AutoZone Glossary | h

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    GLOSSARY TERMS

    H

    H
    The chemical symbol for hydrogen.
    H-Beam Rod
    A connecting rod having an H-beam-shaped cross section.
    H-shift pattern
    An arrangement of shift positions in the form of an H, for a manual transmission.
    H-Valve
    An air-conditioning expansion valve used on some Chrysler and Ford car lines.
    H2O
    The chemical symbol for water.
    Hack
    A Taxicab.
    A passenger car for everyday driving.
    To cut out a section of body work.
    Hairpin
    A turn greater than 90 degrees on a road or race course.
    A crankshaft.
    Half Shaft
    The axle shafts on either side of the differential that transmit power to the drive wheels.
    Half-Moon Key
    A term often used for Woodruff key.
    Halide Leak Detector
    A device consisting of a tank of acetylene gas, a stove, a chimney, and a search hose used to detect CFC refrigerant leaks by visual means.
    Halogen Lamp
    A high-output, white-light lamp used as a headlight. Also known as halogen light.
    Halogen Leak Detector
    An electronic device used to detect leaks in a refrigerant system.
    Halogen Light
    A high-output, white-light lamp used as a headlight. Also known as halogen lamp.
    Halogens
    Refers to any of five nonmetallic and electronegative chemical elements: astatine (At), bromine (Br), chlorine (Cl), fluorine (F), and iodine (I), such as those that may be found in refrigerants.
    Hammer Forming
    To form or shape metal by hitting it with a hammer over, on, or into a base form.
    Hand Shield
    A hand-held protective device that has a filter plate and is used in arc welding, arc cutting, and thermal spraying for shielding the eyes, face, and neck.
    Hand Tight
    To assemble plain or threaded parts as much as can be done by hand, without the aid of a wrench or tool.
    Hand Tools Institute (HTI)
    A trade association that represents automotive hand-tool and tool-box manufacturers.
    Hand Valve
    A valve mounted on the steering column or dash, used by the driver to apply trailer brakes independently of the tractor brakes.
    Handicap
    A system that allows two cars from different classes in a drag race to compete with each other.
    Handler
    A driver who is able to get the most out of his or her car.
    A car that the driver can get the most out of.
    Hang a left
    To make a left turn.
    Hang a right
    To make a right turn.
    Hang it out
    To deliberately throw the rear end into a slide during a turn.
    Hang on unit
    An under-dash, aftermarket air conditioner. May also refer to other under-dash devices, such as a CB radio.
    Hangover
    Undesired speaker-cone oscillation caused by a transient signal.
    Hard Chrome
    A heavy-chrome plating applied to metal to increase its durability and resistance to wear.
    Hard Copy
    A printed copy of machine output in readable form, such as reports, listings, or graphic images.
    Hard Edge
    A sharp corner where two surfaces meet.
    Hard Face
    The application of a hard material, such as chrome, to a surface to increase its durability and wear resistance.
    Hard Parts
    The internal engine parts required to rebuild an engine.
    Hard Pedal
    A loss in braking efficiency so that an excessive amount of pressure is needed to actuate brakes.
    A condition whereby the load literally overrides the brakes.
    Hard Solder
    Any solder requiring more than 1000°F (538°C) to melt.
    Hard Spots
    Areas in the friction surface of a brake drum or rotor that have become harder than the surrounding metal.
    Hard Throttle
    A term used when the accelerator is pressed to the floor.
    Hardener
    A chemical component of an epoxy resin that starts a catalytic reaction and causes the mixture to harden.
    Hardness Test
    The technique used to determine the hardness of a material.
    Hardtop
    A two- or four-door hardtop convertible body type with front and rear seats, generally characterized by the lack of door or B pillars.
    Hardware
    The components that make up a computer system, such as a keyboard, a floppy disk drive, and a visual-display terminal.
    The nuts, bolts, and accessories required to assemble an add-on component.
    Hardwired
    The physical connection of two pieces of electronic equipment by means of a cable.
    Harmonic
    The rhythmic vibration of a moving part or assembly.
    Harmonic Balancer
    A balance shaft or wheel that reduces or eliminates harmonics.
    Harmonic Damper
    A term used for harmonic balancer.
    Harmonic Vibration
    A rhythmic vibration of a moving part or assembly at a specific speed.
    Harness
    A group of electrical conductors laced or bundled in a given configuration, usually with several breakouts.
    Harness Clip
    A metal or plastic clip used to secure a wiring harness to the car frame or body at various points, providing safe and neat routing.
    Harness Connector
    An electrical connector at the end of a wire or harness used to connect the conductor to a device or system.
    Hat
    A pipe-like housing for the injector on a constant-flow fuel-injection system.
    Hat Trick
    Three successive victories in the same annual event.
    Hatchback
    A passenger-car body style where a full opening hatch, including the rear window, lifts for access to the cargo storage area.
    Hauler
    A top-performing car.
    A top-performing driver.
    Hauling the mail
    Performance at peak efficiency.
    Hazard System
    A driver-controlled system of flashing front- and rear-marker lights to warn approaching motorists of a potential hazard.
    Hazardous Waste
    Any product used in a system that is considered harmful to people and/or the environment.
    Any poisonous substance, the byproduct of a poisonous substance, or one that contains carcinogens and is considered unsafe for animal or plant life by the EPA.
    HC
    An abbreviation for hydrocarbon.
    HC-CO meter
    A device using an infrared sensor to measure the amount of hydrocarbons, in parts per million, and carbon monoxide, in percent, in the vehicle exhaust.
    HD
    An abbreviation for heavy duty.
    HDMA
    An abbreviation for Heavy Duty Manufacturers Association.
    HDRA
    An abbreviation for Heavy Duty Representatives Association.
    An abbreviation for High Desert Racing Association.
    Head
    That part of an engine that covers the top of the cylinders and pistons and usually contains the combustion chambers and valve train.
    That part of a compressor that covers the valve plates and separates the high side from the low side of an air-conditioning system.
    Head Bolts
    The bolts that fasten the cylinder heads to the engine block.
    Head Gasket
    A gasket used to seal the cylinder head to the engine block.
    Head Pressure
    Pressure of the refrigerant from the discharge valve through the lines and condenser to the metering-device inlet.
    Head-Gasket Notch
    A series of one to four notches used to measure the thickness of a gasket to control the piston-to-head clearance on a diesel engine.
    Head-Land Ring
    A top compression ring with an L-shaped cross section.
    Head-Saver Shims
    Shims that are used to compensate for material removed during head resurfacing to maintain correct overhead-cam drive-chain tension.
    Headache Rack
    A rack that is provided at the rear of a truck-cab interior to prevent a load from crashing through the rear window.
    Header
    A vehicle crash.
    A free-flowing exhaust manifold used on high-performance cars.
    Header Tank
    The top and bottom tanks or side tanks of a radiator in which coolant accumulates or is received.
    Headlamps
    The lights at the front of a vehicle used to illuminate the road ahead. Also known as headlights.
    Headlight Delay
    A system that turns off the headlamps automatically after the operator leaves the vehicle, though the lights were left on.
    Headlights
    A term used for headlamps.
    Headliner
    Part of the automobile-interior overhead or the covering of the roof inside.
    Heads-Up Display (HUD)
    The images of a vehicle's instrument panel, displayed on the inside surface of the windshield in front of the driver where they can be seen without taking the eyes off the road.
    Heads-Up Racer
    A race driver that is very good.
    Heads-Up Racing
    Direct, no handicap, competition.
    Heat
    A form of energy released by the burning of fuel.
    Heat Dam
    The narrow groove cut into the top of the piston. It restricts the flow of heat down into the piston.
    Heat Exchanger
    An apparatus in which heat is transferred from one medium to another on the principle that heat moves to an object with less heat.
    Heat Intensity
    The measurement of heat concentration with a thermometer.
    Heat Load
    The load imposed on an air conditioner due to ambient temperature, humidity, and all other factors that produce unwanted heat.
    Heat Of Compression
    An increase of temperature brought by the compression of gas, air, or an air/fuel mixture.
    Heat Of Fusion
    The amount of latent heat, in Btu, required to cause a change of state from a solid to a liquid.
    Heat Of Respiration
    The heat given off by ripening vegetables or fruits in the conversion of starches and sugars.
    Heat Quantity
    The amount of heat as measured on a thermometer.
    The amount of heat contained in an object, measured in Btu's or calories.
    Heat Radiation
    The transmission of heat from one substance to another while passing through, but not heating, intervening substances.
    Heat Range
    The measure of a spark plug's ability to transfer heat from the tip of the insulator into the cylinder head.
    Heat Riser
    A channel in an intake manifold through which exhaust gas flows in order to heat the manifold, thus aiding in fuel vaporization.
    Heat Sink
    A finned, aluminum housing that dissipates heat and serves as a base on which electrical components, such as semiconductor devices, are mounted.
    A heat dissipating section of the alternator that holds the insulated diodes or all the diodes, and carries heat away from the devices.
    Heat Tab
    A small aluminum disk attached to the cylinder head or block having a lead center designed to melt at 260°F (127°C) to indicate if the engine has been overheated.
    Heat Transfer
    The movement of heat from a hotter to a colder area by conduction, convection, or radiation.
    Heat Treatable
    A metal alloy that can be hardened by heat treatment after being shaped.
    Heat Treatment
    A sequence of controlled heating and cooling operations applied to a solid metal to provide the desired properties.
    Heat-Control Valve
    A thermostatically controlled valve in the exhaust manifold to divert heat to the intake manifold before the engine reaches normal operating temperature.
    Heat-Riser Valve
    A control valve between the exhaust manifold and exhaust pipe that restricts the flow of exhaust, causing it to flow back through the heat-riser channel under the intake manifold. It acts as an aid to fuel evaporation and to speed engine warm-up.
    Heated-Air System
    A term used for heated air-intake system.
    Heated-Air-Intake System
    A vacuum operated power door in the air-cleaner snorkel that maintains intake air at a near constant temperature by blending ambient air with heated air picked up from a shroud over the exhaust manifold. Also known as heated air system.
    Heated-Exhaust Gas-Oxygen Sensor (HEGOS)
    An oxygen sensor that is heated electrically and by engine exhaust so it warms up to normal operating temperature more quickly, allowing the engine to enter closed-loop operation sooner than with a non-heated sensor.
    Heater
    A device used to warm the vehicle interior.
    Heater Core
    A small water-to-air radiator-like heat exchanger that provides heat for the passenger compartment.
    Heater Valve
    A manual or automatic valve in the heater hose used for opening or closing, providing coolant flow control to the heater core.
    Heating
    The use of an apparatus that produces a relatively high degree of warmth under controlled conditions, by natural or mechanical means, as an aid to ensure personal safety and comfort.
    Heating Value
    The measure of heat released when a fuel is burned.
    Heavy Duty (HD)
    A device or product that is designed for heavy use.
    Heavy Duty Manufacturers Association (HDMA)
    A trade association of original equipment and aftermarket manufacturers of parts, tools, and equipment for servicing heavy trucks.
    Heavy Duty Representatives Association (HDRA)
    An association of independent heavy-duty manufacturers representatives.
    Heavy-Duty Coil spring
    Coil springs having larger wire diameter, and 3 to 5 percent greater load-carrying capability than a conventional spring.
    Heavy-Duty Shock Absorber
    Shock absorbers having improved seals, a single tube to reduce heat, and a rising rate valve for precise spring control.
    Heavy-Duty Truck
    A truck having a gross vehicle weight of over 26,000 pounds (11,794 kilograms).
    Heavy-Duty Vehicle
    Any motor vehicle having a gross vehicle-weight rating over 8,500 pounds (3,856 kilograms), that has a vehicle curb weight over 6,000 pounds (2,722 kilograms), or has a frontal area exceeding 45 square feet (4.2 square meters).
    Heel
    The innermost part of the gear tooth on a ring and pinion gear set.
    The bottommost portion of a cam on a camshaft.
    Heel and Toe
    The act of applying one's right toes to the brake pedal while applying the right heel to the accelerator to slow the vehicle as it enters a turn while keeping the engine revved up.
    Heel to Toe Clearance
    The space between the brake drum and the heel and toe of a brake shoe.
    HEGOS
    An abbreviation for heated exhaust gas-oxygen sensor.
    HEI
    An abbreviation for high-energy ignition.
    Heim Joint
    A spherical-rod end joint that allows precise adjustment of wheel position.
    Heli-Coil
    A trade name for a screw-thread insert used to repair damaged internal threads.
    Heliarc
    The trade name of a tungsten inert-gas form of welding developed by Linde Welding and Cutting Systems.
    Heliarc Welding
    The act of electrically joining two pieces of aluminum or stainless steel using a high-frequency electric weld and an inert gas, such as argon, that is fed around the weld to prevent oxidation.
    Helical
    A device, such as a spring, shaped like a helix or spiral.
    Helical Compression Spring
    An open-pitch spring, in a variety of configurations, used to resist applied compression forces.
    Helical Extension Spring
    A spring, usually made of round wire, that exerts a pulling force with various types of end hooks or loops by which they are attached to the load.
    Helical Gear
    A gear having teeth cut at an angle across its face, or diagonally to the rotational axis.
    Helical Spring
    Springs made of bar stock or wire, coiled into a helical form, including compression, extension, and torsion springs.
    Helmet
    A device worn on the head to protect eyes, face, and neck from arc radiation, radiated heat, and spatter or other harmful matter expelled during arc welding, arc cutting, and thermal spraying.
    Protective head gear.
    Helper Spring
    An additional spring device that permits a greater load on an axle.
    Hemi
    Half of a sphere.
    Hemi Cammer
    An engine having both hemispherical combustion chambers and an overhead camshaft.
    Hemi Head
    A cylinder head with hemispherical combustion chambers providing room for extra large valves or multi-valve arrangements and providing an ideal shape for smooth, even combustion.
    Hemispherical Combustion Chamber
    A type of combustion chamber that is shaped like half of a circle. This combustion chamber has the valves on either side with the spark plug in the center.
    HEPA
    An abbreviation for high-efficiency particulate air filter.
    Heptane
    A hydrocarbon (C7H16) that serves as a primary reference fuel with an index of 0.
    Hermaphrodite Caliper
    A combination tool having a caliper leg and a leg from a divider. Used in testing centered work and laying off distances from the edge of a piece of work.
    Herringbone Gear
    A gear cut in the V-shaped pattern of the small bony extension of a herring's spine.
    Hertz
    A unit of frequency of any regularly occurring event.
    Hesitation
    A momentary lack of throttle response or pause in the rate of acceleration.
    HFC-134a
    A hydrofluorocarbon gas that is not damaging to the environment and ozone layer and that can be used in automotive air-conditioning systems and to replace CFC-12.
    Hg
    The chemical symbol for the element mercury.
    Hhollow
    The concave area of a body panel.
    The inside of a pipe or tube.
    HHV
    An abbreviation for higher heating value.
    Hi-Po
    High performance.
    Hi-Rev Kit
    A set of auxiliary springs to keep roller lifters in contact with the camshaft lobes.
    Hi-Test
    A term referring to pump gas with an octane rating of 92 or higher.
    High CG Load
    Any application in which the load center of gravity of the trailer exceeds 40 inches (102 centimeters) above the top of the fifth wheel.
    High Desert Racing Association (HDRA)
    A professional off-road racing association that stages desert races in Nevada and adjacent states.
    High Gear
    The condition achieved from a planetary gear set when any two members, sun gear and planet gears or planet gears and internal gears, are locked together.
    High Gearing
    The drive ratio that applies maximum engine output at a relatively high road speed.
    High Head
    A term used when the high-side pressure of a system is excessive.
    High Heat Load
    Refers to the maximum amount of heat that can be absorbed by a refrigerant as it passes through the evaporator.
    High Mobility Multipurpose Wheeled Vehicle (HMMWV)
    A GM diesel powered, multipurpose four-wheel-drive vehicle developed by AM General and built in a variety of configurations for civilian and military use.
    High Modulus Belt
    Those belts designed with special reinforcing members with a high degree of elasticity such as steel, fiberglass, or aramid fiber.
    High Output (HP)
    A replacement device having a higher output than the original equipment device.
    High Pedal
    The position of a brake pedal when it is adjusted near the top of its upward gravel.
    The position of the clutch pedal when engaged in high gear in the transmission of a Model T.
    High Riser
    The raised cylinder heads of the 427 cid Ford engine of the 1960s.
    The term given to a manifold that places the carburetor above its original design location.
    High Side
    A term used for discharge side.
    High Spot
    The outward curvature of a normally flat or smooth surface.
    High Suction
    A term used when the low-side pressure of a system is higher than normal due to a malfunction of the system.
    High Tech
    A term for developments in technology resulting from new, or new uses of materials and machinery that produce products fully comprehensible only to experts in that particular technology.
    High Vacuum
    A vacuum below 500 microns (0.009 psia or 0.66 kPa).
    High-Efficiency Particulate Air Filter (HEPA)
    A filter system that is 99.97% efficient in trapping particles.
    High-Energy Ignition (HEI)
    A General Motors high-voltage ignition system.
    High-Lift Cam
    A camshaft that provides increased valve lift.
    High-Lift Rocker Arms
    Rocker arms that provide increased valve lift.
    High-Load Condition
    Those instances when the air conditioner must operate continuously, at maximum capacity, to provide the cool air required.
    High-Mounted Coil Spring Suspension
    A type of suspension having a coil spring located above the upper control arm, with the top end of the spring contacting the car body inside a spring tower.
    High-Performance Speed Rating
    A rating system used for tires designed for high-performance cars such as SR, HR, and VR.
    High-Performance Tire
    A tire having a high-performance speed rating, such as SR, HR, or VR.
    High-Pressure Control
    A term used for high-pressure cut off switch.
    High-Pressure Cutoff Switch
    An electrical switch activated by a predetermined high pressure to open a circuit during certain high-pressure periods.
    High-Pressure Line
    The lines or hoses from the compressor outlet to the metering device inlet of an air conditioner.
    Any line or hose that carries high-pressure liquid or gas, such as a power-steering hose.
    High-Pressure Relief valve
    A mechanical device designed so that it releases the extreme high pressures of the system to the atmosphere.
    High-Pressure Side
    That part of an air-conditioning system, from the compressor outlet to the evaporator inlet, that is under high pressure.
    Any part of a system, such as fuel injection, that is under high pressure.
    High-Pressure Switch
    An electrical switch activated by a predetermined high pressure.
    High-Pressure Vapor line
    A line or hose running from the compressor outlet to the condenser inlet.
    High-Side Pressure
    A term used for discharge pressure.
    High-Side Service Valve
    A device located on the discharge side of the compressor, permitting the service technician to check the high-side pressures and perform other necessary operations.
    High-Speed Steel (HSS)
    An extremely hard alloy.
    High-Speed System
    A carburetor main-metering system that supplies fuel to the engine at speeds above 25 mph (40 km/h).
    High-Swirl Combustion (HSC)
    Ford's name for a cylinder head and valve design that promotes turbulence in the combustion chamber during the power stroke, contributing to complete and efficient burning of the air/fuel charge.
    High-Swirl Port Technology
    A cylinder-head design with high-speed air flow to pack more air/fuel mixture into the combustion chamber.
    High-Tension Cables
    A term used for spark plug wires.
    High-Tension Wire
    The wire that transmits high voltage from the coil to the distributor.
    High-Vacuum Pump
    A two-stage vacuum pump that has the capability of pulling below 500 microns (0.009 psia or 0.66 kPa).
    High-Voltage Cables
    The secondary cables that carry high voltage from the distributor to the spark plugs.
    Highboy
    A hot rod that has not been chopped, channeled, or sectioned.
    Higher Heating Value (HHV)
    The measure of the heat that is released when a fuel is burned.
    Highway Loss Data Institute (HLDI)
    A research organization sponsored by the property-casualty insurance industry.
    Highway Users Federation (HUF)
    A transportation coalition dedicated to preserving freedom of choice in transportation and mobility.
    History
    Something that is damaged or destroyed beyond repair.
    Hitch
    A device that is attached to the rear of a vehicle to allow it to pull a trailer.
    Hitch Ball
    A steel sphere mounted on top of the hitch to receive the cup-like fitting of a trailer.
    Hitch Pin
    A term used for hitch receiver.
    Hitch Receiver
    A heavy square tube into which the removable shank supporting the hitch ball is inserted.
    Hitch Weight
    The load imposed to the tow vehicle's hitch by the trailer tongue, not to exceed 10–15% of the gross trailer weight.
    HLDI
    An abbreviation for Highway Loss Data Institute.
    HMMWV
    An abbreviation for High Mobility Multipurpose-Wheeled Vehicle.
    HO
    An abbreviation for high output.
    Hog
    Among motorcyclists, a Harley Davidson.
    A large vehicle such as an older Lincoln or Cadillac.
    Hog Out
    To enlarge the openings and passages in an engine.
    Hold the Road
    To steer and handle well at high speeds.
    Hold-Down Spring
    A spring tension mechanism that holds a brake shoe against a backing plate.
    Hole
    The starting position for a drag race.
    A cylinder.
    Hole Shot
    Beating a drag-racing competitor right from the start.
    Hollow Flank
    A concave or indented flank on a camshaft lobe.
    Hollow Pushrod
    A tubular pushrod that is hollow inside.
    Hollow-Domed Piston
    A piston with a compression dome with equal thickness across the top.
    Homologated
    A term used when a sufficient number of a particular car have been built to qualify them for racing as a stock-production vehicle.
    Honcho
    Someone in charge.
    Honda System
    An efficient combustion-chamber design that uses a small auxiliary combustion chamber (containing the spark plug) that receives a rich mixture to get an overall lean mixture in the cylinder to fire dependably.
    Honeycomb
    A converter internal-substrate structure, usually made of ceramic, used to support the catalyst material, such as platinum, that allows maximum catalyst surface exposure to exhaust gases.
    Honk
    To beat a competitor, especially in a drag race.
    Honker
    A top-performing car.
    Hood
    An opening body panel in front of the cowl covering the engine in a front-engine vehicle.
    Hood Scoop
    An air duct in the hood to allow air to flow directly across the engine.
    Hook Up
    To enter a draft in oval-track racing.
    To set up a car for the best possible traction off the starting line of a drag race.
    Hooks
    Open loops or ends of extension springs.
    Hooligan
    A consolation event in dirt-track racing featuring cars that failed to make the show for the main event.
    Hop Up
    To modify an engine for increased performance.
    Horizontal Position
    Level or parallel to the horizon.
    The position in which a fillet weld is performed on the upper side of an approximately horizontal surface and against an approximately vertical surface.
    The position of a groove weld in which the weld axis lies in an approximately horizontal plane and the weld face lies in an approximately vertical plane.
    Horn
    An electrical noise making device.
    Horn Brush/Slip-Ring
    An electrical contact ring used in the horn circuit, located in the steering wheel, and used to maintain electrical contact when the steering wheel is turned.
    Horn Relay
    A relay used to carry the heavy-horn current requiring light current to actuate.
    Horsecar
    A Ford Mustang.
    Horsepower (HP)
    A measurement of mechanical power or the rate at which work is done.
    Hose Clamp
    A device used to attach hoses to the engine, heater core, radiator, and water pump. A popular type of replacement clamp is the high-torque, worm-gear clamp with a carbon steel screw and a stainless-steel band.
    Hose-and-Line Tape
    A term used for insulation tape.
    Host computer
    A main computer that monitors and controls other computers.
    Hot Dog
    A top performer in almost all forms of racing.
    Hot Drawn
    A process of forming metal by heating it and pulling it through rollers or dies.
    Hot Gas
    The condition of refrigerant as it leaves the compressor until it gives up its heat and condenses.
    Hot Iron
    An early term for a hot rod.
    Hot Patch
    A patch applied to a tube or tire with the use of heat to vulcanize it onto the damaged surface.
    Hot Rod
    A passenger car that has been modified and rebuilt for high performance and a distinctive appearance used primarily for straightaway speed or acceleration racing.
    Hot Setup
    A combination of modifications and components that enhances maximum performance.
    Hot Shoe
    A top race driver.
    Hot Soak
    A condition that may occur when an engine is stopped for a prolonged period of time after a hot, hard run causing the gasoline to evaporate out of the carburetor.
    Hot Spot
    A localized area where excessive heat can build up, such as around a spark plug, that could cause preignition.
    An area on an auto body where corrosion is most likely to occur.
    Hot Tank
    A large vat containing a hot, caustic, chemical solution that is used to clean the engine block and parts.
    Hot Wire
    Wire that carries current from the ungrounded terminal of the battery to an electric load.
    To bypass the ignition switch to start the vehicle.
    Hot Working
    The mechanical working of metal at a temperature above its recrystallization point and high enough to prevent strain hardening.
    Hot-Gas Bypass line
    The line that connects the hot-gas bypass valve outlet to the evaporator outlet.
    Hot-Gas Bypass valve
    A device used to meter hot gas back to the evaporator, through the bypass line, to prevent condensate from freezing on the core.
    Hot-Gas Defrosting
    The use of high-pressure gas in the evaporator to remove frost.
    Hot-Idle Compensator
    A temperature-sensitive carburetor valve that opens when the inlet air temperature exceeds a certain level to allow additional air to enter the intake manifold, preventing an overly rich air/fuel ratio.
    Hot-Rolled Steel (HRS)
    Steel that is formed while hot.
    Hot-Tank Gloves
    Long-sleeved, thick rubber gloves for use in the hot tank.
    Hot-Wire Sensor
    An electrical device inside the mass-airflow meter that measures air flow and density.
    Hotchkiss drive
    A popular type of drive-line system that features an open or visible drive shaft.
    Hourglass Frame
    A term used for X-frame.
    Housekeeping
    The type of safety in the shop that keeps floors, walls, and windows clean, lighting proper, and containers and tool storage correct.
    Housing Bore
    The inside diameter of a bearing housing.
    Housing Breather
    A venting device that allows air to enter or leave the axle housing.
    HP
    An abbreviation for horsepower.
    HPV
    An abbreviation for human-powered vehicle.
    HRS
    An abbreviation for hot-rolled steel.
    HSC
    An abbreviation for high-swirl combustion.
    HSS
    An abbreviation for high-speed steel.
    HTI
    An abbreviation for Hand Tools Institute.
    Hub
    The center of a wheel.
    Hub Cap
    A wheel covering.
    Hubbed Drum
    A brake drum that is mounted on a hub.
    HUD
    An abbreviation for heads-up display.
    HUF
    An abbreviation for Highway Users Federation.
    Huffer
    A supercharger.
    Hum
    Interference from ac power, normally audible and of low frequency.
    Human-powered vehicle (HPV)
    Any vehicle powered by the operator, such as a bicycle.
    Hummer
    A term used for Humvee.
    Humvee
    An abbreviation for High Mobility Multipurpose-Wheeled Vehicle.
    Hunting
    The tendency of a system to oscillate around its normal position.
    Hunting Shift
    Tendency of an automatic transmission to upshift, downshift, and upshift rapidly.
    Hybrid
    A vehicle having two separate propulsion systems, such as a gasoline engine and an electric motor.
    A vehicle having its major components from more than one source, such as a Ford engine in a Chevrolet chassis.
    Hybrid Battery
    A battery that combines the advantages of both the low maintenance and the maintenance-free battery.
    Hydraulic Brakes
    A braking system using hydraulic pressure to press the brake shoes against the brake drums.
    Hydraulic Clutch
    A clutch that is actuated by hydraulic pressure.
    Hydraulic Lifter
    Valve lifter located between the camshaft and pushrod that uses internal oil pressure to cause the lifter to expand lengthwise.
    Hydraulic Pressure
    A pressure exerted through the medium of a liquid.
    Hydraulic Principles
    The use of a liquid under pressure to transfer force or motion, or to increase an applied force.
    Hydraulic Valve Lifter
    A device using oil pressure to adjust its length and maintain zero valve clearance so that valve noise is at a minimum. Also referred to as hydro.
    Hydraulics
    Hydraulic ram units, installed in a low rider at each suspension point, to lower or raise the ride height. Also referred to as hydro.
    Hydrazine
    A highly explosive jet fuel sometimes used as an additive fuel for drag racing.
    Hydro
    A term used for hydraulics.
    A short term for hydromatic, a transmission built by General Motors.
    Hydro Cam
    A camshaft especially designed to operate with hydraulic valve lifters.
    Hydro-boost
    A type of brake power booster that uses hydraulic pressure provided by the power-steering pump to reduce pedal effort.
    Hydrocarbon (HC)
    A compound containing hydrogen (H) and carbon(C), such as gasoline.
    Hydrocarbon Emissions
    A term used for exhaust emissions or unburned hydrocarbons.
    Hydrocarbon Reactivity
    A measure of the smog-forming potential of a hydrocarbon.
    Hydrochloric Acid
    A corrosive acid produced when water and R-12 are mixed as within an automotive air-conditioning system.
    Hydrochlorofluorocarbon
    A group of refrigerants that contain the chlorine (Cl) atom and the hydrogen (H) atom, which causes the chlorine atom to dissipate more rapidly in the atmosphere.
    Hydrofluorocarbon
    A man-made compound used in refrigerants, such as R-134a.
    Hydrogen
    A colorless, odorless, flammable gas.
    Hydrolyzing Action
    The corrosive action within the air-conditioning system induced by a weak solution of hydrochloric acid formed by excessive moisture chemically reacting with the refrigerant.
    Hydrophilic
    Attraction to water.
    Hydrophilic tread
    A tire tread that is attracted to water.
    Hydroplaning
    A term used for aquaplaning.
    Hydrostatic Lock
    A condition in an engine when coolant leaks into and fills a cylinder to block movement of the piston.
    Hydrostatic Testing
    Testing the integrity of a tank or other such containers by using water under high pressure.
    Hypalon Wire
    A special dielectric wire covering used for high-voltage wires, such as spark-plug wires.
    Hypoid Gear Set
    Two gears that transmit power at a 90 degree angle or other angles having the driving gear below the driven gear centerline to allow for lowering the drive shaft.
    Hypoid Gears
    A spiral-bevel gear that allows the pinion to be placed below the center of the ring gear in a final drive assembly.
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