# PISTON

 A piston consists of a lower part-piston skirt of cast iron and an upper part or crown made of a special heat-resistant steel. They are bolted together and fixed to the piston rod. The position of the piston parts in relation to each other is secured by means of machine turned recesses and a dowel pin in the crown. Each piston is provided with five or six piston rings fitted in chromium plated grooves in the crown. The two or three topmost rings are narrow rings having diagonal cuts, while the next two are broad and are provided with over-lapping end joints. The lowest ring is an oil distributor ring. All the rings are slightly rounded on external top and bottom edges to keep the oil film on the cylinder liner during the running-in period for new piston rings. In order to control thermal stresses, thin-wall intensively cooled pistons are used for some types of modern engines. In this case, the piston has an internal insert in the piston crown, which serves only to direct the cooling liquid flow and, thereby, to intensify the conventional ‘cocktail shaker’ effect.

## Related Terms

### PLUNGER

1. A wooden shaft with a large rubber suction cup at the end, used to clear plumbing traps and waste outlets. 2. The long rod or piston of a reciprocating pump.

### PLAN EQUATION

The mathematical statement that horsepower plan/33,000, where p mean effective pressure (pounds per square inch), l length of piston stroke (feet), a net area of piston (square inches), and n number of cycles completed per minute.

### PISTON VISCOMETER

A device for the measurement of viscosity by the timed fall of a piston through the liquid being tested.

### PISTON SKIRT

That part of a piston below the piston pin bore.

### PISTON PUMP

A pump in which motion and pressure are applied to the fluid by a reciprocating piston in a cylinder. Also known as reciprocating pump.

### PISTON DISPLACEMENT

The volume which a piston in a cylinder displaces in a single stroke, equal to the distance the piston travels times the internal cross section of the cylinder.

### PERCUSSION DRILL

A drilling machine usually using compressed air to drive a piston that delivers a series of impacts to the shank end of a drill rod or steel and attached bit.

### PAIR

1. Two like conductors employed to form an electric circuit. 2. Two parts in a kinematic mechanism that mutually constrain relative motion; for example, a sliding pair composed of a piston and cylinder.

### PACKING DENSITY

The number of devices or gates per unit area of an integrated circuit. packing ring See piston ring.

### OPTICAL INDICATOR

An instrument which makes a plot of pressure in the cylinder of an engine as a function of piston (or volume) displacement, making use of magnification by optical systems and photographic recording; for example, the small motion of a pressure diaphragm may be transmitted to a mirror to deflect a beam of light.

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