On a distorted cylinder, during operation, uneven contact patterns are created resulting in highly polished areas on the dry cylinder liner.
Spotted, uneven contact patterns on the outer wall of the cylinder liner and in the cylinders is always an indication of the cylinder distortion.
If the cylinder is distorted the piston rings can neither provide a perfect seal for the oil nor for combustion gasses. This means the piston rings cannot scrape of the oil at the points where the cylinder is distorted. As the oil accumulates the oil enters the combustion chamber where it is burned off.
Lack of correct sealing of the piston rings also results in increased pressure in the crankcase due to the combustion gasses flowing past the piston rings. This excess pressure will create oil leaks at the seals and intake valve guides.
Typical causes for cylinder distortion:
Uneven and/or incorrect tightening of the cylinder head
Deposits or dirt in the cooling system (creating patches of uncooled areas leading to distortion)
Dirty or distorted threads on the cylinder head bolts
Unsuitable cylinder head gaskets
Faulty shoulder supports
Contact corrosion (frictional corrosion)