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 (SAE)
Figure 1Figure 2
A cylindrical, hollow aluminum, steel 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
That part of the piston above the top ring or between ring grooves. The lands confine and support the piston rings in their grooves. Return to the figures.
The distance from the center of the pin hole to the top of the piston. Where grooves are machined. Return to the figures.
That area between the top of the piston and the pin hole for the installation of piston rings. Return to the figures.
The top piston surface against which the combustion gases exert pressure. The piston head may be flat, concave, convex or of irregular shape. Return to the figures.
Connections between the upper end of the connecting rod and the piston. Pins may be held in one of three ways:
- Anchored in the piston with the bushing in the upper end of the connecting rod oscillating on the pin.
- Clamped in the rod with the pin oscillating in the piston.
- Full floating in both connecting rod and piston with lock rings or other devices preventing the pin from contacting the cylinder wall. Return to the figures.
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. Return to the figures.
An opening through the piston skirt to carry the piston pin. Return to the figures.
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.
That portion of the piston skirt which is opposite the major thrust face. Return to the figures.
If the ring set was selected based on the year, make and engine model, the correct rings will be in the set if shallow oil rings are needed for that engine.
A bushing fitted between piston pin and piston pin hole to obtain a better bearing material. Used particularly with iron pistons. Return to the figures.
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. Return to the figures.
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. Return to the figures.
The distance from the cylinder wall to the bottom of the ring groove with the piston centered in the cylinder. Return to the figures.
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. Return to the figures.
Diameter of the land being measured. In some piston designs all lands are equal diameter. In others they increase from top to bottom. Return to the figures.
The difference between the diameter of the land and cylinder diameter. (R shows 1/2 of total clearance) Return to the figures.
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) Return to the figures.
A groove cut into the piston around its circumference below the pin hole to carry an oil ring. Return to the figures.
On some pistons the pin hole is offset to one side of the piston centerline. Return to the figures.
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.
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.