Piston Wear - Husqvarna K770 Workshop Manual

6.9 Piston wear

The cause of engine failure can be hard to find if the
previous operation and service of the product is not
available. Use the following examples for help.
6.9.1 Usual wear
Usual wear is easiest to see on the piston sections that
point at the exhaust and inlet sides. The picture shows
that the piston has been polished to give a bright
surface. The lines from the production can also be seen.
Oil on the piston rings shows correct lubrication. The
piston rings can move freely in the piston ring groove.
6.9.2 Dust
If dust enters the engine, the engine life decreases. The
effect of dust can be seen with a magnifying glass.
Rounded lines in line with the travel of the piston are
clear signs. The lines after production cannot be seen.
The surface is matt.
The usual cause is a bad filter and/or leakage. Examine
the condition of the filters and gaskets. Also examine the
rubber guides between the cylinder, the carburetor and
the connections. Look for dust from the filter units to the
inlet by the cylinder. Carbon particles on the top of the
piston show that the product is used at short intervals.
The engine has not become fully warm.
701 - 002 -
6.9.3 Scoring
Scoring occurs when the engine is too hot. The scores
are usually on the exhaust side, which is the hottest.
The inlet side can show equivalent damage.
Examine the product to see if it has broken down
because of an incorrect oil mixture, or no oil at all. If
there is oil on a piston that is too hot it can carbonize. If
not, check if the connecting rod or the crankcase has a
thin layer of oil.
If there is sufficient oil, inlet leakage can be the cause.
When the engine leaks air on the inlet side, the fuel/air
mixture changes. This causes scoring, mainly on the
exhaust side. Look for scoring damage in combination
with a piston top that has become too hot.
6.9.4 Too hot piston top
If the engine has been too hot, aluminum particles are
frequently found on the top of the piston. This can cause
the piston to melt.
Too much air in relation to the fuel volume increases the
temperature in the combustion chamber. Examine if the
inlet system is blocked. Examine that the fuel supply has
not been blocked by a defective fuel hose, tank vent or
the pulse hose.
Gasoline with a too low octane grade can cause ignition
at the incorrect position. Spark plugs with incorrect heat
rated value can cause the same problem. They can
increase the temperature in the combustion chamber.
Make sure that the key on the flywheel of the crankshaft
is not damaged. The flywheel position on the crankshaft
controls the ignition point.
Repair and servicing - 49