Figure 3.55. Removing piston rings with a feeler gauge
Clean the pistons and connecting rods from dirt and carbon deposits. Using two or three flat feeler gauges, remove the rings from the pistons up (towards the bottom of the piston) (Figure 3.55). Remove carbon from the piston crown with an aluminum scraper. Do not use sandpaper for this purpose, as the grains of the abrasive are embedded in the aluminum of the piston and subsequently cause scratches. Remove carbon deposits from the piston grooves using a suitable piece of piston ring, being careful not to scratch the groove surface.
Wash the pistons from the removed deposits. Make sure that the holes in the oil scraper grooves are not clogged with dirt.
Inspect the friction surfaces of the pistons. Permissible is wear in the form of vertical «bald patches» on the pressure-receiving surface of the piston skirt and a small gap in the fit of the upper compression ring in the groove. Scoring and scratches on the piston skirt may indicate that the engine has been overheated for any reason: poor cooling, lubrication, too high combustion temperature. Burn marks on the skirt are a sign of gas leakage from the combustion chamber, possibly caused by wear on the cylinder walls or piston rings. Burn marks or pitting on the piston crown are usually signs of an engine running with detonation caused by pre-ignition or low-quality gasoline. For the same reason, in some cases, the piston head may be melted. Re-leaning the mixture due to air leakage at the inlet can also cause the piston to overheat.
All causes of the listed damage must be eliminated before the engine is put into operation, otherwise similar damage to the piston group will occur again.
Figure 3.56. Measuring the piston diameter with a micrometer: E= (42±0,01) mm
Using a micrometer, measure the diameters of all four pistons at a distance (42±0,01) mm from piston crown (Figure 3.56) perpendicular to the axis of the piston pin. Compare results of measurements with specified in tab. 3.4.
Table 3.4. The dependence of the piston diameter on the cylinder diameter
If the piston diameter is out of tolerance, replace the entire piston kit. It should be borne in mind that the cylinder block may have been bored out during a previous overhaul, and oversized pistons may have been installed accordingly. Record the measurement results and use them to check the clearances later when the cylinder diameters are measured.
Measure the gaps between the piston rings and the piston grooves, to do this, install the rings in the grooves and, using a set of flat feelers, take the appropriate measurements in three or four places in each groove. The measured dimensions must not exceed 0.1 mm, otherwise the pistons will need to be replaced.
Carefully inspect the connecting rods, pins, and connecting rod caps for wear, cracks, or other surface damage. Visually check the following:
- condition of connecting rods (they can be twisted or bent);
- density of fit of bearing caps on connecting rods (if necessary, use an emery board to remove burrs to ensure that the caps fit properly).
Figure 3.57. Piston ring gap measurement
Before putting the rings on the pistons, install them one by one in your cylinder and check the gap in the ring lock. Push the ring into the cylinder with the piston so that the plane of the ring is parallel to the top plane of the block (Figure 3.57). Compare the measurement results with those given in Table. 3.3.