Note: During a major engine overhaul, piston rings must be replaced.
2. Carefully spread the rings above the piston and remove them upwards. Be careful - piston rings are fragile and sharp; if stretched too much, they can break and scratch the piston or cut your hands. Using two or three flat feeler gauges will make the job easier and prevent the rings from falling into empty piston grooves and scratching still-good pistons (see Figure 15.2).
Figure 15.2. Carefully spread the rings above the piston and remove them upwards.
Third (oil scraper) the ring contains an expander.
3. Scrape off all traces of carbon deposits from the piston crown. A hand wire brush or a piece of sandpaper will help remove the bulk of the carbon buildup.
4. Remove carbon deposits from the piston grooves using old rings or their fragments. Remove only carbon deposits, do not remove metal or scratch the grooves. Take care of your hands - wear gloves.
5. After finishing cleaning the pistons, wash the pistons/connecting rods with kerosene or another suitable solvent and dry them thoroughly. Make sure that the oil passage holes in the piston grooves and the oil holes in the connecting rods are not clogged with carbon deposits (see Figure 15.5).
Figure 15.5. Check and, if necessary, clean the oil passage hole in the connecting rod housing.
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6. If the cylinder surface is not significantly damaged and the block does not require boring (or replacement of sleeves), you can install the old pistons. Tolerable piston wear appears as uniform vertical bald spots on the friction surface and slight loosening of the upper compression ring in the piston groove. When assembling an engine, mechanics who respect their work always install new piston rings.
7. Carefully inspect the pistons - there should be no cracks near the bosses, on the skirt and in the piston partitions.
8. Check the pistons for scuffs on the rubbing surfaces, burnouts on the bottom and partitions. If the piston has scoring or pitting, the engine may have been overheating and/or the fuel may not be burning properly, causing extremely high combustion chamber temperatures. In such cases, it is necessary to check the cooling and lubrication systems. Burn-through on the piston skirt indicates that the rings were not sealing the piston/cylinder gap. Burn-through on the piston crown indicates possible incorrect ignition timing, injection timing, or engine detonation. If the above problems exist, the cause must be eliminated, otherwise the engine will soon fail again. In gasoline engines, these reasons can be caused by an over-lean mixture due to air being sucked in at the intake, incorrect ignition timing, and a malfunction in the exhaust gas recirculation system. Faulty injectors may also be to blame.
9. Piston corrosion in the form of pitting indicates coolant penetration into the combustion chamber and possibly the crankcase. These problems must be corrected, or they will persist even in a rebuilt engine.
10. If you have any doubts about the condition of the pistons and connecting rods, contact professionals at a specialized workshop. They will carry out the necessary checks and measurements. If you need to purchase new pistons/rings, they will help you select repair sizes and also perform boring (if possible) or cylinder honing.