Dictionary of Automotive Terms: P

PAN: Thin stamped cover bolted to the bottom of crankcase. It forms a sump for engine oil and keeps dirt, etc., from entering engine.

PANCAKE ENGINE: Engine in which cylinders are on a horizontal plane. This reduces overall height and enables them to be used in spots where vertical height is restricted.

PAPER AIR CLEANER: Air cleaner that makes use of special paper through which air to carburetor is drawn.

PARABOLIC REFLECTOR: A light reflector (concave mirror) that emits parallel light rays. Bulb filament must be located at focal point of parabola.

PARALLEL CIRCUIT: Electrical circuit with two or more resistance units so wired as to permit current to flow through both units at same time. Unlike series circuit, current in parallel circuit does not have to pass through one unit to reach the other.

PARALLELOGRAM STEERING LINKAGE: Steering system utilizing two short tie rods connected to steering arms and to a long center link. The link is supported on one end on an idler arm and the other end is attached directly to pitman arm. Arrangement forms a parallelogram shape.

PARKING BRAKE: Hand operated brake which prevents vehicle movement while parked by locking rear wheels, or transmission output shaft.

PARKING LIGHTS: Small lights on both front and rear of vehicle. Usually red or amber color in rear and amber in front. Used so that vehicle will be more visible during dark hours. Lights are turned on whenever headlight switch is operated.

PARTICULATES (Lead): Tiny particles of lead found in engine exhaust emissions when leaded fuel is used.

PART-TIME TRANSFER CASE: Four- wheel drive transfer case that permits either four-wheel or two-wheel drive.

PASCAL’S LAW: “When pressure is exerted on confined liquid, it's transmitted undiminished.”

PAWL: Stud or pin that can be moved or pivoted into engagement with teeth cut on another part—such as parking pawl on automatic transmission that can be slid into contact with teeth on another part to lock rear wheels.

PAYLOAD: Amount of weight that may be carried by vehicle. Computed by subtracting vehicle curb weight from GVW.

PCV (Positive Crankcase Ventilation): System which prevents crankcase vapors from being discharged directly into atmosphere.

PEEN: To flatten out end of a rivet, etc., by pounding with round end of a hammer.

PENETRATING OIL: Special oil used to free rusted parts so they can be removed.

PERIPHERY: Outside edge or circumference.

PERMANENT MAGNET: Magnet capable of retaining its magnetic properties over very long period of time.

PETROL: Gasoline.

PETROLEUM: Raw material from which gasoline, kerosene, lube oils, etc., are made. Consists of hydrogen and carbon.

PHILLIPS HEAD SCREW: Screw having a fairly deep cross slot instead of single slot as used in conventional screws.

PHOSPHOR-BRONZE: Bearing material composed of tin, lead, and copper.

PHOTOCHEMICAL: Relates to branch of chemistry where radiant energy (sunlight) produces various chemical changes.

PHOTOCHEMICAL SMOG: Fog-like condition produced by sunlight acting upon hydrocarbon and carbon monoxide exhaust emissions in atmosphere.

PICKUP COIL: Device in electronic type distributor which senses engine speed (distributor rotation) and sends electrical pulses to control unit.

PIEZOELECTRIC IGNITION: System of ignition that employs use of small section of ceramic-like material. When this material is compressed, even a very tiny amount, it emits a high voltage that will fire plugs. This system does not need a coil, points, or condenser.

PILOT SHAFT: Dummy shaft that's placed in a mechanism as a means of aligning parts. It is then removed and regular shaft in stalled.

PINGING: Metallic rattling sound produced by the engine during heavy acceleration when ignition timing is too far advanced for grade of fuel being burned.

PINION CARRIER: Part of rear axle assembly that supports and contains pinion gear shaft.

PINION (Gear): Small gear either driven by or driving a larger gear.

PIPES: Exhaust system pipes.

PISTON: Round plug, open at one end, that slides up and down in cylinder. It is attached to connecting rod and when fuel charge is fired, will transfer force of explosion to connecting rod then to crankshaft.

PISTON BOSS: Built-up area around piston pin hole.

PISTON COLLAPSE: Reduction in diameter of piston skirt caused by heat and constant impact stresses.

PISTON DISPLACEMENT: Amount (volume) of air displaced by piston when moved through full length of its stroke.

PISTON EXPANSION: Increase in diameter of piston due to normal piston heating.

PISTON HEAD: Portion of piston above top ring.

PISTON LANDS: Portion of piston between ring grooves.

PISTON PIN or WRIST PIN: Steel pin that's passed through piston. Used as base upon which to fasten upper end of connecting rod. It is round and is usually hollow.

PISTON RING: Split ring installed in a groove in piston. Ring contacts sides of ring groove and also rubs against cylinder wall thus sealing space between piston and wall.

PISTON RING (Compression): Ring de signed to seal burning fuel charge above piston. Generally there are two compression rings per piston and they are located in two top ring grooves.

PISTON RING (Oil Control): Piston ring designed to scrape oil from cylinder wall. Ring is of such design as to allow oil to pass through ring and then through holes or slots in groove. In this way oil is returned to pan. There are many shapes and special designs used on oil control rings.

PISTON RING END GAP: Distance left between ends of the ring when installed in cylinder.

PISTON RING EXPANDER: See Ring Expander.

PISTON RING GROOVE: Slots or grooves cut in piston head to receive piston rings.

PISTON RING SIDE CLEARANCE: Space between sides of ring and ring lands.

PISTON SKIRT: Portion of piston below rings. (Some engines have an oil ring in skirt area.)

PISTON SKIRT EXPANDER: Spring device placed inside piston skirt to produce an out ward pressure which increases diameter of skirt.

PISTON SKIRT EXPANDING: Enlarging diameter of piston skirt by inserting an ex pander, by knurling outer skirt surface, or by peening inside of piston.

PITMAN ARM: Short lever arm splined to steering gear cross shaft. Pitman arm transmits steering force from cross shaft to steering linkage system.

PITS: Area at a race track for fueling, tire changing, making mechanical repairs, etc.

PIT STOP: A stop at the pits by racer, for fuel, tires, repairs, etc.

PIVOT: Pin or shaft about which a part moves.

PLANET CARRIER: Part of a planetary gearset upon which planet gears are affixed. Planet gears are free to turn on hardened pins set into carrier.

PLANET GEARS: Gears in planetary gear- set that are in mesh with both ring and sun gear. Referred to as planet gears in that they orbit or move around central or sun gear.

PLANETARY GEARSET: Gearing unit consisting of ring gear with internal teeth, sun or central pinion gear with external teeth, and series of planet gears that are meshed with both the ring and the sun gear.

PLATES ( Battery): Thin sections of lead peroxide or porous lead. There are two kinds of plates — positive and negative. The plates are arranged in groups, in an alternate fashion, called elements. They are completely submerged in the electrolyte.

PLATINUM: Precious metal sometimes used in the construction of breaker points. It con ducts well and is highly resistant to burning.

PLAY: Movement between two parts.

PLEXIGLAS: Trade name for an acrylic plastic, made by the Rhom and Haas Co.

PLIES (Tire): Layers of rubber impregnated fabric that make up carcass or body of tire.

PLUG GAPPING: Adjusting side electrode on spark plug to provide proper air gap between it and the center electrode.

PLY RATING (Tires): Indication of tire strength (load carrying capacity). Does not necessarily indicate actual number of plies. Two-ply four-ply rating tire would have load capacity of a four-ply tire of same size but would have only two actual plies.

P-METRIC (Tire): Tire size designation based on international standards. Example: P 155/80R13. P = passenger car use. 155 = section width in millimeters. 80 = height to width ratio. R = radial construction. 13 = wheel rim diameter in inches.

POLARITY (Battery Terminals): Indicates if the battery terminal (either one) is positive or negative (plus or minus) (+ or -).

POLARITY (Generator): Indicates if pole shoes are so magnetized as to make current flow in a direction compatible with direction of flow as set by battery.

POLARITY (Magnet): Indicates if end of a magnet is north or south pole (N or S).

POLARIZING (Generator): Process of sending quick surge of current through field windings of generator in direction that will cause pole shoes to assume correct polarity. This will insure that the generator will cause current to flow in same direction as normal.

POLE (Magnet): One end, either north or south, of a magnet.

POLE SHOES: Metal pieces about which field coil windings are placed. When current passes through windings, pole shoes become powerful magnets. Example: pole shoes in a generator or starter motor.

PONY CAR: Small, sporty car along the lines of the Mustang, Firebird, Camaro, etc.

POPPET: A spring-loaded ball engaging one or more indentations. Used to hold one object in position in relation to another.

POPPET VALVE: Valve used to open and close valve port entrances to engine cylinders.

PORCELAIN (Spark Plug): Material used to insulate center electrode of spark plug. It is hard and resistant to damage by heat.

POROSITY: Small air or gas pockets, or voids, in metal.

PORT: Openings in engine cylinder blocks for exhaust and intake valves and water connections. To smooth out, align, and somewhat enlarge intake passageway to the valves.

POSITIVE TERMINAL: Terminal (such as on battery), to which current flows.

POST ( Battery): Round, tapered lead posts protruding above top of battery to which battery cables are attached.

POTENTIAL: An indication of amount of available energy.

POTENTIOMETER: Variable resistor with three connections. One connection (called wiper) slides along resistive unit. Can be used as a voltage divider.

POUR POINT: Lowest temperature at which fluid will flow under specified conditions.

POWER: Time rate at which energy is converted into work.

POWER BOOSTER (Brakes): Engine vacuum or power steering fluid operated device on fire wall which increases brake pedal force on master cylinder during stops.

POWER PLANT (Auto): The vehicle engine.

POWER STEERING: Steering system utilizing hydraulic pressure to increase the driver’s turning effort. Pressure is utilized either in gearbox itself or in hydraulic cylinder attached to steering linkage.

POWER STEERING PUMP: Belt driven pump which produces pressure for power steering system.

POWER or FIRING STROKE: Portion of piston’s movement devoted to transmitting power of burning fuel mixture to crank shaft.

PPM (Parts-Per-Million): Term used in deter mining extent of pollution existing in given sample of air.

PRACTICAL EFFICIENCY: Amount of horsepower delivered to drive wheels.

PRECISION INSERT BEARING: Very accurately made replaceable type of bearing. It consists of an upper and lower shell. The shells are made of steel to which a friction type bearing material has been bonded. Connecting rod and main bearings are generally of precision insert type.

PREHEATING: Application of some heat prior to later application of more heat. Cast iron is preheated to avoid cracking when welding process is started. A coil (ignition) is preheated prior to testing.

PREHEATING (Metal): Process of raising temperature of metal to specific level before starting subsequent operations such as welding, brazing, etc.

PREIGNITION: Fuel charge being ignited before proper time.

PRELOADING: Adjusting antifriction bearing so it's under mild pressure. This prevents bearing looseness under a driving stress.

PRESS-FIT: Condition of fit (contact) between two parts that requires pressure to force parts together. Also referred to as drive or force fit.

PRESSURE BLEEDER: Device that forces brake fluid, under pressure, into master cylinder so that by opening bleeder screws at wheel cylinders, all air will be removed from brake system.

PRESSURE CAP: Special cap for radiator.. It holds a predetermined amount of pressure on water in cooling system. This enables water to run hotter without boiling.

PRESSURE DIFFERENTIAL SWITCH: Hydraulic switch in brake system that operates brake warning light in dashboard.

PRESSURE RELIEF VALVE: Valve de signed to open at specific pressure. This will prevent pressures in system from exceeding certain limits.

PRIMARY CIRCUIT (Ignition System):

Low voltage (6 or 12 volt) part of ignition system.

PRIMARY, FORWARD, or LEADING BRAKE SHOE: Brake shoe installed facing front of car. It will be a self-energizing shoe.

PRIMARY WINDING (Coil): Low voltage (6 or 12 volt) winding in ignition coil. The primary winding is heavy wire; secondary winding uses fine wire.

PRIMARY WIRES: Wiring which serves low voltage part of ignition system. Wiring from battery to switch, resistor, coil, distributor points.

PRINTED CIRCUIT: Electrical circuit made by connecting units with electrically conductive lines printed on a panel. This eliminates actual wire and task of connecting it.

PROGRESSIVE LINKAGE: Carburetor linkage designed to open throttle valves of multiple carburetors. It opens one to start and when certain opening point is reached, it will start to open others.

PRONY BRAKE: Device utilizing friction brake to measure horsepower output of engine.

PROPANE (LPG): Petroleum product, similar to and often mixed with butane, useful as engine fuel. May be referred to as LP-Gas.

PROPELLER SHAFT: Shaft connecting transmission output shaft to differential pinion shaft.

PROPORTIONING VALVE (Brakes): Valve in brake line which keeps rear wheels from locking up during rapid stops.

PROTON: Positive charge particle, part of atom.

PSI: Pounds per square inch.

PTO: Power take off. A spot or place on transmission or transfer case from which an operating shaft from another unit (such as a winch) can be driven. Usually consists of a removable plate that exposes a drive gear.

PULL IT DOWN (Engine): Term often used in reference to dismantling and overhauling an engine.

PULSATION DAMPER: Device to smooth out fuel pulsations or surges from pump to carburetor.

PULSE AIR INJECTION: Emission control system which feeds air into exhaust gases by using pressure pulsations of exhaust system.

PULSE WIDTH: Often used to describe length of time a fuel injector is held open. Pulse being electric current applied to the injector winding and width being length of time current is allowed to flow. The wider the pulse, the more fuel delivered.

PUMP: A device designed to cause movement of water, fuel, air, etc., from one area to another.

PUMPING THE GAS PEDAL: Forcing accelerator up and down in an endeavor to provide extra gasoline to cylinders. This is often cause of flooding.

PURGE: Removing impurities from system. See Bleeding.

PUSH ROD: Rod that connects valve lifter to rocker arm. Used on valve-in-head installations.

PYLON: Marker for controlling traffic.