When assembling the engine, always use new rings.
1. Carefully, using a special tool, remove the rings, pay attention to the mark (arrow) the top of the ring. Be careful not to leave nicks or dents on the pistons.
2. Remove any deposits from the piston crown. After the bulk of the carbon has been removed with a scraper, a hand wire brush or a piece of fine sandpaper can be used. Under no circumstances use a wire brush in an electric drill to remove carbon deposits from pistons. The piston is made of soft material and can be damaged by it.
3. Use a groove cleaner to remove carbon deposits from the ring grooves. Be very careful and only remove carbon deposits - do not remove metal or leave nicks or scratches on the side walls of the grooves.
If a groove cleaner is not available, then this job can be done with a piece of an old piston ring, however, take care of your hands - the rings can be sharp.
4. After removing carbon deposits, wash pistons / connecting rods with solvent and dry with compressed air (if available). Make sure the oil return holes in the sides of the ring grooves are clean.
5. If the cylinder walls and pistons are not damaged or excessively worn, and if the cylinder block has not been ground, then new pistons will not be required. Normal piston wear is manifested as uniform wear of the thrust surfaces (at a 90°angle to the finger hole) and in the form of a slight play in the groove of the upper ring. However, when rebuilding an engine, always install new rings.
6. Carefully inspect each piston for cracks around the skirt, on the pin bosses, and in the ring grooves.
7. Inspect skirt thrust surfaces for gouges and wear, bottom for holes, and bottom edge for burnt areas. If the skirt is striated or worn, the engine may have been overheated and/or there is an abnormal compression ratio that causes elevated operating temperatures.
8. Carefully check the cooling system and lubrication system. A hole in the piston crown indicates that there was an abnormal compression ratio (early ignition). Burn marks on the edge of the bottom are usually a sign of explosive combustion (detonation). If any of these problems exist, then the cause must be eliminated, otherwise the malfunction will occur again. The fault may be intake manifold leaks, incorrect fuel/air mixture, or incorrect ignition setting. On later models with complex exhaust control systems involving catalytic converters, the problem may originate from the EGR system (exhaust gas recirculation), if applied.
9. Piston corrosion, which looks like small depressions, indicates penetration into the combustion chamber and / or cooler crankcase, in the same way, the cause must be eliminated, otherwise the problem will arise again.
10. Measure the piston ring backlash by installing a new piston ring in the groove and inserting a feeler gauge between it and the groove side.
11. Check clearance in three or four places around each groove. Make sure you check the correct ring - rings are different. If the clearance is greater than the allowable value, the piston must be replaced.
12. Check the piston clearance in the cylinder by subtracting the piston diameter from the cylinder diameter. Make sure the pistons and cylinders fit together. Measure the piston diameter at skirt level along the pin and at a 90°angle to it.
13. Any differences in these measurements are indicative of ovality. Subtract the piston diameter from the cylinder diameter to get the clearance. If the gap is more than acceptable, the block should be ground and new pistons and rings should be installed.
14. Check up backlash between the piston and a rod, turning them in opposite directions. Any noticeable play is an indication of excessive wear that must be corrected. The piston/connecting rod assembly must be sent to a workshop for inspection. After repair, check the connecting rods for bending and twisting, as most machine shops have special equipment for this.
If new pistons and/or connecting rods are not being installed, do not separate the pistons from the connecting rods.
15. Check up in rods existence of cracks and other damages. Temporarily remove the connecting rod bearing caps, remove the old bearing shells, wipe the bearing surfaces in the connecting rod and in the cap, and inspect them for nicks, dents, and scratches. After checking the connecting rods, install new bearings, put the caps in place and tighten the nuts by hand.