Dictionary of Automotive Terms Abbreviations

DICTIONARY OF AUTOMOTIVE TERMS - "He"

Head

The upper part of something.
A term used to express the increase of energy content in a fluid pumped, expressed in units of energy per unit of mass, usually simply feet.
A part of the engine which covers the piston and creates the combustion chamber. The proper name is cylinder head.
Pressure, usually expressed in feet of water, inches of mercury, or millimeters of mercury.
The product of the water's weight and a usable difference in elevation gives a measurement of the potential energy possessed by water.

Also See
blown head gasket
button head
cheese head
comet head
composite headlight
crossflow cylinder head
cylinder head
double-overhead cam
exhaust header
F-head
F-head engine
five-valve head
fixed head
flanged head
flathead
gib-head key
headers
head gasket
headlight
headliners
head restraint
headset
heads up display
head tube
hemi head
high compression head
I-head
I-head engine
interchangeable head torque wrench
Jaguar Drophead
jet head
L-head
L-head engine
Lagonda Drophead
mixing head
negative suction head
octagonal head bolt
overhead cam
overhead valves
Phillips head screw
piston head
rear head restraints
round head
round head bolt
sealed beam headlight
spark head
speedometer head
steering head
T-head
test head
thermal head
two-valve head
valve-in-head engine
valve head
X-flow cylinder head

Headache rack

A colloquial term for the roll bar (safety bar) above the head of the driver
Heavy protective barrier mounted behind a truck tractor's cab. Designed to prevent "headaches" caused by load shifting forward from the trailer and crushing the cab.

Head bolt: See
cylinder head bolt
octagonal head bolt
round head bolt

Head engine: See
F head engine
I head engine
L head engine
T head engine

Header

The action of diving or crashing into something.
An exhaust manifold. See headers. The British term is "headpipe."
A part at the top of another part.
Length of pipe or vessel, to which two or more pipe lines are joined, that carries fluid from a common source to various points of use.

Also See
exhaust header
high-performance header
tuned header
twin header
windshield header

Header bar: The front lateral bar of a roof which attaches to the top of the windshield frame; usually made of sheet steel shaped to match the top front edge to the curvature of the windshield

Header bow: The front lateral bar of a roof which attaches to the top of the windshield frame; usually made of sheet steel shaped to match the top front edge to the curvature of the windshield

Header panel: An exterior body panel located just ahead of the hood and surrounds the radiator.

Also See
windshield header panel

Headers

The down pipe connecting the exhaust manifold to the front of the muffler or to the connector pipe. The British term is "headpipe."
A special exhaust manifold that replace the stock manifold. It is designed with smooth flowing lines to prevent back pressure caused by sharp bends, rough castings, etc.

Also See
exhaust manifold

Header tank: The top radiator tank in a thermosyphon water cooling system

Head friction: Head required to overcome friction of the interior surface of a conductor and between fluid particles in motion.

Head gasket: The seal at the top of the cylinder which sits between the cylinder block and the head. It keeps the coolant out of the combustion chamber and retains compression in the cylinder. Also called cylinder head gasket.
Also See
blown head gasket

Heading angle: The yaw angle.

Head key: See
gib head key

Headlamp: In Britain, "headlamp" refers to the actual unit, whereas headlight" is used for the unit as well as for its function and where emphasis is on the actual light produced by the lamp. In North America the terms are interchangeable.

Also See
circular headlamp

Headlamp delay system: Automatically controls headlamp ON-OFF operation after ignition and main lighting switch are turned OFF

Head land: The uppermost piston land, subject to the highest thermal load

Headlight: The main light on the front of a vehicle. In Britain, "headlamp" refers to the actual unit, whereas "headlight" is used for the unit as well as for its function and where emphasis is on the actual light produced by the lamp. In North America the terms are interchangeable.

Also See
composite headlight
concealed headlights
ellipsoidal headlight
ff headlight
gas discharge headlight
gaseous discharge headlight
halogen headlight
hide-away headlights
high beam
homofocal headlight
inner headlight
low beam
mf headlight
multi-focal headlight
outer headlight
pe headlight
polyellipsoidal headlight
pop-up headlights
rectangular headlight
retractable headlights
sealed beam headlight
sloping headlight
twin headlight

Headlight adjusting screw: A screw which moves the headlight up or down, left or right to align the beam of light

Headlight beam setting: British term for headlight adjusting screw

Headlight body: A sheet metal pot welded or screwed to the front fender that provides the housing for the headlight and its bulbs and wiring

Headlight bucket: The headlight housing of cars or motorcycles having separate headlights not integrated into the body line

Headlight cover

A rigid or flexible protection against dirt or stone damage when lights are not being used. On some vehicles, when the lights are turned on, the covers are flipped up by an electric or vacuum operated device.
A clear protective cover which is permanently mounted in front of the headlights to protect the headlight itself from damage.

Headlight dimmer switch: A switch which changes the headlight from high beam to low beam or from low beam to high beam. In older vehicles, the switch was located on the floor to the far left and operated by the driver's left foot. In newer vehicles, it is found on a stalk projecting from the steering column beneath the steering wheel. In Britain it is called the "headlight dipper switch." Also called, "dimmer switch."

Headlight dipper switch: British term for headlight dimmer switch. Also called "dipswitch"

Headlight flasher: On some vehicles, when you flick the direction indicator lever upwards against spring pressure, the headlights flash on and off quickly. It is used to remind on-coming traffic to dim their lights or to warn them about possible hazards on the road.

Headlight leveling: A system that compensates for a heavy load in the trunk which pushes the front of the car up and causes the headlights to project upward. The leveling system levels out the projection of the light. Some will do it automatically while others have a manual control.

Headlight leveling control: Usually a manual control with a vertical thumbwheel that adjusts the height of the headlight beam

Headlight mounting panel: A sheet metal panel for rectangular headlights that is spot-welded to the front section of modern cars where the radiator grille and the headlights are mounted adjacent to each other; provides the mounting points for the headlight

Headlight on/off delay system: A system with two functions when activated, it can automatically turn ON the headlights during darkness and OFF during daylight; it can also be set to keep the headlights ON for up to approximately three minutes after leaving the parked vehicle; useful in dark, high-risk areas. The system is controlled by a photovoltaic cell on the dashboard

Headlight retractor indicator lamp: A light on the dash that illuminates when the headlight covers are opening or closing

Headlight shell: The headlight housing of cars or motorcycles having separate headlights not integrated into the body line

Headlight visor

A partial lid which encircles only the top part of the headlight.
An attachment between headlight rim and lens, used only on some classic cars

Headlight wash/wipe: A system that cleans the headlights with a jet of water and a small wiper blade

Headliner: Fabric or vinyl upholstery on the interior of the roof of a vehicle. The British term is "headlining."

Headlining: A British term for headliner a soft lining on the inside of a car roof.

Head nut: See
cylinder head nut

Head-on collision: An accident in which two cars hit each other's front end. Also called "head-on crash."

Head-on crash: An accident in which two cars hit each other's front end. Also called "head-on collision."

Headpipe: A British term for the header—the down pipe connecting the exhaust manifold to the front of the muffler or to the connector pipe.

Also See
twin headpipe

Head pressure

Pressure which exists in condensing side of refrigerating system.
Force caused by the weight of a column or body of fluids. Expressed in feet, inches, or psi.
Refrigerant pressure in the lines and condenser between the discharge reed valve and the expansion valve orifice

Head pressure control: Pressure-operated control which opens electrical circuit if high-side pressure becomes too high.

Head pressure safety cutout: Motor protection device wired in series with motor; will shut off the motor when excessive head pressures occur.

Headrest: Another term for head restraint

Head restraint: An extension on the upper portion of the seatback. Some are permanently attached and other are adjustable. They are designed to reduce whiplash injuries caused by sudden stopping or collisions. They limit the rearward motion of the head and neck. Some head restraints contain radio speakers or even TV sets.

Also See
rear head restraints

Headroom: The distance between the top of a passenger's head and the inside of the vehicle roof

Head screw: See
phillips head screw

Headset: On a bicycle or motorcycle, the component that secures the forks to the frame and consists of cups, cones, and ball bearings that creates the bearing mechanism that allows the fork column to rotate inside the head tube. It thus permits the fork to turn for steering and balancing. Headsets can be either threaded or threadless.

Headset sizing: The nominal size of a headset is based on the outside diameter of the steerer. This is a source of confusion, because the steerer is not visible on an assembled bicycle. In the case of a bicycle with a traditional expander/wedge type stem, the stem shaft will be 1/8" smaller than the steerer. Sometimes people measure the stem diameter and assume, incorrectly, that this is the size headset they have

Headset star fangled nut: A part of a threadless headset that is inserted into the fork steerer tube. The top cap bolt of a threadless headset is threaded into the star fangled nut.

Headset threaded: See
threaded headset

Headset threadless: See
threadless headset

Head, static: Pressure of fluid expressed in terms of height of column of the fluid, such as water or mercury.

Heads up display: (HUD) A system of mounting gauges so that the read-out is shown on the windshield. In this way the driver does not have to take his eyes off the road to see how his vehicle is performing.

Head tester: See
cylinder head tester

Head torque: See
interchangeable head torque wrench

Head torque wrench: See
interchangeable head torque wrench

Head, total static: Static head from the surface of the supply source to the free discharge surface.

Head trim: The lining of the roof.

Head tube: The vertical tube (visually identified as the shortest tube) in the main triangle of a bicycle frame, the one inside of which the fork column (fork steerer tube) is inserted and rotates. The top tube and down tube are attached to it. The fork steerer tube is inserted into the head tube.

Head velocity: Height of fluid equivalent to its velocity pressure in flowing fluid.

Healey

See Austin Healey.
A brand of vehicle of which the 1949-50 Silverstone models are milestone cars.

Heat

That which increases the internal energy of a body by changing the motion of the molecules. This causes a change in temperature, volume, or state of the body.
Form of energy which acts on substances to raise their temperature; energy associated with random motion of molecules.
The energy associated with the motion of atoms or molecules in solids, which can be transmitted through solid and fluid media by conduction, through fluid media by convection, and through empty space by radiation. All substances with temps above absolute zero contain heat

Also See
grass heat shield
manifold heat control valve

Heat-affected zone: That part of the base metal which has been altered by the heat from the welding, brazing, or cutting operation.

Heat balance: See
thermal equilibrium.

Heat capacity: See
BTU

Heat checking: Small cracks on a brake disc or drum friction surface caused by heat. Heat check can usually be removed by resurfacing

Heat checks: Cracks in the clutch pressure plate

Heat conductivity: Speed and efficiency of heat energy movement through a substance.

Heat content: The amount of heat energy available to be released by the transformation or use of a specified physical unit of an energy form (e.g., a ton of coal, a barrel of oil, a kilowatthour of electricity, a cubic foot of natural gas, or a pound of steam). The amount of heat energy is commonly expressed in Btu's. Heat content of combustible energy forms can be expressed in terms of either gross heat content (higher or upper heating value) or net heat content (lower heating value), depending upon whether or not the available heat energy includes or excludes the energy used to vaporize water (contained in the original energy form or created during the combustion process). The Energy Information Administration typically uses gross heat content values.

Heat control: See
manifold heat control valve

Heat control valve: See
heat-control valve
manifold heat control valve

Heat-control valve: A valve which is controlled by temperature changes. When the ambient temperature is cold or the engine has not warmed up, it is closed so that some of the hot exhaust gases passes by the intake manifold to pre-heat the fuel mixture going to the cylinders. As the engine warms up, the valve opens up and no exhaust gases pass by the intake manifold. If the valve does not operate properly, the engine has difficulty in start up or the fuel may cause vapor locking.

Heat cracking: A pattern of small, irregular cracks (e.g., on brake discs)

Heat crossover: A passage from one exhaust manifold up, over, and under the carburetor and on to the other manifold. Usually found on V-8 or V-6 engines. This crossover provides heat to the carburetor during engine warmup.

Heat curing adhesives: Adhesives that require a definite period of time above room temperature to develop full bond strength. They can be of one or 2 part composition. The term is usually applied to those adhesives that require 82°C or above to effect a cure

Heat dam: A thin groove cut into the head of a piston between the top ring groove and the top of the piston. The heat, instead of passing through the aluminum of the piston to the ring, encounters the heat dam. This helps to minimize heat transfer.

Heat dissipation: The transfer of heat. In brake systems the heat produced by braking is transferred to the air.

Heated air inlet system: (HAI) a system that operates during cold weather and cold start. Brings warm, filtered air into the engine to control the volume of air entering the engine, vaporize the fuel better and reduce HC and CO emissions

Heated exhaust gas oxygen sensor: (HEGO) an EGO sensor with a heating element

Heated intake: A device that helps to vaporize the gasoline in a cold engine. The thermostat in the air cleaner horn is connected to the outside of the exhaust manifold. When the engine is cold, the thermostat closes a damper in the air cleaner horn so that air going to the carburetor does not come through the open end of the horn. Instead the air comes from above the hot exhaust manifold. As the engine warms up, the thermostat opens the damper so that air is taken in from the end of the horn.

Heated oxygen sensor: An oxygen sensor (O2S) which is heated to improve sensor performance

Heated rear window: A rear window with a heating element that defogs (demists) inside and defrosts (de-ices) the outside either by fine wires embedded or etched in the glass or by a heater and fan.

Heated tool welding: A welding process in which the parts to be welded are pressed against a heated plate and subsequently pressed together to produce a fusion weld

Heated windshield: Windshield with a heating element to facilitate defrosting (de-icing)

Heater: A device which gives off heat. In automobiles, it heats the interior of the vehicle. In a water-cooled engine the coolant is channelled through the heater in the passenger compartment. Some cars used an electric or gasoline heater because they had no engine coolant (e.g., the Volkswagen Beetle).

Also See
backlight heater
block heater
cylinder block heater
engine block heater
inlet manifold heater
intake manifold heater
manifold heater
parking heater
rear window heater system

Heater air pipe: A channel section incorporated into the side member or other structural sections that is designed to provide a flow of warm air into the interior of the vehicle, above all into the footwells (e.g., as used on the VW Beetle)

Heater blower: Electric fan to boost heating and ventilation. Also called "heater fan"

Heater core: A finned unit located in the passenger compartment and through which coolant from the engine flows to heat the unit. A fan blows air over the unit to heat the passenger compartment

Heater fan: Electric fan to boost heating and ventilation. Also called "heater blower"

Heater flange: Heating element for preheating the intake air in small diesel engines

Heater plug: See
glow plug

Heater-plug indicator: See
glow plug indicator

Heater system: See
rear window heater system

Heat exchanger: A device which transfers the heat of one substance to another (i.e., from a warm or hot surface to a cold or cooler one) such as evaporators, condensers, and radiators.

Heat fade: Temporary reduction of brake effectiveness due to a loss of friction between braking surfaces, resulting from heat

Heat index: A number representing the effect of temperature and humidity on humans by combining the two variables into an apparent temperature. Introduced as a replacement for the temperature-humidity index. Later replaced by the Canadian introduced Humidex

Heating: See
rear window heating

Heating coil: Heat transfer device consisting of a coil of piping, which releases heat.

Heating control: Device which controls temperature of a heat transfer unit which releases heat.

Heating element: All parts which transmit heat from flames or flue gases to the medium being heated and in contact with both.

Heating equipment: Any equipment designed and/or specifically used for heating ambient air in an enclosed space. Common types of heating equipment include: central warm air furnace, heat pump, plug-in or built-in room heater, boiler for steam or hot water heating system, heating stove, and fireplace. Note: A cooking stove in a housing unit is sometimes reported as heating equipment, even though it was built for preparing food.

Heating value: Amount of heat which may be obtained by burning a fuel. The heating value is usually expressed in BTU per lb., BTU per gal., or k_J/kg.

Heat intensity: Heat concentration in a substance as indicated by the temperature of the substance through use of a thermometer.

Heat lag: The time it takes for heat to travel through a substance heated on one side.

Heat leakage: Flow of heat through a substance.

Heat load: Amount of heat, measured in BTU or watts, which is removed during a period of 24 hours.

Heat of Combustion: See
Gross heat of combustion Net heat of combustion

Heat of compression: Mechanical energy of pressure changed into energy of heat.

Heat of condensation: The heat liberated by a unit mass of gas at its boiling point as it condenses to a liquid. Equal to the heat of vaporization

Heat of fusion: Heat released from a substance to change it from a liquid state to a solid state. The heat of fusion of ice is 144 BTU per pound (335 k_J/kg).

Heat of respiration: Process by which oxygen and carbohydrates are assimilated by a substance; also when carbon dioxide and water are given off by a substance.

Heat of vaporization: The heat absorbed per unit mass of a given material at its boiling point that completely converts the material to a gas at the same temp. Equal to heat of condensation

Heat path: The path along which heat passes from the spark plug tip to the water jacket

Heat pipe, gas forced-air: High efficiency gas furnace that uses vertical liquid filled pipes. The pipes are heated by a burner at their base, and the liquid boils and vaporizes within the pipe. The furnace blower circulates air over the pipes for heating.

Heat pump: Compression cycle system used to supply heat to a temperature-controlled space. Same system can also remove heat from the same space.

Heat quantity: See
BTU

Heat range: Refers to the operating temperature of a given style of spark plug. Plugs are made to operate at different temperatures depending upon the thickness and length of the porcelain insulator as measured from the sealing ring down to the tip. In this way it transfers heat from the combustion chamber to the cylinder head. The speed at which it transfers heat is considered in terms of hot and cold plugs. A hot plug transfers heat slowly, causing the plug to operate at a higher temperature. A cold plug transfers heat more quickly, thus operating at a lower temperature. If a plug is too cold, it will foul. If it is too hot, it will cause preignition.

Heat range reserve: The distance to the start of pre-ignition under further increasing thermal loading of the spark plug. This reserve is expressed in degrees crankshaft, the amount by which the factory-set ignition timing can be further advanced without preignition occurring

Heat reactivate: To soften a dried thermoplastic adhesive film to a sticky state by application of heat. Used as a method of bond

Heat reactivation: To restore the tackiness of the adhesive with heat, and then to bond under pressure

Heat recovery system: Produces and stores hot water by transferring heat from condenser to cooler water.

Heat riser

An area, surrounding a portion of the intake manifold, through which exhaust gases can pass to heat the fuel mixture during warmup.
The flapper in the exhaust manifold that is closed when the engine is cold, causing hot exhaust gases to heat the inlet manifold, thus providing better cold engine operation; a thermostatic spring opens the flapper when the engine warms up

Heat shield: A sheet metal part, usually reflective, often asbestos-lined. It protects heat-sensitive components from heat radiation of the exhaust system. Also called "heat shroud."

Also See
grass heat shield

Heat shroud: See
heat shield

Heat shrinking: Shrinking dents in panels using a gas welder to heat local areas of the dents

Heat shrink tube: A thin, rubber-like hose into which bare electrical wire is inserted. When the tube is heated, it shrinks to fit the wire, thus protecting the wire from touching other wires, etc.

Heat sink

Heat removing component, such as the fins on an air-cooled cylinder
Relatively cold surface capable of absorbing heat.

Heat soak: Heat from the engine warming the carburetor and other parts of the fuel system which can cause vapor lock and may make restarting difficult

Heat transfer

The transfer of thermal energy from one material to another by means of thermal conduction, convection, or radiation; heat transfer is possible only from a high to a low temperature level
Movement of heat from one body or substance to another. Heat may be transferred by radiation, conduction, convection, or a combination of these three methods.

Heat transfer module: Primary system of heat transfer in a glycol water solution forced-air furnace. The heat transfer module contains the ignitor, burner, and primary solution circulating coil. hermetic compressor Compressor which has the driving motor sealed inside the compressor housing. The motor operates in an atmosphere of the refrigerant.

Heat transmission: The flow of heat from one substance to another

Heat treatment: A combination of heating and cooling operations timed and applied to a metal in a solid state in a way that will produce desired properties.

Heaves: See
frost heaves

Heavy: See
tail heavy

Heavy braking: The action of applying the brakes fast and hard especially during an emergency stop. Opposite to light braking

Heavy cable: Cable that is used for severe service where high strength is required. Opposite to light cable

Heavy clutch: Having a stiff operation, e.g., requiring considerable effort to push down the pedal on cars and trucks; or squeeze the lever on motorcycles. Opposite to light clutch

Heavy crude: Crude oil with a high specific gravity and a low API gravity due to the presence of a high proportion of heavy hydrocarbon fractions and metallic content.

Heavy-duty: (HD) Something that is built for heavy loads or severe use. Opposite to light-duty

Heavy-duty diagonal cutting pliers: Diagonal cutting pliers with a special joint and handle design for extra cutting power

Heavy-duty end cutting pliers: End cutting pliers with a special joint and handle design for extra cutting power

Heavy-duty ring wrench: A strong single end box wrench for use with a tubular handle. It can be used without the handle for quickly spinning on nuts or, with the handle slipped on, for final tightening or reaching otherwise inaccessible nuts

Heavy-duty truck: Vehicle weighing from 26,001 to 33,001 lbs. Also included off-highway trucks.

Also See
medium-duty truck
light-duty truck

Heavy-duty wrench: A tool which can withstand severe use (and abuse) before breaking

Heavy film build: Excessive thickness of paint coating

Heavy foot: To drive at full throttle or high speeds as though the driver's foot were very heavy so that the natural pressure of the foot against the throttle pedal caused high speeds

Heavy gas oil: Petroleum distillates with an approximate boiling range from 343.9°C to 537.8°C.

Heavy goods vehicle: (HGV) A British term expressing a vehicle capable of carrying heavy loads and requiring a special licence to drive HGV licence

Heavy metals: Metallic elements, including those required for plant and animal nutrition, in trace concentration but which become toxic at higher concentrations. Examples are mercury, chromium, cadmium, and lead.

Heavy oil: The fuel oils remaining after the lighter oils have been distilled off during the refining process. Except for start-up and flame stabilization, virtually all petroleum used in steam plants is heavy oil. Includes fuel oil numbers 4, 5, and 6; crude; and topped crude.

Heavy paint: Thickly coated. Opposite of light paint

Heavy phosphating: Application of extremely heavy phosphate coatings

Heavy rail: An electric railway with the capacity for a "heavy volume" of traffic and characterized by exclusive rights-of-way, multi-car trains, high speed and rapid acceleration, sophisticated signaling, and high platform loading. Also known as "subway," elevated (railway), "metropolitan railway (metro)."

Heavy sheet metal: Thick sheet metal for severe use beyond the average. Opposite of light sheet metal

Heavy side pattern: An oval spray pattern that is thicker toward the left or right hand side, i.e., it takes the shape of a crescent oriented towards the right or left. This is often caused by a clogged horn hole at the air cap of the spray gun

Heavy steering: Having a stiff operation, e.g., requiring considerable effort to turn the steering wheel. Opposite of light steering

Heavy traffic: A road with a lot of moving vehicles. Opposite of light traffic

Heavy water: Water containing a significantly greater proportion of heavy hydrogen (deuterium) atoms to ordinary hydrogen atoms than is found in ordinary (light) water. Heavy water is used as a moderator in some reactors because it slows neutrons effectively and also has a low cross section for absorption of neutrons.

Heavy yellow boot: A Denver boot

Heel

The end of the brake shoe which rests against the anchor pin.

Also See
brake shoe heel.
The end of the brake shoe furthest from where the braking force is applied.
The wide end of a tapered gear tooth such as found in the differential gears.

Also See
tooth heel.
The rubbing block on the contact breaker lever.
A bead heel.
The inclination of a ship to one side.

Also See
cam heel

Heel and toe: A driving technique where the driver places the left side or the toes of his right foot on the brake pedal and the right side or the heel of his right foot on the throttle pedal so that he can simultaneously brake and blip the throttle for a downshift. This is done to lessen the strain on the gearbox and drivetrain and makes for smoother driving.

Heel and toe wear: Uneven wear of tread blocks on a tire. The trailing edge of the block often tends to wear at a faster rate that the leading edge.

Heelboard: The vertical transverse sheet metal panel running across the width of the car interior at the front edge of the rear seat well; this panel links the rear seat well to the floorpan and provides rigidity for both panels. Also called "heel plate." Compare toeboard

Heel dolly: A dolly in the form of a heel of a foot to shape and straighten dented panels, usually by holding the dolly behind the metal to be shaped and hammering the metal.

Heel plate: The vertical transverse sheet metal panel running across the width of the car interior at the front edge of the rear seat well; this panel links the rear seat well to the floorpan and provides rigidity for both panels. Also called "heelboard." Compare toeboard

HEGO: Acronym for heated exhaust gas oxygen.

Also See
HEGO sensor

HEGO sensor: A device which detects heated exhaust gas oxygen.

HEI: Acronym for high energy ignition

HEI-EST: Acronym for high energy ignition system with electronic spark timing

Height: See
clearance height
crush height
flange height
installed height
metacentric height
ride height
section height
thread height

Height adjustable steering column: A steering column which can be lengthened or shortened to suit the individual driver

Height corrector: An automatic leveling control in hydropneumatic suspension systems

Height hamper pitch control: See
bump stop

Height regulator: An automatic leveling control in hydropneumatic suspension systems

Heim joint: An extremely rigid articulating joint, commonly known as a "spherical rod-end," used in any precision linkage. Heim joints are often used in the suspension links of race cars because they locate wheels very precisely.

Helical: A spiralling shape such as that made by a coil spring. In the shape of a helix.

Also See
double helical gear

Helical differential: Virtually all gears in modern cars are cut with a spiral helix angle rather than straight meshing. Straight gears are simpler to manufacture, but are extremely noisy.

Helical gear: A gear that has the teeth cut at an angle to the center line of the gear. This kind of gear is useful because there is no chance of intermittent tooth-to-tooth operation because there are at least two teeth engaged at any time. Also helical gears tend to operate quieter than straight-cut gears.

Also See
double helical gear

Helical spring lock washer: A locking device for threaded fasteners

Helical teeth: Curved gear teeth on the edge of a gearwheel, cut at an angle to its axis

Helicoil®: A trade name for a coil-type thread insert, commonly used to replace a stripped spark plug thread

Heliostat: A mirror that reflects solar rays onto a central receiver. A heliostat automatically adjusts its position to track daily or seasonal changes in the sun's position. The arrangement of heliostats around a central receiver is also called a solar collector field.

Helium leak test: A pressure test using helium

Helix: A spiral, like the thread on a screw or a coil spring in a suspension system

Helm: Steering wheel installed on the bridge or wheelhouse of a ship to turn the rudder during manoeuvering and navigation

Helmet

A protective device for the head of bicycle and motorcycle riders as well as race car drivers.
A protecting hood which fits over the arc welder's head, provided with a lens of safety glass through which the operator may safely observe the electric arc.

Also See
full face helmet
open face helmet

Helmet connector: A special type of lug for connecting a battery with tapered terminal posts. Also called "helmet lug."

Helmet hair: The condition of a person's hair after wearing a helmet for a period of time. Short hair tends to stand on end while long hair tangles and/or becomes flat.

Helmet lug: A special type of lug for connecting a battery with tapered terminal posts. Also called "helmet connector."

Helmhotz free energy: (A or F in US) Similar to Gibbs free energy but with internal energy substituted for enthalpy. A negative change in A is indicative of a spontaneous change in a closed system at constant volume.

Helper leaf: An additional spring device (usually another leaf spring) which permits a greater load on the axle. Also called "helper leaf."

Helper spring: An additional spring device (usually another leaf spring) which permits a greater load on the axle. Also called "helper leaf."

Hemi: Engine using hemispherical-shaped (half of a globe or sphere) combustion chambers. The valves are cocked at 45 degrees from the piston top. Mopars, despite their fame, are not the only cars with hemi heads.

Also See
engine type.

Hemi head: See
hemi.

Hemispherical combustion chamber: Click image to supersize

A round, dome-shaped combustion chamber. This shape permits larger valves and straighter intake and exhaust ports for improved breathing. Its small surface area in comparison to volume reduces the amount of heat loss. It is used in high performance cars and racing engines.

Also See
bathtub combustion chamber
wedge combustion chamber
squish combustion chamber
piston-crown combustion chamber

Henry: (H) A unit of inductance, equal to the inductance of a circuit in which the variation of current at the rate of one ampere per second induces an electromotive force of one volt. The term was named after US physicist, J. Henry (1797-1878)

Hermetic compressor: Compressor in which the driving motor is sealed in the same dome or housing as the compressor.

Hermetic motor: Compressor drive motor sealed within same casing which contains compressor.

Hermetic system: Refrigeration system which has a compressor driven by a motor contained in compressor dome or housing.

Herringbone gear: See
herringbone gears.

Herringbone gears: Two helical gears operating together and so placed that the angle of the teeth form a "V" shape.

Also See
double helical gear

Herringbone pattern: The characteristic pattern cut by a tool bit when a brake drum is machined without the proper use of a dampening belt.

Hertz: (hz) Correct terminology for cycles per second.

Hesitation: The term used to describe a problem in drivability. The engine momentarily fails to respond to a push on the accelerator. The cause is usually a lean fuel-air mixture (i.e., more air than fuel) or retarded timing.

HEUI: Acronym for Hydraulic-Electronic Unit Injector

HEV: Acronym for Hybrid-Electric Vehicle

Hexagon: See
double hexagon socket

Hexagonal: Having six sides

Hexagonal bolster: A tool that allows a screwdriver to be turned with a wrench for extra torque to loosen tight screws

Hexagonal collar: A tool that allows a screwdriver to be turned with a wrench for extra torque to loosen tight screws

Hexagon bit: A screwdriver bit with six sides

Hexagon bolt: A bolt with a six-sided head

Hexagon key: A key for hexagon recess screws, 90° offset at one end. Also called an Allen key

Hexagon screwdriver: See
ball end hexagon screwdriver

Hexagon socket: See
double hexagon socket

Hex bit: A screwdriver bit with six sides

Hex bolt: A bolt with a six-sided head

Hex key: A key for hexagon recess screws, 90° offset at one end. Also called an Allen key

Hex wrench: An L-shaped tool that works like a screwdriver, designed to remove screws with hexagonal holes in their heads.
See
Allen Wrench.