EXPLANATION OF PISTON AND RING TERMS
Clear, accepted definitions are as necessary to a knowledge and understanding of
pistons and piston rings as they are for any other subject. The following nomenclature has
been developed in over half-a-century of usage within the industry. The majority of these
terms have been adopted and approved by the Society of Automotive Engineers.
PISTON TERMS are defined in Figures 1 and 2. Click on a letter to see
the definition.
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Figure 1 |
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Figure 2 |
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
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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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Hastings
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