearance values exceed the specified limits, replace the piston rings or the piston complete with the piston pin and rings.
