This piston is 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 color 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 required.

Always check piston to cylinder wall clearance to make certain the wear limit has not been exceeded.

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.

This piston is 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 color 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 required.

Always check piston to cylinder wall clearance to make certain the wear limit has not been exceeded.

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.

Seized Pistons

The piston shows yellow spots on the dome and seizure marks on the skirt but there is no sign of metal aluminum.

If you inspect an engine and find yellow or yellow-orange deposits on the piston dome, that engine has experienced an abnormal combustion condition known as detonation. In this situation, combustion begins normally with ignition from the spark plug but as the flame front moves across the combustion chamber, rather than burning smoothly, the unburned portion of the charge spontaneously ignites. This results in extremely high temperatures and a shock wave heard as "pinging." What has happened is combustion has been completed in about 29 degrees or crankshaft rotation instead of the normal 50.

The oxide of calcium, an element found in 2 - stroke oil additives, is normally near white in colour. At temperature very near that of the melting point of the piston, the oxide will change colour from white to yellow-orange and is an indicator that the engine was overheated. The excessive heat results in rapid piston expansion and possible failure of the lubrication film.

Possible causes:

· too low an octane fuel used or fuel with too much alcohol used

· jetting too lean or failure of the fuel system, i.e., clogged fuel line or filter, fuel tank not venting, problem with fuel pump, carburetor,m air leak into crankcase

· spark plug heat range too hot

· ignition timing too far advanced, possible failure of the c.d. amplifier box

· restricted exhaust system: back pressure too high

· overheated, loosely-installed spark plug

Seized Pistons

The piston shows yellow spots on the dome and seizure marks on the skirt but there is no sign of metal aluminum.

If you inspect an engine and find yellow or yellow-orange deposits on the piston dome, that engine has experienced an abnormal combustion condition known as detonation. In this situation, combustion begins normally with ignition from the spark plug but as the flame front moves across the combustion chamber, rather than burning smoothly, the unburned portion of the charge spontaneously ignites. This results in extremely high temperatures and a shock wave heard as "pinging." What has happened is combustion has been completed in about 29 degrees or crankshaft rotation instead of the normal 50.

The oxide of calcium, an element found in 2 - stroke oil additives, is normally near white in colour. At temperature very near that of the melting point of the piston, the oxide will change colour from white to yellow-orange and is an indicator that the engine was overheated. The excessive heat results in rapid piston expansion and possible failure of the lubrication film.

Possible causes:

· too low an octane fuel used or fuel with too much alcohol used

· jetting too lean or failure of the fuel system, i.e., clogged fuel line or filter, fuel tank not venting, problem with fuel pump, carburetor,m air leak into crankcase

· spark plug heat range too hot

· ignition timing too far advanced, possible failure of the c.d. amplifier box

· restricted exhaust system: back pressure too high

· overheated, loosely-installed spark plug

Piston dome damage


The piston shows melted areas on the dome and seizure marks on the skirt.

Detonation results in extremely high combustion chamber temperatures. That rapid heat rise, if allowed to continue, will heat combustion chamber deposits and spark plug electrodes to the point that they will ignite the charge before the spark at the plug does.

This condition is called runaway surface ignition or pre-ignition. Once pre-ignition occurs, the temperature rise is so rapid that melting point of the piston is reached while the engine is still running. Metal will melt away beneath the spark plug or in areas of high heat retention such as at the ring positioning pin.

A complete breakdown of the lubrication film also occurs resulting in severe seizure. Detonation always precedes pre-ignition and the causes for this abnormal combustion are the same as those for detonation.

Piston dome damage


The piston shows melted areas on the dome and seizure marks on the skirt.

Detonation results in extremely high combustion chamber temperatures. That rapid heat rise, if allowed to continue, will heat combustion chamber deposits and spark plug electrodes to the point that they will ignite the charge before the spark at the plug does.

This condition is called runaway surface ignition or pre-ignition. Once pre-ignition occurs, the temperature rise is so rapid that melting point of the piston is reached while the engine is still running. Metal will melt away beneath the spark plug or in areas of high heat retention such as at the ring positioning pin.

A complete breakdown of the lubrication film also occurs resulting in severe seizure. Detonation always precedes pre-ignition and the causes for this abnormal combustion are the same as those for detonation.

Piston dome damage


The piston shows melted areas on the dome and seizure marks on the skirt.

Detonation results in extremely high combustion chamber temperatures. That rapid heat rise, if allowed to continue, will heat combustion chamber deposits and spark plug electrodes to the point that they will ignite the charge before the spark at the plug does.

This condition is called runaway surface ignition or pre-ignition. Once pre-ignition occurs, the temperature rise is so rapid that melting point of the piston is reached while the engine is still running. Metal will melt away beneath the spark plug or in areas of high heat retention such as at the ring positioning pin.

A complete breakdown of the lubrication film also occurs resulting in severe seizure. Detonation always precedes pre-ignition and the causes for this abnormal combustion are the same as those for detonation.

 Piston shows imprints of small cylindrical parts on the dome

Possible Cause

· needles of broken wrist pin bearing trapped in the squish area between the piston and head, destroy the dome, ring land and rings.

· make certain to check the condition of the big end rod bearing and cage

 Small ball shaped imprints on piston dome

Possible Cause

· foreign material, e.g. a pop rivet head, has entered the engine and was trapped in the squish area, before leaving through the exhaust port. In some cases the engine continues to run normally until the ring fails

Piston shows a vertical groove on the skirt connected to the wrist pin hole.

Possible cause:

· A wrist pin circlip has come loose and was trapped in the transfer port window.

· Note always use NEW circlips when rebuilding an ultralight aircraft engine.

Piston shows scoring marks below the rings on the intake side.

Possible Cause

· Snow or water has entered the engine and destroyed the oil film

                · if the piston has scoring on the intake and exhaust sides but the dome                 shows no signs of

abnormal combustion, the engine has been run with too little or no oil. Check the operation of the oil injection system or the amount of oil in the premixed fuel. Also, insure proper piston clearance.

· if the piston has scoring on the intake and exhaust sides, no signs of abnormal combustion on the dome and is very dark in color, there has been a failure of the cooling system. Check the fan belt, or collant level etc.

  Piston shows dark brown deposits all over the skirt.

Possible Cause

· wrong oil quality, e.g. gear oil, automotive oil

· unnecessary additives have been used in the fuel, i.e. octane boosters, high performance additives, upper cylinder lubricants, etc.

Piston shows scuffing marks on the exhaust side of the skirt without corresponding marks on the intake side. The lubrication film failed under thermal load on the exhaust side of the piston

Possible cause:

· Poor quality of oil.

Possible Cause

· improper break in period

· poor oil quality

· not enough oil in premixed fuel

· low delivery rate from oil injection system

· connecting rod breakage

· damage due to bad handling,

· crankshaft locking via piston blocking

· engine has ingested foreign material