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H: A letter rating for tires to indicate that they are theoretically rated for speeds up to 210 kph (130 mph), as in P185HR13. The next higher rating is V and the one lower rating is T

half link: Every chain has rollers which are connected by side plates or keepers. When counting the number of links in a length of chain, it is easiest to count the number of side plates on one side of the chain and multiplying the number by two. When a chain needs to be an odd number, a half link is included. The term "half link" is a misnomer. It should be called a single link. The side plate on a half link is not flat but has a step down shape.

half moon key: See half-moon key.

half-moon key: A driving key serving the same purpose as the regular key but it is shaped somewhat like a half circle. Also called a "woodruff key."

half shaft: A rotating shaft that transmits power from the final drive unit to one side of the drive wheels, but usually refers to the two shafts that connect the road wheels to the final drive with independent rear suspension or front-wheel drive as opposed to the axle shafts of a live rear axle.

half step gearing: See half-step gearing.

half-step gearing: A gearing system of a bicycle in which a shift between chainrings in a double chainring set is equivalent to half a gear step on the freewheel.

half wave rectifier: See rectifier.

half-wave rectifier: See rectifier.

hand crank: Before Cadillac invented and produced electric starters, engines were started by means of a handle which was inserted into the front of the engine and rotated manually. After 1930 it became obsolete.

handlebar: A steering device found on bicycles and motorcycles.

handlebar riser: !!! Definition under construction !!!.

handling: The relative ability of a vehicle to negotiate curves.

harden: [1] The action of turning from a liquid to a solid. [2] The action of making the surface a metal tougher. See case harden.

hardener: Chemical added to plastic filler to induce hardening as used in auto body repair.

hardness: The toughness of the surface of a metal. See Rockwell hardness.

hand shield: An eye and face protector held in the hand. It enables a person to look directly at the electric arc through a special lens without being harmed.

hardtop: A two-door or four-door vehicle without a center door post, i.e., no B-post. It gives the impression of uninterrupted glass along the side of the car. The term is derived from "hardtop convertible."

hardtop convertible: An automobile with a fixed roof that does not retract into the trunk, but gives the appearance of being a convertible.

hard-top trailer: !!! Definition under construction !!!

harmonic balancer: Also called vibration damper. It usually is a solid crankshaft fan belt pulley that has a weight ring bonded by rubber to the inner crankshaft-mounted ring. The outer ring absorbs and cancels out crankshaft vibrations that otherwise might cause the crankshaft to break. Also see crankshaft pulley and damper.

hatch: The rear opening of a vehicle which allows passage into its cab.

hatchback: A car design in which the rear trunk and lid are replaced by a rear hatch that includes the backlight (i.e., rear window). Usually the rear seat folds down to accommodate more luggage.

HB: Acronym for "hatchback."

HC: Unburned hydrocarbons. They are at their worst with very rich or very lean mixtures. They combine with NOx in sunlight to form photochemical smog.

HCV: Acronym for "Heavy Commercial Vehicles."

HD: Acronym for "heavy duty."

hdtp: Abbreviation for "hardtop."

head: [1] The upper part of something. [2] A part of the engine which covers the piston and creates the combustion chamber. The proper name is cylinder head. See blown head gasket composite headlight, double-overhead cam exhaust header F-head engine, flathead gib-head key head gasket head restraint head tube heads up display headlight headliners headset headers high compression head, I-head engine, Jaguar Drophead L-head engine Lagonda Drophead overhead cam overhead valves Phillips head screw piston head sealed beam headlight T-head valve-in-head engine.

head ache rack: A colloquial term for the roll bar (safety bar) above the head of the driver

head gasket: The gasket at the top of the cylinder and sits between the cylinder and the head. It keeps the coolant out of the cylinders and retains compression in the cylinder. Also see blown head gasket.

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.

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

header: [1] The action of diving or crashing into something. [2] An exhaust manifold. See headers.

headers: Special exhaust manifolds that replace the stock manifold. They are designed with smooth flowing lines to prevent back pressure caused by sharp bends, rough castings, etc. Also see exhaust manifold.

headlamp: See headlight.

headlight: The front light of a vehicle. See composite headlight, high beam, low beam and sealed beam headlight.

headlight visor: A partial lid which encircles only the top part of the headlight.

headliner: Fabric or vinyl upholstery on the interior of the roof of a vehicle.

headliners: See headliner.

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.

headset: On a bicycle, the combination of cups, cones, and ball bearings that creates the bearing mechanism that allows the fork column to rotate inside the head tube.

head tube: The shortest tube in the main triangle of a bicycle, the one inside of which the fork column rotates.

Healey: See Austin Healey. 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.

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

heat control valve: See manifold heat control valve.

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 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 exchanger: A device, such as a radiator, used either to cool or heat by transferring heat from one object to another.

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 riser: An area, surrounding a portion of the intake manifold, through which exhaust gases can pass to heat the fuel mixture during warmup.

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.

heat control valve: See 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.

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.

heater: A device which gives off heat. In automobiles, the engine coolant is channelled through the heater in the passenger compartment. Some cars used an electric or gasoline heater because they hand no engine coolant (e.g., the Volkswagen Beetle). Also see block heater.

heavy braking: The action of applying the brakes fast and hard especially during an emergency stop.

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

heel: [1] The end of the brake shoe which rests against the anchor pin. See brake shoe heel. [2] The wide end of a tapered gear tooth such as found in the differential gears. See tooth 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.

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.

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.

helmet: [1] A protective device for the head of bicycle and motorcycle riders as well as race car drivers. [2] 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.

helmet hair: The condition of a person's hair after wearing a helmet for a period of time.

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. See engine type.

hemi head: See hemi.

hemispherical combustion chamber: 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.

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

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.

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.

hex wrench: See Allen Wrench.

hide-away headlights: See concealed headlights

hiding: The ability of a paint to obscure the surface to which it is applied.

high beam: All cars have at least two levels of lighting for night driving. The low beam is used when there are other vehicles approaching or when you are following another vehicle. The high beam is used when driving on poorly lit roads where there is no visible traffic. The high beam light may be an integral part of the headlight assembly or is the inboard light when there are two lamps on each side of the vehicle. When the lamps are stacked vertically, the high beam is the upper one. When the high beam switch is activated, the low beam lights may or may not be switched off. In a system with factory installed day-time running lights, the high beam light is illuminated at a lower intensity.

high beam indicator: A light on the instrument panel which comes on when the high beams are activated. Also called "beam indicator"

high compression head: A cylinder head with a smaller combustion chamber area thereby raising the compression. The head can be custom built or can be a stock head milled (cut) down.

high gear: The top gear or the cruising gear of a transmission. It is usually the highest number, i.e., fifth gear of a five-speed transmission.

high lift rocker arm: See high lift rocker arms.

high lift rocker arms: custom rocker arms designed so that a standard lift of the push rod will depress or open the valve somewhat more than the stock lifter.

high tension lead: High voltage wire from the ignition coil. May also indicate the secondary wire from the ignition coil to the distributor and wires from the distributor to the spark plugs.

high tension leads: See high-tension leads.

high-tension leads: The wire which goes from the ignition coil to the distributor and the wires which go from the distributor to each of the spark plugs. See high tension lead.

highrise manifold: See high-rise manifold.

high-rise manifold: An intake manifold designed to mount the carburetor or carburetors, considerably higher above the engine than is done in the standard manifold. This is done to improve the angle at which the fuel is delivered.

hi perf: Abbreviation for "high performance," also called "hi-po" or "hi po."

hi po: See hi-po.

hi-po: See hi perf.

Hispano Suiza: The 1925-1948 models are classic cars.

hitch ball: A metal ball with a bolt attached to it. It is secured to a bracket on a vehicle in order to mount a trailer hitch. British term is "towball"

hi-way bars: !!! Definition under construction !!!.

hog: [1] A colloquial term for a Harley Davidson motorcycle. [2] A person who persists in driving by using the full lane or both lanes at a slow speed so that it is impossible to pass him.

hold down clamp: See distributor hold-down clamp.

hold-down clamp: See distributor hold-down clamp.

hole circle: See wheel bolt hole circle.

hone: [1] To remove metal with a fine grit abrasive stone in order to meet precise tolerances. [2] The device or tool which is rotated in a cylinder to remove slight imperfections in the cylinder wall. See cylinder hone.

hood: The part of the vehicle body which covers the top of the engine in front engine vehicles. In mid-engine cars the panel which conceals the engine is called the "engine cover" or access panel. In rear-engine cars the panel which conceals the engine is called a "deck lid."

hood badge: An emblem reflecting the name or logo of the car manufacturer and is secured to the front end of the hood. Some are bolted or pasted flat to the front, others stand upright on fixed or pivoting rods.

hood bump rubber: A thick rubber strip which is secured to the vertical panel on which the hood is lowered

hood bumper: See hood bump rubber

hood landing panel: The panel on which the hood is lowered. It also houses the hood locking mechanism

hood liner: A material secured to the underside of the hood to provide sound insulation. It is usually made of polyethylene, polypropylene, polyurethane, or a fabric of polyester

hood lock: A mechanism which is made of a peg on the front end of the underside of the hood and a sliding latch on the panel above the radiator. The sliding latch is released by a lever under the hood or by a lever under the dash in the passenger compartment. See hood release

hood pin: A pin designed to hold a hood closed.

hood pin kit: A pair of hood pins and brackets which secure the hood of competition cars and "would-be" competition cars

hood pins: Pins designed to hold a hood closed.

hood release: A device which releases the hood lock. The trigger, located under the dash in the passenger compartment, is attached to a cable which controls the hood lock.

hood rod: A bar or rod which keeps the hood in an open position. The rod is located under the hood. Once the hood is raised, the rod is swung upright and the free end is placed in a notch or hole in the hood to keep it in place

hood tape: A fabric covered beading which keeps the hood on older cars from rubbing on the paint below them.

hooke joint: See cardan joint.

hooke universal: See cardan joint.

hook: The curved, dropped section of a set of turned-down handlebars of a bicycle.

hooks: The curved, dropped sections of a set of turned-down handlebars of a bicycle.

hooped: A colloquial expression meaning damaged or out of order as in Your engine is hooped.

hop: See wheel hop.

hopping up: Increasing engine performance through various modifications.

Horch: The 1925-1948 models are classic cars.

horizontally opposed engine: An engine possessing two banks of cylinders that are placed flat or 180 degrees apart. This configuration gives a lower center of gravity which improves handling. As well it has a lower hood height to improve aerodynamics. Also called a "boxer" engine. Also see flat engine.

horizontal position: A weld performed on a horizontal seam at least partially on a vertical surface.

horn: A device for sounding an alarm. On some entry level vehicles, a single horn makes a high pitch beep. On more expensive vehicles, the sound is made with two or more horns. The sound is made by an electrical charge which activates and deactivates a coil. The sound is then magnified by the shape of the horn. Large truck horns make a very loud noise because the sound is made by passing compressed air through the sounding device. A Claxton horn makes a sound that resembles the word "A-hoo-gah." Also see air cleaner horn or air horn.

horn switch: !!! Definition under construction !!!

horsepower: (HP) A measurement of the engine's ability to perform work. One horsepower is defined as the ability to lift 33,000 pounds one foot in one minute. To find horsepower, the total rate of work in foot pounds accomplished is divided by 33,000. If a machine was lifting 100 pounds 660 feet per minute, its total rate of work would be 66,000 foot pounds per minute. Divide this by 33,000 foot pounds per minute to arrive at 2 horsepower. In metric terms, it is the ability to raise 250 kilograms a distance of 30 centimeters in one second. It is also equal to 746 watts. See brake horsepower, friction horsepower, indicated horsepower, SAE horsepower, SAE gross horsepower, and SAE net horsepower.

horsepower weight factor: See horsepower-weight factor.

horsepower-weight factor: The relationship between the total weight of the vehicle and the horsepower available. By dividing the weight by the horsepower, the number of pounds to be moved by one horsepower is determined. This factor has a great effect on acceleration, gas mileage, and all around performance.

hose: A flexible tube used to convey liquid. In most automobiles, hoses connected independent components like the radiator, water pump, and heater.

hose clamp: An adjustable metal ring placed around a hose where it connects to another part, to prevent leaks and to keep the hose in place.

hose clamps: Adjustable metal rings placed around a hose where it connects to another part, to prevent leaks and to keep the hose in place.

hot cap: The conventional method of retreading in which uncured rubber is added to a buffed casing and cured in the mold at temperatures of approximately 290°-300°. This temperature allows uncured rubber to flow in the matrix forming the tread design during vulcanization. Also see cold cap

hot grip: !!! Definition under construction !!!.

hot plug: A spark plug which has a long insulator nose which absorbs more heat and dissipates heat slowly. A colder plug is used in a hot engine while a hot plug is used in a cold engine. Thus if the plugs are fouling too much, try a hotter plug. If the plugs are coming out white, try a colder plug. The ideal color of the center insulator nose should be a light chocolate brown.

hot rod: A production car that has been modified by the owner in the attempt to increase acceleration and top end speed. Although the term can be applied to any modified car, it is usually reserved for vehicles produced from 1930 to 1940's. Typically the engine is modified, and some body panels removed. Many were painted with a design of flames behind the front wheels to give the appearance that this vehicle was "hot" -- thus the name.

hot spot: [1] Refers to a comparatively thin section or area of the wall between the inlet and exhaust manifold of an engine, the purpose being to allow the hot exhaust gases to heat the comparatively cool incoming mixture. [2] Also used to designate local areas of the cooling system which have above average temperature.

hot wire: [1] The positive wire coming from the battery or generating system. [2] As a verb, it indicates the starting procedure when by-passing the ignition key and normal starting procedure.

Hotchkiss: The 1925-1948 models with required application are classic cars.

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