piston damage causes 2

7/29/2019 Piston Damage Causes 2

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This piston is NORMAL in appearance.The deposits on the piston dome are fromthe 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


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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


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ration and reduce heat transfer.

Seized PistonsThe 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 theflame 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 failureof the lubrication film.Possible causes: too low an octane fuel used or fuel with too


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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 PistonsThe 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 inextremely 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


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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 ofthe 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 toomuch 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


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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 surfaceignition 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 lubricationfilm 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.


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Piston dome damage




















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Piston dome damage




















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Piston damage by foreign materialPiston shows imprints of small cylindrical

parts on the domePossible 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

Piston damage by foreign materialSmall ball shaped imprints on piston domePossible Cause foreign material, e.g. a pop rivet head, has

entered the engine and was trapped in the

squish area, before leaving through theexhaust port. In some cases the engine

continues to run normally until the ring fails


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Piston damage by foreign materialPiston 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 with lubrication failure

(no signs of detonation on dome)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 ofabnormal 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


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cooling system. Check the fan belt, or

collant level etc.

Piston with lubrication failure

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 inthe fuel, i.e. octane boosters, high

performance additives, upper cylinder

lubricants, etc.

Piston with lubrication failurePiston 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 pistonPossible cause: Poor quality of oil.


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Piston ring scuffing

Cylinder liner shows scoring areas, which are

exactly limited to the ring travel zone.

Beside the blue colored vertical stripes, thesurface is in good condition. The rings show

corresponding dark colored marks. The

reason is a breakdown of the lubrication.Possible Cause improper break in period poor oil quality not enough oil in premixed fuel low delivery rate from oil injection system

Piston fracture

Material fatigue fracture can sometimes e

observed on pistons of high speed engines,however, skirt fracture is usually the result

of excessive piston to cylinder wall

clearance.

Other possible causes: connecting rod breakage damage due to bad handling, crankshaft locking via piston blocking engine has ingested foreign material

piston damage causes 2

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