Audi, Porsche, Mercedes, and Volkswagen Repair, Maintenance, and Performance Related Engine Glossary Portland Oregon

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Accumulator: A tank located in the outlet of the evaporator to receive the refrigerant that leaves the evaporator. A component used to store or hold liquid refrigerant in an air-conditioning system. This device is constructed to ensure that no liquid refrigerant enters the compressor.

Adjusting shim: A metal shim, available in various thicknesses, used to change the valve clearance in some overhead cam engines.

[spark] Advance: The moving ahead of the ignition spark in relation to the piston position.

Air box: An enclosed chamber to direct air into a carburetor or intake manifold.

Air charge temperature (ACT): The temperature of incoming air in a fuel-injection system.

Air cleaner: A device connected to the carburetor in a manner that all incoming air must pass through it. Its purpose is to filter dirt and dust from the air before it passes into the engine.

Air conditioning (A/C): The process of adjusting and regulating, by heating or refrigerating, the quality, quantity, temperature, humidity, and circulation of air in a space or enclosure; to condition the air.

Air-cooled engine: An engine that is cooled by the passage of air, generally forced, around and over the cylinders.

Air door: A door in the duct system that controls the flow of air in the air conditioner and/or heater.

Air filter: A filter that removes dust, dirt, and particles from the air passing through it.

Airflow sensor (AS): An instrument for measuring airflow in an electronic fuel-injection system to be processed by the electronic control module with other sensory data to calibrate the air/fuel mixture.

Air/fuel mixture: The proportion of air to fuel provided by the fuel-injection system.

Air/fuel ratio (A/F, AFR): The relative proportions of air and fuel entering an engine's cylinders as produced by the fuel-injection system; the measure of the amount of air and fuel needed for proper combustion. The ideal or stoichiometric ratio for gasoline is 14.7: 1 air to fuel by weight. A higher ratio would contain more air and less fuel, and would be considered a lean mixture. A lower ratio with more fuel and less air would be a rich mixture. The air/fuel ratio is determined by the orifice opening and fuel pulse duration of a fuel injector.

Air temperature sensor: A unit consisting of an aspirator, bimetallic sensing element, and a vacuum modulator used to sense in-car temperature.

Align bore: A stationary or portable boring machine used to correct an engine's out-of-round and/or warped main bearing housings.

Align hone: A stationary machine operation that corrects an engine's out-of-round and/or warped main bearing housings using a special honing mandrel.

Antifreeze: A chemical solution added to the coolant (water) to prevent freezing; usually ethylene glycol and anticorrosion chemicals. Ethylene glycol resists evaporation, but the anticorrosion elements in the antifreeze may be used up in one year, depending on the amount and type of driving.

Anti-knock additive: A compound that may be added to gasoline to increase its octane, thereby decreasing its knock.

Anti-knock index: The average of the motor octane number (MON) and the research octane number (RON); a measure of a fuel's anti-knock characteristics.

Apex seal: A seal used to retain the combustion pressure at all three tips of the rotor in a rotary engine.

Armored ring groove: A metal ring groove cast into a piston during manufacturing to increase resistance to wear.

Assembly lube: A special lubricant used to coat parts that rub or rotate against each other during initial assembly.

Atomization: The breaking down of a liquid into small particles, like a mist, by the use of pressure.

An explosion in the exhaust system of a motor vehicle caused when an unburned air/fuel mixture is ignited, usually upon deceleration.
An explosion of the air/fuel mixture in the intake manifold, which is evident at the throttle body and may be caused by improper ignition timing, crossed spark plug wires, or an intake valve that is stuck open.

Balancer (also harmonic balancer): A heavy crankshaft pulley that aids in overall crankshaft balance as it rotates.

Balancing: The process of proportioning weight or force equally on all sides of an object. Most crankshafts, for example, are balanced both statically and dynamically.

Base circle: The low portion of each cam on the camshaft, concentric with the journal, which is not part of the lobe.

Bearing crush: The additional height, manufactured into each bearing half, to ensure complete contact of the bearing back with the housing bore when the engine is assembled.

Bearing groove: A channel cut into the surface of a bearing to ensure oil distribution.

Belt: A device used to drive the water pump and/or other auxiliary devices, such as the alternator, off the engine.

Belt tension: The tightness of a drive belt, generally measured in ft-lb (N*m).

Billet crankshaft: A crankshaft machined from a billet of steel, usually used for racing applications.

Black smoke: The exhaust that is produced when the air/fuel mixture is too rich.

Block: Main casting of the engine that contains the cylinders; often made of cast iron or aluminum.

Blow-by: Byproducts of combustion, mostly hydrocarbons, that leak out of the combustion chamber, past the piston and piston rings, into the crankcase during the compression and power strokes. In modern engines, blowby vapors are drawn into the intake through the positive crankcase ventilation (PCV) system and are burned in the engine.

An engine that has a supercharger or turbocharger.
A seriously damaged engine.

Blown head gasket: A broken head gasket that leaks water, oil, or air and reduces engine performance.

Blueprint: To rebuild an engine to original equipment manufacturer (OEM) design specifications.

The diameter of an engine cylinder.
To increase the diameter of a cylinder.

Bore align: To machine an engine's main bearing journals to assure they are in perfect alignment.

Bored and stroked: A combination of an enlarged cylinder bore and a lengthened piston stroke to increase an engine's displacement.

Bottom end: The crankshaft main bearing and connecting-rod bearing assembly in an engine.

Boundary layer: The thin layer of air along the inner walls of an intake port.

Brake horsepower (bhp): A measurement of horsepower (hp) delivered at the engine crankshaft. A brake or an engine dynamometer is used to determine brake horsepower.

Brake specific fuel consumption: The measure of an engine's fuel efficiency during dynamometer testing.

Brake torque: The engine output at the flywheel as measured on a dynamometer.

Cam duration: The amount of time, measured in crankshaft degrees, that a camshaft holds an exhaust or intake valve open.

Cam lift: The distance, in thousandths of an inch, a cam lobe raises the valve lifter off the base circle.

Cam-lobe face and nose taper: The slant, about 0.002 inch (0.051 mm), designed across the cam-face contacting surface, from the cam front to rear edge, to promote lifter rotation.

Cam sensor: A camshaft-mounted sensor that signals when cylinder number one is at TDC.

Camshaft: A shaft having lobes driven by the crankshaft via gears, chains, or belts that, in turn, opens and closes the intake and exhaust valves at proper intervals.

Camshaft duration: The amount of time, measured in degrees of crankshaft rotation, an intake or exhaust valve is held open.

Camshaft lift: How far a valve is pushed open, as determined by the height of the cam lobe and the geometry of the rocker arms on a push rod engine, or the cam followers on an overhead cam engine.

Camshaft plug: A plug found in the rear of the engine block, at the camshaft, to retain and direct oil to the rear camshaft bearings.

Carbon dioxide (CO2): A harmless, odorless gas composed of carbon (C) and oxygen (O); a product of complete combustion.

Carbon monoxide (CO): An odorless gas composed of carbon (C) and hydrogen (H), formed by the incomplete combustion of any fuel containing carbon. This major air pollutant is potentially lethal if inhaled, even in small amounts.

Cavitation: The presence of air in a liquid during pumping, which can inhibit the flow of the liquid.

An abbreviation for cubic centimeter.
The measure of the volume of a combustion chamber.

CC-ing: To measure or calculate the volume of a combustion chamber in cubic centimeters.

Chain guide: A device used to support and guide a chain, such as a timing chain, to reduce or prevent whip.

Chain tensioner: A device used to maintain chain tension, such as a guide rail or a hydraulic pressure piston with a rubbing shoe.

Check-engine light: A warning light, generally located in the instrument cluster, that indicates a potential engine or system problem.

Clearance: The space between mating parts, such as between a journal and a bearing, that allows freedom of movement or prevents interference.

The basic principle of electronic engine management in which input from an oxygen sensor allows the engine-control computer to determine and maintain a nearly perfect air/fuel ratio.
A computer condition in which the air/fuel ratio is being controlled on the basis of various inputs to the computer.
A continuous circuit from beginning to end, and beyond.

Closed-loop fuel system: A computerized air/fuel metering system based on monitoring the temperature and composition of the exhaust gases.

Cold-start injector: An electronic fuel-injection system that supplies extra fuel to the engine for cold starting.

Cold-start valve: A valve that permits additional air into the intake manifold during a cold start on a fuel-injected engine.

Combustion chamber: Area above a piston at TDC, primarily distinguished by a recessed cylinder head, where combustion takes place.

Combustion pressure: The pressure in the cylinder from expanding gases immediately after the air/fuel mixture is ignited, which is about four times greater than compression pressure.

The process of squeezing a vapor (gas) into a smaller space.
The upward stroke of a piston that compresses the air/fuel mixture into the combustion chamber prior to ignition.
Short for compression ratio.

Compression height: A distance, as measured from the crown of the piston to the center of the wrist pin.

Compression ratio (CR): A measurement of how much the air/fuel mixture is compressed inside an engine cylinder. If compressed to 1/10 of its original volume, the compression ratio is 10 to 1.

Compression ring: A piston ring that seals pressure during the compression and power strokes. There are usually two compression rings per piston.

Connecting rod: A component used to attach the piston, with pin, to the crankshaft rod journal.

Connecting-rod cap: The half-round, lower, bolt-on portion of the connecting rod.

Continuous-injection system (CIS): A continuous-flow, mechanically controlled, fuel-injection system.

Continuous-injection system--electronic (CIS-E): A continuous-flow, electronically controlled, fuel-injection system.

Coolant: The fluid, consisting of water and antifreeze, that circulates throughout a liquid-cooling system, around hot engine parts, to remove the heat and prevent damage.

Coolant fan: An electrically- or mechanically-driven fan to increase air flow across the radiator to facilitate heat removal.

Coolant-fan relay: A control device that is used to activate the electric-coolant fan at a pre-determined, high-coolant temperature.

Coolant jacket: Hollow passages surrounding the cylinders in the block and the combustion chambers in the cylinder head.

Coolant passages: Coolant passages, called coolant jackets or water jackets, surround each cylinder in the block to provide a means to carry away unwanted engine heat.

Coolant pump: A term often used for water pump. A centrifugal-type pump used to circulate coolant through the cooling system.

Coolant reservoir: A tank used for storing excess coolant; connected to the cooling system with a small-diameter overflow hose.

Coolant-temperature sensor: A thermistor that sends a signal to the electronic control unit relative to the coolant temperature in a computerized engine-control system.

Coolant thermostat: A unit found in the coolant outlet of the engine to help prevent over-cooling conditions, especially during short trips.

Cooling-system fan: An electrically- or mechanically-driven rotating device, having four to seven pitched blades to move air past a heat exchanger, such as the radiator.

Copper gasket: A gasket made of copper and used in high-temperature and pressure conditions.

Core plug: A metal, cup-shaped disc in a cast component, such as the engine block, to seal openings required by the casting tools that may pop out to protect it from freezing damage. Also known as freeze or expansion plug.

Counterbalance: A weight added to a rotating shaft or wheel to offset vibrations.

Counterweight: A weight that is cast opposite each offset connecting-rod journal to provide the necessary balance.

Crankcase: The lower section of the engine that supports the crankshaft, confined by the lower block casting and the oil pan.

Crankcase breather: A tube or vent that allows excessive crankcase pressure to escape.

Crankcase pressure: The pressure produced in the crankcase from blow-by gases.

Crankcase ventilation: A tube or vent that allows excessive crankcase pressure to escape.

A revolving part mounted in the lower portion of the engine block.
That part of a reciprocating compressor on which the wobble plate or connecting rods are attached.

Crankshaft counterweight: A weight that is cast opposite each offset connecting-rod journal, to provide the necessary balance.

Crankshaft end-play: A specified crankshaft endwise motion controlled by side flanges on one of the main bearings.

Crankshaft gear: A gear or sprocket found on the front end of the crankshaft that is used, directly or indirectly, to drive the camshaft.

Crankshaft main journal: That part of the crankshaft, ground round and polished smooth, around which the closely-fitted main bearings surround the journals and support the crankshaft.

Crankshaft oil passage: Holes drilled through the crankshaft to permit oil to flow from the main bearings to the connecting-rod bearings.

A rubber-like seal around an engine crankshaft to prevent the leakage of oil.
A rubber-like and/or ceramic seal surrounding the compressor crankshaft to prevent the leakage of oil and/or refrigerant.

Crank signal: An electrical signal sent to the computer to tell it that the engine is cranking and to enrich the air/fuel ratio for easier starting.

Crosshatch: A honing pattern that is required in a cylinder to maintain the correct amount of oil retention and to facilitate ring rotation.

Cross-flow head: A cylinder head having its intake ports on one side and the exhaust ports on the other side.

Crosshatch: A honing pattern that is required in a cylinder to maintain the correct amount of oil retention and to facilitate ring rotation.

Cubic-inch displacement (cid): The English measure for the volume of space displaced by the piston as it moves from BDC to TDC.

Cylinder: The round hole(s) inside an engine block that provide space for the reciprocating piston(s).

Cylinder block: The basic framework of an engine to which all other parts and assemblies are installed or attached. Also called an engine block.

Cylinder head: That part of the engine that covers the cylinders and pistons.

Cylinder-head gasket: The gasket used to seal the head to the block to promote compression and to ensure a leak-free bond.

Cylinder sleeve: A round, replaceable, cylindrical tube that fits into the cylinder bore.

A device that reduces or eliminates vibration or oscillation.
A balancer on the front of the crankshaft to reduce or eliminate harmonic vibration.

The flat, mating surfaces of an engine block and head.
To machine an engine block deck flat.

Deck (torque) plate: A heavy metal plate that is bolted to an engine block during cylinder-honing operations.

Degreeing a cam: Using instruments to determine the actual timing of valves opening and closing for a particular camshaft.

Detonation: A phenomenon of internal combustion where the compressed air/fuel charge explodes violently instead of burning smoothly, usually due to the creation of a second flame front in the combustion chamber, away from the spark plug.

Dieseling: A condition in which an engine (usually carbureted) continues to run after the ignition is shut off.

Differential pressure valve: A device that maintains a constant pressure to fuel injectors.

Dip stick: A thin, steel strip of metal used to measure the fluid level or quantity in the engine, transmission, or compressor.

Direct injection: A type of fuel-injection system that injects fuel directly into the cylinders.

Displacement: The volume within an engine's cylinders, usually expressed in cubic centimeters (cc).

Drilled oil passage: Holes drilled in various parts of the engine for pressurized oil to flow through.

Drive belt: Flexible belt or belts used to connect a drive pulley on the crankshaft to the coolant pump and accessories. Two basic types of drive belts are the serpentine or multiple-ribbed belt and the V-belt.

Drive by wire: To use an electronic throttle control rather than mechanical linkage.

Dry-film lubricant: A petroleum-based chemical used to lubricate operating parts during assembly, prior to engine start up, to provide lubrication until oil is circulated after start up.

Duration: The time, in crankshaft degrees, that a camshaft holds an exhaust or intake valve open.

Dwell: The degree of distributor-shaft rotation while the ignition breaker points are closed.

The crankshaft of a Wankle engine.
A shaft with eccentric offsets, such as a camshaft.

EGR system: An EGR valve, mounted on the intake manifold, that meters a small amount of exhaust gas into the intake manifold to dilute the air/fuel mixture. This keeps combustion temperatures below 2,500°F and reduces the formation of NOX. The amount of exhaust gas recirculated into the engine is only a few percent.

Electronic fuel-injection system: A fuel-injection system that injects gasoline into a spark-ignition engine that includes an electronic control to time and meter the fuel flow.

End clearance: The distance a shaft moves longitudinally as the crankshaft in an engine. Also known as end play.

Engine: A device that burns fuel to produce mechanical power; to convert heat energy into mechanical energy.

Engine displacement: The volume swept within an engine as its cylinders make one stroke, generally expressed in cubic inches, cubic centimeters, or liters.

Engine overhaul: To perform more than minor repairs on a powerplant, though not as extensive as an engine rebuild.

Engine rebuild: To perform an extensive engine repair including machining, reboring, and honing to factory-stock specifications.

Engine torque: A rotating, twisting action developed by the engine that is measured in pounds, feet, or kilowatt hours.

Engine tuneup: The inspection, testing, and adjusting of an engine; the replacement of any parts required to ensure maximum performance.

Evaporator: The heat exchanger of an air-conditioning system that removes heat from the air passing through it.

Exhaust-gas recirculation (EGR): An emissions-control system that reduces an engine's production of oxides of nitrogen by diluting the air/fuel mixture with exhaust gas so that peak combustion temperatures in the cylinders are lowered.

Exhaust port: In a rotary combustion engine, peripheral opening in the rotor housing that allows the burned gases to leave the engine.

Exhaust stroke: The upward motion of the piston forcing burned gases out the open exhaust valve.

Exhaust valve: Valve that, upon opening, allows the burned gases to leave the combustion chamber during the exhaust stroke.

A coolant-recovery tank.
An auxiliary tank, usually connected to the inlet tank or a radiator, that provides additional storage space for heated coolant.

Fan belt: A flexible V-, or flat poly-groove-type drive belt that transfers power from the crankshaft pulley to the water pump and/or accessories, such as the alternator.

Fan blade: In an engine cooling-system fan, four to six wings on the fan, usually spaced unevenly to reduce vibration and noise.

Fan clutch: A device installed between the water-pump pulley and fan of an engine-driven fan that is sensitive to engine speed and underhood temperature.

Fan shroud: Plastic or metal housing inside which the fan rotates; on certain vehicles, this allows the fan to pull more air past radiator finned tubing and prevents air recirculation.

Flooded: A condition whereby the air/fuel mixture in a cylinder is too rich to burn.

A cycle of engine operation whereby the combustion occurs in every cylinder on every second revolution of the crankshaft.
A complete cycle includes intake or induction, compression, combustion or expansion, and exhaust.
Also known as four cycle.

Fuel distributor: A mechanical or electro-mechanical device used to route fuel to the injectors.

Fuel filter: A device located in the fuel line to remove impurities from the fuel before it enters the carburetor or injector system.

Fuel-injection system: A system that sprays fuel under pressure into the intake manifold or directly into the cylinder intake ports, allowing more precise control of the air/fuel mixture for improved performance, fuel economy, and reduced exhaust emissions.

Fuel injector: A mechanical or electro-mechanical device that meters fuel into an engine.

Carry fuel from the fuel tank to the carburetor or injector.
Return fuel not used to cool the carburetor and/or injectors.

Fuel pump: A mechanical or electrical device used to move fuel from the fuel tank to the carburetor or injectors.

Fuel rail: A conduit to deliver fuel from the distributor to the injectors of an FI system.

Fuel system: The system that delivers fuel to the cylinders, consisting of a fuel tank and lines, gauge, fuel pump, carburetor or injectors, and intake manifold or fuel rail.

The wide open throttle position.
The accelerator pressed to the floorboard.

A piece of thin compressible material such as cork, rubber, or soft metal, placed between two mating surfaces to form a seal.
A metal ring used on some spark plugs to seal pressure between the plug shell and the cylinder head.

Glow plug: An electrical plug used to preheat the combustion chamber to aid in starting a cold diesel engine.

Guide rail: A device that keeps the timing chain aligned and in the correct position.

Gun-drilled oil holes: Holes that are drilled to allow oil to be fed to the piston pin. Also known as rifle-drilled oil holes.

Harmonic balancer: A balance shaft or wheel that reduces or eliminates harmonics. Also known as a harmonic damper.

Head: That part of an engine that covers the top of the cylinders and pistons and usually contains the combustion chambers and valve train.

Heat range: The measure of a spark plug's ability to transfer heat from the tip of the insulator into the cylinder head.

Hot-wire sensor: An electrical device inside the mass-airflow meter that measures air flow and density.

Hydraulic lifter: Valve lifter located between the camshaft and valve stem that uses internal oil pressure to cause the lifter to expand lengthwise.

Idler pulley: A pulley that is used to adjust the belt in a belt-drive system.

Impeller: A rotor that transmits motion such as a centrifugal pump or fluid coupling.

The tube or nozzle through which fuel is injected into the intake airstream of the combustion chamber.
A term used for fuel injector.

Installed height: The distance from the valve-spring end to the valve-spring seat.

Intake manifold: A metal component used to duct: Air in an injected engine to the intake ports.

Intake ports: Passages in the cylinder head that direct air/fuel mixture from the intake manifold to the intake valves.

Intake stroke: A downward stroke of a piston that draws the air/fuel mixture into the cylinder.

Keeper grooves: The grooved area on a valve stem to accommodate the keepers.

Keepers: Key-like, tapered-metal locking devices used to hold valve retainers in place on the valve stem.

KE-Jetronic: A continuous electronic fuel-injection system by Bosch that has been modified using a lambda oxygen sensor.

Keyway: A groove milled or machined onto a shaft or into a bore to accept a square, half moon, or round piece of metal.

K-Jetronic: A continuous fuel-injection system; a forerunner of the KE-Jetronic system.

Knock: A noise within an engine generally caused by detonation or preignition.

Knock sensor: A sensor that signals the engine-control computer when detonation is detected, momentarily retarding ignition timing until detonation ceases.

Laminar airflow: The movement of a body of air/fuel mixture through the intake manifold and ports as it flows over the boundary layer.

Leakdown test: A test using 100 psi (689.5 kPa) air pressure injected in each cylinder via the spark-plug hole or injector port to determine the leakage past the rings, gaskets, or valves.

Lean misfire: A condition caused by a vacuum leak or open EGR valve that results in an air/fuel mixture too lean to sustain combustion, causing one or more cylinders to pass unburned fuel into the exhaust system, resulting in an increase in hydrocarbon (HC) emissions.

LH-Jetronic: An electronic fuel-injection system by Bosch that uses a mass airflow sensor with a digital control unit.

A part between the camshaft and valve stem on an OHC engine.
Also known as follower.

L-Jetronic: A Bosch pulsed, electronic, fuel-injection system that uses input on the volume of intake air to calculate fuel delivery.

Lobe: That concentric part of a camshaft that causes the valves to open and close by actuating a valve lifter.

Lobe centers: The number of degrees of a camshaft between intake and exhaust lobes.

Long block: An assembled engine block that contains all of the components from the intake manifold to the exhaust ports.

Longitudinal engine: An engine mounted lengthways in a vehicle, parallel with the vehicle, such as in most rear-drive vehicles.

LU-Jetronic: A United States version of a Bosch pulsed-electronic, fuel-injection system having a lambda sensor.

Magnaflux: A dry, nondestructive magnetic test to check for cracks or flaws in iron or steel parts.

Magnetic pick up: The system of an electronic ignition that triggers the amplifier to generate voltage to fire the plugs.

Main bearings: The bearings that locate and support the crankshaft in an engine block.

Main cap: The structural device that holds the crankshaft in place in an engine block.

Main journals: The journals that fit into the engine block to support the crankshaft.

Major overhaul: An engine overhaul just short of a rebuild, where all worn or damaged parts are rebuilt or replaced.

Major tune-up: A conventional, ignition-engine maintenance procedure that may include points, capacitor, cap, rotor, plugs, plug wires, and a valve adjustment.

Manifold absolute pressure (MAP): The pressure in an intake manifold relative to atmospheric pressure.

Manifold absolute-pressure (MAP) sensor: A variable resistor used as a sensor to signal an engine-control computer relative to the vacuum conditions in the intake manifold.

Manifold air temperature: The temperature of the intake stream in the intake manifold, as increased by a heat riser or an Early Fuel-Evaporation system, and/or converted to an engine-control computer input by a sensor.

Manifold pressure: A positive pressure in the intake manifold measured in pounds per square inch or bars.

Manifold vacuum: The negative pressure in an engine's intake manifold produced by the engine's pumping action and measured in inches of mercury.

Mass airflow sensor (MAF): A device found at the inlet of the intake manifold of an electronic fuel-injection system which supplies the computer with input as to the volume of air entering the manifold, using the temperature differential between a heated platinum wire or a plastic film and the passing air, to generate a signal of varying voltage.

Minor overhaul: An engine-repair procedure that includes a valve job, replacing the piston rings, the rod bearings, and the gasket set.

Minor tune up: An engine-maintenance procedure performed on distributor ignition vehicles that includes only the replacement of parts found to be defective.

Misfire: An intermittent or continuous failure to ignite the air/fuel mixture in one or more cylinders.

Moly lube: A lubricant popular for coating engine parts during rebuilding to avoid an initial dry start up.

Motor mounts: Supports made of hard rubber for the engine and transmission to be secured to the vehicle's frame.

Multi (multiple) -viscosity oil: An engine oil that has a low viscosity when cold and a high viscosity when hot.

Multi-point fuel injection: A type of fuel-injection system that has a separate fuel injector for each of the engine's cylinders to deliver better performance and lower emissions than throttle-body injection (TBI) systems.

Naturally aspirated: An engine that uses atmospheric pressure to force the air into the cylinders.

Nikasil: A trade name of a popular coating applied to the walls of cylinder sleeves to reduce friction and promote sealing at high temperatures.

Nitrous oxide: A non-flammable, non-explosive gas (N2O) used as an oxidizing agent with gasoline or methanol to increase the rate and efficiency of combustion thereby increasing the horsepower.

Normally aspirated: An engine that is not equipped with a forced means of inducing air.

Number of cylinders: The total number of cylinders (1, 2, 4, 6, or 8) contained in an engine.

Oil clearance: The small space between the main bearing and crankshaft journal, usually 0.001 to 0.003 inch (0.025 to 0.076 mm), for lubricating oil to circulate.

Oil control ring: The bottom piston ring that scrapes the oil from the cylinder wall.

Oil cooled piston: A piston that is cooled by a jet of oil sprayed under the dome of some diesel- and endurance-race engines.

Oil filter: A component, located near the oil pump, that removes abrasive particles from the motor oil by a straining process as the oil circulates through the lubrication system.

Oil-fouled plug: A wet, oily deposit on a spark plug that may be caused by oil leaking past worn piston rings.

Oil galley: Passages drilled or cast into the cylinder heads, engine block, and crankshaft to receive pressurized oil from the oil pump for distribution throughout the engine.

Oil pressure: The pressure developed by the oil pump to force oil through the lubrication system.

Oil pump: A pump, driven directly or indirectly by the camshaft, that draws oil from the oil pan and forces it, under pressure, through the engine lubrication system.

Oil ring: Piston ring that scrapes oil from the cylinder wall to control cylinder wall lubrication and prevent excessive oil loss past the piston and into the combustion chamber.

Oil scraper ring: A ring used to scrape oil off the cylinder wall that may have been left by the oil control ring.

Oil seal: A seal around a rotating shaft or other moving part to prevent oil leakage.

Oil sludge: An accumulation of thickened oil, water, carbon, and dust particles inside an engine.

Open-chamber head: A cylinder head in which the cylinders have no quench area.

Open loop: In engines with a computer and oxygen-sensor control system, a mode of operation during which the computer ignores the signal from the oxygen sensor, typically before the engine reaches normal operating temperature.

Otto cycle: The basic principle of operation of the four-stroke piston engine: intake, compression, power, and exhaust.

Out of round: The condition of a circular part when it is not circular, as in slightly oval shaped.

Overbore: To enlarge the cylinders of an engine to a size larger than stock diameters.

A tank at the top of a radiator to receive heated coolant and vent trapped air due to expansion.
A device found in some fuel tanks to prevent gas escaping due to expansion.
A term used for expansion tank.

Overhead camshaft engine: An engine in which the camshaft is mounted over the cylinder head.

Overlap: The interval of valve timing when the intake valve starts to open before the exhaust valve is fully closed.

Pancake engine: An engine having opposed pistons, such as a flat four or a flat six. Also known as a boxer engine.

PCV (positive crankcase ventilation) valve: A vacuum-controlled metering device that regulates the flow of crankcase fumes in the positive crankcase ventilation system by allowing more flow at high speed than at low speed, and acts as a system shutoff in case of engine backfire to prevent an explosion in the crankcase.

Pintle: A valve-like part of a fuel injector that controls the fuel-spray pattern.

Pintle valve: The ball and seat found inside a thermostatic expansion valve, attached to the diaphragm, which causes it to open and close in response to pressure changes.

Piston: An engine part that reciprocates in the cylinder and transfers the force of the expanding gases via the piston pin and connecting rod to the crankshaft.

Piston oiler: A device that injects oil into the piston as an aid to carry away heat from the piston crown.

Piston pin: A round precision-ground part, usually hollow, used to attach the piston to the connecting rod by press fit or held in place with internal snap rings. Commonly called a wrist pin.

Piston rings: Rings that fit into grooves on the outer wall of a piston, just below the crown, that seal the combustion chambers and scrape oil from the cylinder walls.

Piston skirt: The lower part of the piston that makes contact with the cylinder wall.

Piston-skirt clearance: The allowable space between the piston skirt and cylinder wall.

Piston slap: The noise that is made by an undersize or loose piston skirt as it makes contact with the cylinder wall.

Piston speed: The velocity of a piston, given in feet per minute, as it reciprocates in a cylinder.

Piston-to-valve clearance: The distance between the intake and exhaust valves and the piston crown when the valves are at overlap period and the piston is at top dead center.

A chamber containing air under pressure.
A chamber containing a gas such as air or an air/fuel mixture under higher than atmospheric pressure.

Port injection: A fuel-injection system that injects fuel directly into the individual intake ports.

Port runner: A partition in the intake manifold that directs the air/fuel mixture to the individual cylinders.

Pounds-feet: An English measure of torque. One pound (lb) raised one foot (ft) is equal to one pounds-feet (lb-ft).

Pre-ignition: The ignition of the air/fuel mixture in the combustion chamber by means other than the spark; usually caused by hot spots in the combustion chamber due to sharp edges, carbon accumulation, or spark plugs with a heat range that is too hot.

Pressure cap: A cap placed on the radiator to increase the pressure of the cooling system, reduce cavitation, protect the radiator hoses, and prevent or reduce surging.

Pressure regulator: A device that operates to prevent excess pressure buildup in a system (ie fuel pressure regulator (FPR)).

Pressure relief valve: A valve held closed by a spring or other means, which automatically relieves pressure in excess of its setting.

Pulley: A wheel-shaped, belt-driven device used to drive engine accessories.

Pulse width: The time, in milliseconds, the injectors are energized and held open, which determines the amount of fuel injected. Also known as a pulse period and pulse time.

Quench area: Any internal portion of a combustion chamber which causes combustion to cease because of the temperature drop in the air/fuel charge where it meets this area.

Radiator: A heat exchanger used to remove heat from the coolant in the cooling system containing a vertical- or horizontal-finned tubing section connected between two tanks.

Radiator core: The center of the radiator, made of tubes and fins, used to transfer heat from the coolant to the air.

Radiator pressure cap: A cap that seals in pressure from hot expanding coolant until a predetermined limit is reached, then the valve opens, allowing excess pressure to escape, generally to a coolant-recovery tank.

Reciprocating: A back-and-forth, up-and-down, or to-and-fro motion, such as that of a piston in a cylinder.

Reciprocating engine: A powerplant in which the pistons move in a reciprocating motion.

Refrigerant: The chemical compound used in a refrigeration system to produce the desired cooling effect.

Refrigerant HFC-134a (R-134a): The refrigerant of choice, tetrafluoroethane (CH2FCF3), to be used in automotive air conditioners as R-12 is being phased out.

Restrictor plate: A plate with holes drilled in it to restrict the flow of air.

[spark] Retard: To cause ignition spark to occur later in an engine's cycle.

Reversion: The interval during valve timing overlap when both intake and exhaust valves are open and a small mist of air/fuel is forced out back through the carburetor.

A governor.
A mechanical or electrical device that limits engine speed.

Revolutions per minute (rpm): The rate of speed of a rotating part, such as a crankshaft.

Rich mixture: An air/fuel mixture having insufficient air or excessive fuel.

Rigid motor mount: A solid mount for attaching the motor to the frame without any provisions for vibration dampening.

Ring gap: The gap between the ends of a piston ring installed in the cylinder generally 0.004 inch (about 0.1 mm) for each inch (25.4 mm) of cylinder diameter.

Rod big end: The end of the connecting rod that attaches around the crankshaft.

Rod-length ratio: The center-to-center length of a connecting rod divided by the piston stroke.

Rod small end: The end of the connecting rod through which the piston(wrist) pin passes to connect the piston to the connecting rod.

Scavenging: The forced removal of exhaust gases from a cylinder during the overlap period.

Scraper ring: The second ring from the top of a piston used to scrape oil from the cylinder wall.

Sensor plate: A plate used in the air intake of a continuous-flow, fuel-injection system to measure airflow.

Sequential-port fuel injection (SPFI): A type of multi-port injection system where individual fuel injectors are pulsed sequentially, one after another in the same firing order as the spark plugs, rather than being pulsed simultaneously. This allows more precise fuel control for lower emissions and better performance. Also known as sequential fuel injection.

Serpentine belt: A wide, flat belt having multi-V-grooves to provide frictional contact with the pulleys that winds through all of the engine accessories to drive them off the crankshaft pulley.

Shroud: A metal or plastic duct that directs ambient air to the radiator cooling fan.

Shrouding: An obstruction, such as carbon buildup, around a valve in the combustion chamber that interferes with the proper air flow.

Single overhead camshaft engine: An engine having a single camshaft mounted over each cylinder head.

Skin effect: A thin, unburned layer of air/fuel mixture next to the combustion camber surface.

Slipper skirt piston: A piston that has a cutaway skirt so that the piston can come closer to the counterweights reducing the overall size of the engine. Also known as slipper skirt.

Sludge: A buildup of combustion by-products that can clog oil lines and interfere with proper lubrication.

Snap ring: A circular retaining clip used inside or outside a shaft or part to secure a shaft, such as a floating wrist pin.

Sodium-filled valve: A partially hollow valve containing metallic sodium that melts at a low temperature (208°F) 97.8°C and when in its liquid state at operating temperatures splashes around inside the valve, transferring heat away from the valve head.

Sonic testing: A procedure for testing the integrity of engine blocks, using sound waves.

Spin on filter: An oil filter having a threaded attachment for the ease and speed of replacement.

[valve] Spring: Small steel coils that are used to close the intake and exhaust valves when the cam lobes release pressure on the valve stem.

Sprocket: A sheet-like disk with teeth around its outer perimeter that mesh with a belt or chain.

Spun bearing: Any bearing on the crankshaft that has seized on the journal and turned in the housing bore.

Square engine: An engine in which the bore and stroke dimensions are the same.

Squish: The action where some compressed air/fuel mixture is pushed out of a decreasing space between the piston and cylinder head of the combustion chambers in some engines.

Staggered timing camshaft: A camshaft ground so as to provide a longer duration for the cylinders farthest away from the main intake tract to equalize the amount of fuel mixture reaching all of the cylinders.

Stoichiometric ratio: The ideal air to fuel ratio of 14.7:1, in terms of mass, to achieve the most complete combustion possible in an internal-combustion engine.

Stratified charge: A type of combustion having a small amount of rich air/fuel mixture near the spark plug with a leaner mixture throughout the remainder of the combustion chamber.

Stress relieve: An engine block or component that has been relieved of the stress caused by casting and machining.

Stress riser: An area of a part that is most likely to crack due to mishandling and/or misuse.

The distance traveled by the piston from top dead center (TDC) to bottom dead center (BDC).
To increase the stroke in an engine.

Stroker kit: A special crankshaft and connecting rod kit used to increase the displacement of an engine by lengthening the stroke of the pistons.

Surge: A condition where the engine speed increases and decreases slightly with no throttle action.

Surge tank: The reservoir of a cooling system where coolant condenses before being returned to the radiator.

Swirl: A cylinder-head design that causes the air/fuel mixture to enter the combustion chamber at a high rate of speed, increasing its atomization.

Synthetic oil: A type of engine lubricant consisting of highly polymerized chemicals.

System pressure: The average pressure in a system, such as the fuel-injection system.

Tall-deck (tall block) engine: An engine-block design height that permits a longer crank throw, or stroke.

Taper-bored piston (tapered wrist) pins: Piston (wrist) pins having tapered bores to reduce weight and add strength.

Temperature sending unit: A device in contact with the engine coolant that changes resistance, depending on its temperature, and sends a voltage signal to the gauge or lamp.

Thermostat: A temperature-sensitive component located in a housing at the coolant outlet of the engine that restricts coolant flow to the radiator to maintain the desired engine-operating temperature.

Throttle plate (valve): A flap valve that controls the amount of air admitted into the induction system.

Throw a rod: A broken connecting rod that has been forced through the block or oil pan.

Thrust bearing: A bearing or a part of the main bearing that limits end-to-end movement of the crankshaft.

Thrust surface: The area of a crank or block that absorbs end-to-end thrust pressure.

Timing belt: The belt through which the crankshaft drives the camshaft or camshafts in an overhead valve or overhead cam engine.

Timing chain: The chain through which the crankshaft drives the camshaft or camshafts in an overhead valve or overhead cam engine.

Timing gear: The gear or gears through which the crankshaft drives the camshaft or camshafts in an overhead valve or overhead cam engine.

High engine rpm; a point that horsepower is the greatest.
The upper end of the engine, consisting of cylinder head, etc.

Torque: The measure of a force-producing tension and rotation around an axis.

Torque plate: A thick, metal plate bolted to the deck of a block during cylinder boring and honing to reproduce the stress caused when the heads are installed and the bolts are properly torqued.

Transverse engine: An engine mounted sideways in a vehicle, such as in most front- drive vehicles.

Tube-and-fin radiator: A type of heat exchanger that consists of tubes to which fins are attached to facilitate the transfer of heat from the fluid passing through it to the air passing over it.

Tune up: A routine maintenance procedure that includes replacing the rotor and spark plugs; in older cars, replacing the rotor, spark plugs and adjustment of the timing.

Turbulence: A violent disturbance such as the rapid swirling motion of the air/fuel mixture entering a combustion chamber.

Twin-plug head: A cylinder head having provisions for two spark plugs per cylinder.

Two-bolt main: An engine block in which the mains are held in place with two bolts each.

Unleaded gasoline: Gasoline that contains less than 0.0018 ounces (0.05 grams) of tetraethyl lead per gallon (3.785 liters).

Vacuum: An enclosed space from which all air has been removed, having an absolute atmospheric pressure of near zero.

Vacuum line: A rubber-like, plastic or nylon tube used to transmit a vacuum reading from one place to another.

Valve: A circular-stemmed device used to control the flow of air/fuel mixture in and the flow of burned gasses out of the engine.

Valve clearance (lash): The specified clearance between a valve stem end and the lifter/camshaft on an OHC engine; necessary to allow for heat expansion.

Valve face: The tapered section of the valve head making contact with the valve seat.

A valve that does not close completely.
A valve that does not close at the proper time.

Valve guide: The bore in the cylinder head through which the valve stem passes.

Valve keeper: A small part that fits into the retainer groove located near the tip of the valve stem to secure the valve and valve spring. Also known as a keeper or valve key.

Valve lift: Distance that the valve moves from the closed to the open position.

Valve lifter: A cylindrically shaped hydraulic or mechanical device in the valve train that rides on the camshaft lobe to lift the valve off its seat. Also known as a lifter or cam follower.

Valve overlap: The time that the closing of the exhaust valve overlaps the opening of the intake valve at the end of the exhaust stroke and at the beginning of the intake stroke, when the intake and exhaust valves are partially open at the same time depending on the spacing of the lobe centers on the camshaft and the cam's duration.

Valve relief (pocket): An area cut into the top of a piston to provide valve clearance.

Valve spring: A small coil spring that closes the valve and keeps the lifter in contact with the camshaft.

Valve-spring retainer: A device on the valve stem that holds the spring in place.

Valve-spring retainer lock: A device on the valve stem that locks the valve-spring retainer in place.

Valve-spring seat: The area in the cylinder where the fixed end of the valve spring is attached.

Valve-stem groove: That part of the valve stem used to position the keepers and locate the valve retainer.

Valve timing: The actual opening and closing of the valves in relationship to the number of degrees of crankshaft rotation.

Valvetrain: The many parts making up the valve assembly and its operating mechanism.

Vibration damper: A device designed to dampen or reduce crankshaft torsional (twisting) vibrations that might otherwise cause crankshaft cracks or breakage.

Viscosity rating: A numerical indicator of the viscosity of an engine oil established by the American Petroleum Institute.

Volumetric efficiency (VE): A percentage ratio that varies with the engine's rpm measuring the difference between the air-fuel mixture actually entering a cylinder and the amount that can enter under ideal conditions.

V-pulley: Used in automotive applications to drive the accessories, such as a water pump, generator, alternator, power steering, and air-conditioner compressor.

X-drilled crankshaft: A crankshaft having two oil passages at approximately 90 degrees apart in the main journals.

X-drilled rotor: A term used for a cross-drilled brake rotor, in which holes are drilled in the rotor to help evacuate built-up gases caused under heavy braking.