CAUTION: Install new piston rings whenever the engine is disassembled.
Using ring installers, carefully remove the old rings from the pistons. Be careful not to scratch or damage the pistons in the process.
Clean all traces of carbon from the top of the piston using a hand wire brush or sandpaper. For heavy carbon buildup, a piece of an old piston ring can be used.
The text of the article was obtained from the website MitsubishiMan.ru
Never use a wire brush attached to a drill, as it may damage the piston, which is made of soft material.
Using a piece of old piston ring, clean away any carbon deposits in the ring grooves. Clean away only the carbon deposits and do not remove metal or scratch the sides of the grooves.
After you have cleaned off the carbon deposits, clean the pistons and connecting rods with solvent and dry them. Make sure the oil return holes on the sides of the ring grooves are clean
If the pistons and cylinder walls are not too worn or damaged, and if the cylinders have not been rebored, old pistons can be used. Normal piston wear shows up as vertical wear stripes on the piston bearing surfaces and a slight slack in the top ring groove.
Check for nicks and scratches on the skirt bearing surfaces, in the piston pin bore, and for burnt areas on the edge of the head.
If the skirt is scratched or worn, the engine may be suffering from overheating and/or abnormal combustion, which leads to excessively high operating temperatures. Such pistons should be replaced. The lubrication and cooling systems should be carefully checked. A hole in the piston crown indicates abnormal combustion (premature (early) ignition). Burnt areas on the edge of the crown usually indicate detonation. If any of these problems occur, their causes must be eliminated, otherwise the damage will recur. Causes may include intake air leaks, improper air-fuel mixture, incorrect ignition timing, and improper operation of the exhaust gas recirculation system.
Typical burnt piston due to abnormal combustion
Piston corrosion in the form of small pits indicates that coolant is leaking into the combustion chamber and/or crankcase. This fault should also be corrected.
Measure the piston ring lateral clearance by placing a new ring (2) in each groove and inserting a feeler gauge (1) next to it. Measure the clearance at three or four points along the diameter of each groove. Make sure you are using the right ring - they are different. If the lateral clearance is greater than the permissible value, the piston must be replaced.
Check the clearance between the piston and the cylinder wall by measuring the cylinder bore diameter and the piston diameter. Measure the piston diameter across the skirt coaxially and at an angle of 90° to the piston pin. The clearance is determined as the difference between the diameters of the cylinder bore and the piston. If the clearance is greater than the permissible bore, the cylinders should be bored and new pistons and rings should be used.
Check the clearance between the piston and connecting rod by sliding them in opposite directions. Any noticeable play means there is excessive wear that must be corrected. The pistons, along with the connecting rod, should be machined and new rings of the oversize installed.
Check the connecting rod for warpage. To do this, secure the connecting rod tightly to the fixture and measure the deviation of the connecting rod from the plane. If there is a deviation, the connecting rod must be replaced, selecting it according to weight.
ATTENTION: The difference in weight in a set of connecting rods should not exceed 4 grams.
Check the connecting rods for cracks and other damage. Periodically remove the connecting rod caps, pull out the old bearing shells, wipe the bearing surfaces in the connecting rods and caps clean, and check for scratches, burrs, etc. After checking the connecting rods, replace the old bearing shells, install the caps, and tighten the nuts with your fingers.