These two pistons are NORMAL in appearance.
The deposits on the piston dome are from the oil and ash, fuel
contaminants and unburned carbon. As the thickness increases with operating time, the
colour gets brighter because of higher surface temperatures.
Depending on gasoline and oil used, normal deposits may also be brown, from light tan
colour to almost black dark brown.
Heavy black deposits on the piston dome are unburned carbon accumulated because of too
low temperatures resulting from light load operation or too rich a mixture.
Brown or black varnish on piston side below rings is baked oil caused by combustion
gases blow-by. Improper oil or poor ring sealing is the cause.
Very light scoring of the piston skirt is likely from foreign material passing through
the engine. It does not suggest a problem in the engine and piston replacement is not
Always check piston to cylinder wall clearance to make certain the wear limit has not
Deposits should be removed from the piston dome and cylinder head using a wood or
plastic scraper. Excessive deposits collection will increase the compression ration and
reduce heat transfer.