NORMAL PISTON

Ideal Condition   

The crown of the piston shows an ideal carbon deposit pattern. The transfer ports on the engine are 

flowing evenly and the carbon deposits are a dark chocolate brown color. This indicates that the  

engine is running great.

 

 

Carbon on underside of piston 

Symptom - Underside of the piston has a carbon build up. 

 

Cause - The black spot is a carbon deposit caused by oil burning onto the underside of the piston because the crown was too hot. Main reasons for this occurring are overheating due the carbs being jetted too lean, or maybe coolant system failure. This piston is normal.

(NOTE) You will always see some carbon under the piston and much more on inverted engines.

 

 

 

 

 

 

 

 

 

Debris Marks 

Symptom - The piston crown and cylinder head show signs of damage where material has been 

crushed between the two surfaces after entering the combustion chamber.  

 

Cause - Most common causes are broken bearings in the big or small end of the con rod, broken rings and dislodged ring centering pins. Failure to adhere to Rotax's specified service intervals, incorrectly fitted rings, incorrect parts fitted. When this does occur it's extremely important to diagnose the exact cause and trace where the debris has originated from. Crankcases must be stripped and flushed to remove any remaining debris that could cause the same problem. If the cause was big end failure then the crank must be replaced along with the main bearings and seals. 

 

 

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Shattered Skirt 

Symptom - Shattered piston skirt.  

 

Cause - Piston to cylinder clearance too large or engine  over-revved.  

If the piston to cylinder clearance is too large it allows the piston to rattle inside the cylinder 

bore, with time the piston develops stress fractures and eventually cracks. The piston to cylinder clearance should be checked every time you rebuild the top end. To check the clearance measure your cylinder with a bore gauge and micrometer, then measure the piston  and check the readings against the limits in the Rotax service manual.  

 

 

 

 

 

Four corner seizure 

Symptom - The piston has vertical seizure marks at four equally spaced points around the circumference. 

 

Cause - A four corner seizure is caused when the piston expands faster than the cylinder and the 

clearance between the piston and cylinder is reduced. The main reasons for this problem are too quick warm-up, too lean carb jetting (main jet), or a spark plug with too high a heat setting being used. 

 

 

     

 

 

 

Multi point seizure (The most common in aircraft pistons) 

Symptom - Piston has multiple vertical seizure marks around its circumference. 

 

Cause - This occurs if the piston was heated too fast expanding quickly before the cylinder is hot enough. 

As soon as the engine is started and the piston started its thermal expansion, the piston pressed up 

against the cylinder walls and seized. The optimum piston to cylinder wall clearances is for different sizes of engines For the clearance for your engine look to the specs that are listed in the Rotax service manual. 

 

 

 

 

Intake side seizure 

Symptom - Piston has seized on the intake side 

 

Cause - There is only ever one cause of a seizure on the intake side and that is loss of lubrication 

Reason - Loss of lubrication is down to three things - no pre-mix or two-stroke oil due to incorrect pre-mix ratio being used or oil pump failure/blockage, separation of the fuel and pre-mix oil in the fuel tank, 

water passing through the air filter and down into the carbs which in turn washes the oil film off the 

piston skirt. 

 

 

Hole Piston

 

 

 

Hole in piston top 

Symptom - Hole in the piston crown, collapsed ring grooves on the exhaust side 

 

Cause - Piston has severely overheated. As the temperature is higher on the exhaust side problems 

will appear there first. Air leak on the magneto side of the crankshaft seal, too lean carb jetting, ignition timing advanced too far or faulty ignition box, too hot of a spark plug range being used, too high of a compression ratio, or too low octane fuel. 

 

 

 

 

Blow by 

Symptom - Deposit on piston skirts 

 

Cause - Piston rings worn past the maximum ring end gap spec, glazed cylinder walls 

This is the most common problem we see. If the rings are worn past the maximum specification 

they allow combustion chamber gases to seep past the rings and down the piston skirt causing a distinct 

deposit pattern. If the cylinder walls are glazed or too far worn the same thing will happen and even fitting new rings will not provide a good enough seal.