The process of examining a used piston can tell a mechanic helpful information on the condition of an engine. When an engine failure occurs, the piston is likely to take the brunt of the damage. A careful examination of the piston can help a mechanic trace the source of a mechanical or tuning problem. This technical article serves as a guide for the most common mechanical problems that plague engines. The pistons shown in the pictures came from motorcycle engines but the principles are the same.
Perfect brown crown
The crown of this piston shows an ideal carbon pattern. The colour of
the carbon pattern is chocolate brown. That indicates that this engine's
carburettor is jetted correctly.
Black spot hot
The underside of this piston has a black spot. The black spot is a
carbon deposit that resulted from oil burning on to the piston because
the piston's crown was too hot. The main reasons for this problem are
overheating due to too lean carb jetting or coolant system failure.
Ash trash
This piston crown has an ash colour, which shows that the engine has run
hot. The ash colour is actually piston material that has started to
flash (melt) and turned to tiny flakes. If this engine was run any
longer, it probably would've developed a hot spot and hole near the
exhaust side and failed. The main causes of this problem are too lean
carb jetting, too hot spark plug range, too far advanced ignition
timing, too much compression for the fuel's octane, or a general
overheating problem.
Smashed
debris
This piston crown has been damaged because debris entered the combustion
chamber and was crushed between the piston and the cylinder head. This
engine had a corresponding damage pattern on the head's squish band. The
common cause of this problem are broken ring ends. When a problem like
this occurs, its important to locate where the debris originated. Also
the crankcases must be flushed out to remove any left over debris that
could cause the same damage again.
Chipped
Crown Drowned
This piston crown chipped at the top ring groove because of a head
gasket leak. The coolant is drawn into the combustion chamber on the
down-stroke of the piston. When the coolant hits the piston crown it
makes the aluminium brittle and it eventually cracks. In extreme cases
the head gasket leak can cause erosion at the top edge of the cylinder
and the corresponding area of the head. Minor leaks of the gasket appear
as black spots across the gasket surface. An engine that suffers from
coolant being pressurized and forced out of the radiator cap's vent
tube, is a strong indication of a head gasket leak. In most cases the
top of the cylinder and the face of the cylinder head must be resurfaced
when a leak occurs.
Shattered
skirt
The skirts of this piston shattered because the piston to cylinder
clearance was too great. When the piston is allowed to rattle in the
cylinder bore, it develops stress cracks and eventually shatters.
Four-corner
seizure
This piston has vertical seizure marks at four equally spaced points
around the circumference. 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 causes for this problem are too
quick warm-up, too lean carb jetting (main jet), or too hot of a spark
plug range.
Multi-point
seizure
This piston has many vertical seizure marks around the circumference.
This cylinder was bored to a diameter that was too small for the piston.
As soon as the engine 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 for different types of
cylinders vary greatly. For the best recommendation on the optimum
piston to cylinder clearance for your engine, look to the specs that
come packaged with the piston or consult your supplier.
Burnt out blow-hole
This piston was overheated so badly that a hole melted through the crown
and collapsed the ring grooves on the exhaust side. Normally the piston
temperature is higher on the exhaust side so catastrophic problems will
appear there first. There are several reasons for a failure like this,
here are the most common; air-leak, too lean carb jetting or too far
advanced ignition timing
Blow-by
This piston didn't fail in operation but it does show the most common
problem, blow-by. The rings were worn past the maximum ring end gap
spec, allowing combustion pressure to seep past the rings and down the
piston skirt causing a distinct carbon pattern. Its possible that the
cylinder walls cross-hatched honing pattern is partly to blame. If the
cylinder walls are glazed or worn too far, even new rings won't seal
properly to prevent a blow-by problem.
This article, by Malcolm Watts, originally appeared in CA7C Seven Focus in Jan 2006 pp16-18.
See also:
Crankshaft
Conversion
Every Piston
Tells a Story
Front Main Bearing
Retaining Lip
Gearbox Oil
Seals
Good Sump
Joints
Little End Bolt Failure
Replacing a
3-speed gearbox with 4-speed
Synchromesh
Set-up
Wear in Pistons,
Cylinders and Rings