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
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
(NOTE) You will always see some carbon under the piston and much more on inverted
Symptom - The piston crown and cylinder head show signs of damage where material has
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.
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
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
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
Multi point seizure (The most common in aircraft
Symptom - Piston has multiple vertical seizure marks around its
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
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
Hole in piston top
Symptom - Hole in the piston crown, collapsed ring grooves on the exhaust
Cause - Piston has severely overheated. As the temperature is higher on the exhaust
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.
Symptom - Deposit on piston skirts
Cause - Piston rings worn past the maximum ring end gap spec, glazed cylinder
This is the most common problem we see. If the rings are worn past the maximum
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.