Pistons: Cast vs. Forged
GLM offers an extensive line of pistons for all
major brands and models of outboard engines. GLM pistons, like OEM
pistons, are cast as opposed to forged.
We have all heard the advantages of forged pistons.
However, when you consider the disadvantages of forged pistons it
becomes very clear why GLM cast pistons are the ultimate pistons
for your rebuilding and repair projects.
The major disadvantages to forged pistons are
actually a result of the forging process itself. Forging results
in a piston that is considerably heavier, than cast pistons, and
is limited in the aluminum alloys that can be used to produce the
piston. Additionally, the forging process also limits the design
configuration of the piston itself.
The forging process and its limited choice of
aluminum alloys result in a dramatically higher expansion rate for
the forged piston. This means that the set up characteristics are
very different from the original engine manufacturers' and that
break-in and warm up periods are crucial for the forged piston.
An example of the dramatic expansion of forged
pistons is the piston skirt clearances in the cylinder. A typical
forged piston has a skirt clearance of approximately .0007 to .0009.
A cast piston has a skirt clearance of .005 to .007. The forged
piston has 10 times more slop in the cylinder. This results in less
ring stability against the cylinderical wall, more piston noise
and extra blowby.
In all fairness, after the forged piston has reached
operating temperature, its dramatic expansion makes up for these
extra clearances. However, this should remind us of the typical
customer that a dealer services. Can we expect the casual weekend
boater to strictly observe the extended break in period and the
critical warm up procedures required for a forged piston? Let your
own experience answer this question.
Most forged pistons are quality products, but
they are better suited to racing and professional applications.