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Needle Bearing - A bearing which consists of a number (usually a large number) of long, thin rollers.

Ohm - The unit used to measure the resistance of conductor-to-electrical flow. One ohm is the amount of resistance that limits current flow to one ampere in a circuit with one volt of pressure.

Ohmmetter - An instrument used for measuring the resistance, in ohms, in an electrical circuit.

Output Shaft - The shaft which transmits torque from a device, such as a transmission.

Overdrive - A gear assembly which produces more shaft revolutions than that transmitted to it.

Overhead Camshaft (OHC) - An engine configuration in which the camshaft is mounted on top of the cylinder head and the camshaft is located in the cylinder block. The camshaft operates the valves via lifters and pushrods.

Overhead Valve (OHV) - An engine configuration in which all of the valves are located in the cylinder head and the camshaft is located in the cylinder block. The camshaft operates the valves via lifters and pushrods.

Oxides of Nitrogen (NOx) - Chemical compounds of nitrogen produced as a byproduct of combustion. They combine with hydrocarbons to produce smog.

Oxygen Sensor - Use with the feedback system to sense the presence of oxygen in the exhaust gas and signal the computer which can reference the voltage signal to an air/fuel ratio.

Pinion - The smaller of two meshing gears.

Piston Ring - An open-ended ring with fits into a groove on the outer diameter of the piston. Its chief function is to form a seal between the piston and cylinder wall. most automotive pistons have three rings: two for compression sealing one for oil sealing.

Preload - A predetermined load placed on a bearing during assembly or by adjustment.

Primary Circuit- The low voltage side of the ignition system which consists of the ignition switch, ballast resistor or resistance wire, bypass, coil, electronic control unit and pick-up coil as well as the connecting wires and harnesses.

Press Fit - The mating of two parts under pressure, due to the inner diameter of one being smaller than the outer diameter of the other, or vice versa; and interference fit.

Race - The surface on the inner or outer ring of a bearing on which the balls, needles or rollers move.

Regulator - A device which maintains the amperage and/or voltage levels of a circuit at predetermined values.

Relay - A switch which automatically opens and/or voltage levels of a circuit at predetermined values.

Resistance - The opposition to the flow of current through a circuit or electrical device, and is measured in ohms. Resistance is equal to the voltage divided by the amperage.

Resistor - A device, usually made of wire, which offers a preset amount of resistance in an electrical circuit.

Ring Gear - The name given to a ring-shaped gear attached to a differential case, o affixed to a flywheel or as part of a planetary gear set.

Roller Bearing - A bearing made up of hardened inner and outer races between which hardened steel rollers move.

Rotor - 1. The disc-shaped part of a disc brake assembly, upon which the brake pads bear; also called brake disc. 2. The device mounted atop the distributor shaft, which passes current to the distributor cap tower contacts.

Secondary Circuit - The high voltage side of the ignition system, usually above 20,000 volts. The secondary includes the ignition coil, coil wire, distributor cap and rotor, spark plug wires and spark plugs.

Sending Unit - A mechanical, electrical hydraulic or electro-magnetic device which transmits information to a gauge.

Sensor - Any device designed to measure engine operating conditions or ambient pressures and temperatures. Usually electronic in nature and designed to send a voltage signal to an on-board computer, some sensors may operate as a simple on/off switch or they may provide a variable voltage signal (like a potentiometer) as conditions or measured parameters change.

Shim - Spacers of precise, predetermined thickness used between parts to establish a proper working relationship.

Slave Cylinder - In automotive use, a device in the hydraulic clutch system which is activated by hydraulic force, disengaging the clutch.

Solenoid - A coil used to produce a magnetic field, the effect of which is to produce work.

Spark Plug - A device screwed into the combustion chamber of a spark ignition engine. The basic construction is a conductive core inside of a ceramic insulator, mounted in an outer conductive base. An electrical charge from the spark plug wire travels along the conductive core and jumps a preset air gap to a grounding point or points at the end of the conductive base. The resultant spark ignites the fuel/air mixture in the combustion chamber.

Splines - Ridges machined or cast onto the outer diameter of a shaft or inner diameter of a bore to enable parts to mate without rotation.

Tachometer - A device used to measure the rotary speed of an engine, shaft, gear, etc., usually in rotations per minute.

Thermostat - A valve, located in the cooling system of an engine, which is closed when cold and opens gradually in response to engine heating, controlling the temperature of the coolant and rate of coolent flow.

Top Dead Center (TDC) - The point at which the piston reaches the top of its travel on the compression stroke.

Torque - The twisting force applied to an object.

Torque Converter - A turbine used to transmit power from a driving member to a driven member via hydraulic action, providing changes in drive ratio and torque. In automotive use, it links the driveplate at the rear of the engine to the automatic transmission.

Transducer - A device used to change a force into an electrical signal.

Transistor - A semi-conductor component which can be actuated by a small voltage to perform an electrical switching function.

Tune-Up - A regular maintenance function, usually associated with the replacement and adjustment of parts and components in the electrical and fuel systems of a vehicle for the purpose of attaining optimum performance.

Turbocharger - An exhaust driven pump which compresses intake air and forces it into the combustion chambers at higher than atmostpheric pressures. The increased air pressure allows more fuel to be burned and results in increased horsepower being produced.

Vacuum Advance - A device which advances the ignition timing in response to increased engine vacuum.

Vacuum Guage - An instrument used to measure the presense of vacuum in a chamber.

Valve - A device which controls the pressure, direction of flow or rate of flow of a liquid or gas.

Valve Clearance - The measured gap between the end of the valve stem and the rocker arm, cam lobe or follower that activates the valve.

Viscosity - The rating of a liquid's internal resistance to flow.

Voltmeter - An instrument used for measuring electrica force in units called volts. Voltmeters are always connected parallel with the circuit being tested.

Wheel Cylinder - Found in the automotive drum brake assembly, it is a device, actuated by hydraulic pressure, which through internal pistons, pushes the brake shoes outward against the drums.