Dictionary of Automotive Terms: D

DAMPER: A unit or device designed to remove or reduce vibration, oscillation, etc., of a moving part, fluid, air, etc.

DASHBOARD: Part of body containing driving instruments, switches, etc.

DASHPOT: Unit utilizing cylinder and piston, or cylinder and diaphragm, with small vent hole, to retard or slow down movement of some part.

DC (Electrical): Direct Current.

DC (Piston Position): Dead Center. Piston at extreme top or bottom of its stroke.

DEAD AXLE: Axle that does not rotate but merely forms base upon which to attach wheels.

DEAD CENTER (Engine): Point at which piston reaches its uppermost or down-most position in cylinder. Rod crank journal would be at 12 o’clock UDC or 6 o’clock LDC.

DECELERATION: The process of slowing down in rotational speed, forward speed, etc.

DECELERATION VALVE: It feeds air into intake manifold to prevent backfiring during deceleration.

DECIBEL: A unit of measurement used to indicate a sound level or to indicate the difference in specific sound levels.

DEFLECTION RATE (Springs): Measurement of force, in lbs., required to compress leaf spring a distance of one inch.

DEGLAZER: Abrasive tool used to remove glaze from cylinder walls so a new set of rings will seat.

DEGREE (Circle): 1/360 part of a circle.

DEGREE WHEEL: Wheel-like unit attached to engine crankshaft. Used to time valves to a high degree of accuracy.

DEHYDRATE: To dry out. Remove moisture.

DEMAGNETIZE: Removing residual magnetism from an object.

DEPOLARIZE: Removal of residual magnetism thereby destroying or removing the magnetic poles.

DESICCANT: Material, such as silica-gel, placed within a container to absorb and retain moisture.

DETENT BALL AND SPRING: Spring loaded ball that snaps into a groove or notch to hold some sliding object in position.

DETERGENT: Chemical added to engine oil to improve its characteristics (sludge control, non-foaming, etc.).

DETONATION: Fuel charge firing or burning too violently, almost exploding.

DIAGNOSIS: Process of analyzing certain symptoms, readings, etc., in order to deter mine underlying reason for trouble at hand.

DIAL GAUGE OR INDICATOR: Often used precision micrometer type instrument that indicates exact reading via needle moving across dial face.

DIAPHRAGM: Flexible cloth-rubber sheet stretched across an area thereby separating two different compartments.

DICHLORODIFLUOROMETHANE: Re frigerant-12 used in air conditioning system.

DIE (Forming): One of a matched pair of hardened steel blocks that are used to form metal into a desired shape.

DIE (Thread): Tool for cutting threads.

DIE CASTING: Formation of an object by forcing molten metal, plastic, etc., into a die.

DIESEL ENGINE: Internal combustion engine that uses diesel oil for fuel. True diesel does not use an ignition system but injects diesel oil into cylinders. Piston com presses air so tightly that air is hot enough to ignite diesel fuel without spark.

DIESEL FUEL GRADE: Classification of diesel fuel operating characteristics, 1-D is for cold weather and 2-D is for normal conditions.

DIESEL INJECTION PUMP: Mechanically operated fuel pump which develops high pressure to force fuel out of injectors and into combustion chambers.

DIESELING: Condition in which engine continues to run after ignition key is turned off. Also called “running on.”

DIFFERENTIAL: Unit that will drive both rear axles at same time but will allow them to turn at different speeds when negotiating turns.

DIFFERENTIAL CASE (Carrier): Steel unit to which the ring gear is attached. Case drives spider gears and forms an inner bearing surface for axle end gears.

DIFFERENTIAL WINDING: A secondary winding in an electrical device that's wound in a reverse manner as related to the primary (main) windings.

DIMMER SWITCH: Foot operated switch that operates headlight low and high beams.

DIODE: Unit having ability to pass electric current readily in one direction but resisting current flow in the other.

DIPSTICK: Metal rod that passes into oil sump. Used to determine quantity of oil in engine.

DIRECT CURRENT (DC): Electric current that flows steadily in one direction only.

DIRECT DRIVE: Such as high gear when crankshaft and drive shaft revolve at same speed.

DIRECT FUEL INJECTION: Fuel is sprayed into combustion chamber.

DIRECTIONAL STABILITY (Steering): Ability of car to move forward in straight line with minimum of driver control.

DISCHARGE ( Battery): Drawing electric current from battery.

DISCHARGE PRESSURE (Air Conditioning): Pressure of refrigerant as it leaves compressor.

DISCHARGE SIDE (Air Conditioning): The high pressure section of the air conditioning system extending from the compressor to the expansion valve.

DISC WHEEL: Wheel constructed of stamped steel.

DISPLACEMENT: Total volume of air displaced by piston traveling from BDC to TDC.

DISTILLATION: Heating a liquid and then catching and condensing the vapors given off by heating process.

DISTRIBUTION TUBES (Cooling System): Tubes used in engine cooling area to guide and direct flow of coolant to vital areas.

DISTRIBUTOR (Ignition): Unit designed to make and break the ignition primary circuit and to distribute resultant high voltage to proper cylinder at correct time.

DISTRIBUTOR CAP (Ignition): Insulated cap containing central terminal with series (one per cylinder) of terminals that are evenly spaced in circular pattern around central terminal. Secondary voltage travels to central terminal where it's then channeled to one of outer terminals by the rotor.

DIVERTER VALVE: A device used in the air injection system to divert air away from the injection nozzles during periods of deceleration. Prevents “backfiring.”

DOHC: Refers to an engine with double (two) overhead camshaft.

DOT: Department of Transportation.

DOUBLE FLARE: End of tubing, especially brake tubing, has a flare so made that flare area utilizes two wall thicknesses. This makes a much stronger joint and from safety standpoint, it's a must.

DOWEL PIN: Steel pin, passed through or partly through, two parts to provide proper alignment.

DOWNDRAFT CARBURETOR: A carburetor in which air passes downward through carburetor into intake manifold.

DOWNSHIFT: Shifting to lower gear.

DRAG LINK: A steel rod connecting pitman arm to one of steering knuckles. On some installations drag link connects pitman arm to a center idler arm.

DRAW (Electrical): Amount of electrical current required to operate electrical device.

DRAW (Forming): To form (such as wire) by pulling wire stock through series of hardened dies.

DRAW (Temper): Process of removing hard ness from a piece of metal.

DRAW-FILING: Filing by passing file, at right angles, up and down the length of work.

DRIER (Receiver-Drier): Tank, containing desiccant, inserted in air conditioning system to absorb and retain moisture.

DRILL: Tool used to bore holes.

DRILL PRESS: Non-portable machine used for drilling.

DRIVE-FIT: Fit between two parts when they must be literally driven together.

DRIVELINE: Propeller shaft, universal joints, etc., connecting transmission output shaft to axle pinion gear shaft.

DRIVE OR PROPELLER SHAFT SAFETY STRAP: A metal strap or straps, surrounding drive shaft to prevent shaft from falling to ground in event of a universal joint or shaft failure.

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

DRIVE TRAIN: All parts that generate power (engine) and transmit it to road wheels (transmission, clutch, drive shaft, differential, drive axles).

DRIVING LIGHTS: Auxiliary headlights, often very bright, that can be used to in crease amount of illumination provided by regular headlights.

DROP CENTER RIM: Center section of rim being lower than two outer edges. This allows bead of tire to be pushed into low area on one side while the other side is pulled over and off the flange?

DROP FORGED: Part that has been formed by heating steel blank red hot and pounding it into shape with a powerful drop hammer.

DROPPED AXLE: Front axle altered so as to lower the frame of car. Consists of bending axle downward at outer ends. (Solid front axle.)

DRY CELL or DRY BATTERY: Battery (like flashlight battery) that uses no liquid electrolyte.

DRY CHARGED BATTERY: Battery with plates charged but lacking electrolyte. When ready to be placed in service, electrolyte is added.

DRY FRICTION: Resistance to movement between two un-lubricated surfaces.

DRY SLEEVE: Cylinder sleeve application in which sleeve is supported in block metal over its entire length. Coolant does not touch sleeve itself.

DRY SUMP: Instead of letting oil throw-off drain into a regular oil pan sump, system collects and pumps this oil to a remote (separate) container or sump.

DRY WEIGHT: Weight of vehicle without fluid (oil, fuel, water) in various units.

DUAL BRAKES: Tandem or dual master cylinder to provide separate brake system for both front and rear of car.

DUAL BREAKER POINTS (Ignition): Distributor using two sets of breaker points to increase cam angle so that at high engine speeds, sufficient spark will be produced to fire plugs.

DUALS: Two sets of exhaust pipes and mufflers — one for each bank of cylinders.

DWELL: See Cam Angle.

DYNAMIC BALANCE: When centerline of weight mass of a revolving object is in same plane as centerline of object, that object would be in dynamic balance. For example, weight mass of the tire must be in the same plane as centerline of wheel.

DYNAMO: Another word for generator.

DYNAMOMETER: Machine used to mea sure engine horsepower output. Engine dynamometer measures horsepower at crank shaft and chassis dynamometer measures horsepower output at wheels.