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PISTON
A cylindrical, hollow aluminum or iron
part, closed on top and open at the bottom, fitting closely
within the engine cylinder or sleeve and capable of being
driven alternately up and down in the cylinder. The piston
transmits the force of expanding combustion gases through
the piston pin to the connecting rod and crankshaft throws.
The piston serves as a carrier for the piston rings
.
A) LAND
That part of the piston above the
top ring or between ring grooves. The lands confine and
support the piston rings in their grooves.
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B) HEAT DAM
A narrow groove cut in the top
land of some pistons to reduce heat flow to the top ring
groove. This groove fills with carbon during engine operation
and reduces heat flow to the top ring.
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C) COMPRESSION DISTANCE (or height)
The distance from the center of
the pin hole to the top of the piston. where grooves are
machined.
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D) RING BELT
That area between the top of the
piston and the pin hole for the installation of piston
rings.
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E) PISTON HEAD
The top piston surface against
which the combustion gases exert pressure. The piston
head may be flat, concave, convex or of irregular shape.
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F) PISTON PINS (Wrist pins or gudgeon pins)
Connections between the upper end
of the connecting rod and the piston. Pins may be held
in one of three ways:
1. Anchored in the piston with
the bushing in the upper end of the connecting rod oscillating
on the pin.
2. Clamped in the rod with the
pin oscillating in the piston.
3. Full floating in both connecting
rod and piston with lock rings or other devices preventing
the pin from contacting the cylinder wall.
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G) SKIRT
That part of the piston located
between the first ring groove above the pin hole, and
the bottom (open end) of the piston. The skirt forms a
bearing area in contact with the cylinder wall.
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H) PIN HOLE
An opening through the piston skirt
to carry the piston pin.
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I) MAJOR THRUST FACE
That portion of the piston skirt
which carries the greatest thrust load. This is on the
right side when viewing the engine from the flywheel end
with the crankshaft rotating counterclockwise.
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J) MINOR THRUST FACE
That portion of the piston skirt
which is opposite the major thrust face.
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K) PISTON PIN BUSHING
A bushing fitted between piston
pin and piston pin hole to obtain a better bearing material.
Used particularly with iron pistons.
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L) OIL RING GROOVE
A groove cut into the piston around
its circumference, at the bottom of the ring belt or at
the lower end of piston skirt. Oil ring grooves are usually
wider than compression ring grooves and generally have
holes or slots through the bottom of the groove for oil
drainage to the interior of the piston.
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M) COMPRESSION RING GROOVE
A groove cut into the piston around
its circumference, in the upper part of the ring belt.
The depth of groove varies depending on piston size and
types of rings used.
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N) SCUFF BAND
A narrow raised band or bands machined
on the top land of some pistons to reduce the possibility
of scuffing.
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O) GROOVE DEPTH
The distance from the cylinder
wall to the bottom of the ring groove with the piston
centered in the cylinder.
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P) GROOVE ROOT DIAMETER
Diameter of the piston measured
at the bottom of the groove. The root diameter of each
groove on any given piston may differ, depending on the
type of ring to be installed.
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Q) LAND DIAMETER
Diameter of the land being measured.
In some piston designs all lands are equal diameter. In
others they increase from top to bottom.
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R) LAND CLEARANCE
The difference between the diameter
of the land and cylinder diameter. (R shows 1/2 of total
clearance)
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S) SKIRT CLEARANCE
The difference between piston skirt
diameter measured in a plane perpendicular to the piston
pin) and cylinder diameter. (S shows 1/2 of total clearance)
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T) SKIRT GROOVE
A groove cut into the piston around
its circumference below the pin hole to carry an oil ring.
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U) OFFSET PIN HOLE
On some pistons the pin hole is
offset to one side of the piston centerline.
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V) TOP GROOVE SPACER
Badly worn top grooves must be
remachined before installation of new rings. A steel spacer
is installed above the ring in a reconditioned groove
to reduce the side clearance to the recommended dimension.
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PISTON SKIRT TAPER - The difference
between the diameter of the piston at the top of the skirt
and at the bottom of the skirt with the diameters being measured
in the thrust direction.
PISTON CAM - The circumferential shape
in which a piston skirt is manufactured to provide proper
cylinder contact and running clearance under all normal conditions
of temperature and load.
CAST-IN GROOVE INSERT-A steel or cast
iron insert, in an aluminum piston, either chemically or mechanically
bonded during the manufacturing process, into which the top
groove is machined to provide a longer wearing surface for
the top ring
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(P3)
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