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Dead Center - The extreme top or bottom of the piston stroke.

Detonation - An unwanted explosion of the air/fuel mixture in the combustion chamber caused by excess heat and compression, advanced timing, or an overly lean mixture. Also referred to as "ping".

Diaphragm - A thin, flexible wall separating two cavities, such as in a vacuum advance unit.

Dieseling - A condition in which hot spots in the combustion chamber cause the engine to run on after the key is turned off.

Differential - A geared assembly which allows the transmission of motion between drive axle, giving one axle the ability to turn faster than the other.

Diode - An electrical device that will allow current to flow in one direction only.

Disc Brake - A hydraulic braking assembly consisting of a brake disc, or rotor, mounted on an axle, and a caliper assembly containing, usually two brake pads which are activated by hydraulic pressure. The pads are forced against the sides of the disc, creating friction which slows the vehicle.

Distributor - A mechanically driven device on an engine which is responsible for electrically firing the spark plug at a predetermined point of the piston stroke.

Dowel Pin - A pin, inserted in mating holes in two different paths allowing those parts to maintain a fixed relationship.

Drum Brake - A braking system which consists of two brake shoes and one or two wheel cylinders, mounted on a fixed backing plate, and a brake drum, mounted on an axle, which revolves around the assembly.

Dwell - The rate, measured in degrees of shaft rotation, at which an electrical circuit cycles on and off.

Electronic Control Unit (ECU) - Ignition module, module, amplifier or igniter. See Module for definition.

Electronic Ignition - A system in which the timing and firing o the spark plugs is controllec by an electronic control unit, usually called a module. These systems hav no points or condenser.

End-Play - The measured amount of axial movement in a shaft.

Engine - A device that converts heat into mechanical energy.

Exhaust Manifold - A set of cast passages or pipes which conduct exhaust gases from the engine.

Feeler Guage - A blade, usually metal, or precisely predetermined thickness, used to measure the clearance between two parts.

Firing Order - The order in which combustion occurs in the cylinders of an engine. Also the order in which spark is distributed to the plugs by the distributor.

Flooding - The presence of too much fuel in the intake manifold and combustion chamber which prevents the air/fuel mixture from firing, thereby causing a no-start situation.

Flywheel - A disc shaped part bolted to the rear end of the crankshaft. Around the outer perimeter is affixed the ring gear. The starter drive engages the ring gear, turning the flywheel, which rotates the crankshaft, imparting the initial starting motion to the engine.

Foot Pound (ft. lbs. or sometimes, ft.lb.) - The amount of energy or work needed to raise an item weighing one pound, a distance of one foot.

Fuse - A protective device in a circuit which prevents circuit overload by breaking the circuit when a specific amperage is present. The device is constructed around a strip or wire of a lower amperage rating than the circuit it is designed to protect. When an amperage higher than that stamped on the fuse is present in the circuit, the strip or wire melts, opening the circuit.

Gear Ratio - The ratio between the number of teeth on meshing gears.

Generator - A device which converts mechanical energy into electrical energy.

Heat Range - The measure of a spark plug's ability to dissipate heat from its firing end. The higher the heat range, the hotter the plug fires.

Hub - The center part of a wheel or gear.

Hydrocarbon (HC) - Any chemical compound made up of hydrogen and carbon. A majo pollutant formed by the engine as a byproduct of combustion.

Hydrometer - An instrument used to measure the specific gravity of a solution.

Inch Pound (inch lbs.; sometimes in.lb. or in. lbs.) - One twelfth of a foot pound.

Induction - A means of transferring electrical energy in the form of a magnetic field. Principle used in the ignition coil to increase voltage.

Injector - A device which receives metered fuel under relatively low pressure and is activated to inject the fuel into the engine under relatively high pressure at a predetermined time.

Input Shaft - The shaft to which torque is applied, usually carrying the driving gear or gears.

Intake Manifold - A casting of passages or pipes used to conduct air or a fuel/air mixture to the cylinders.

Journal - The bearing surface within which a shaft operates.

Key - A small blocked usually fitted in a notch between a shaft and a hub to prevent slippage of the two parts.

Manifold - A casting of passages or set of pipes which connect the cylinders to an inlet or outlet source.

Manifold Vacuum - Low pressure in an engine intake manifold formed just below the throttle plates. Manifold vacuum is highest at idle and drops under acceleration.

Master Cylinder - The primary fluid pressurizing device in a hydraulic system. In automotive use, it is found in brake and hydraulic clutch systems and is pedal activated, either directly or, in a power brake system, through the power booster.

Module - Electronic control unit, amplifier or igniter of solid state or integrated design which controls the current flow in the ignition primary circuit based on input from the pick-up coil. When the module opens the primary circuit, high secondary voltage is induced in the coil.

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