Dictionary of Automotive Terms: H

HALF-MOON KEY: Driving key serving same purpose as regular key but it's shaped somewhat like a half circle.

HALOGEN BULB: Light bulb in which tungsten filament is surrounded by a halogen gas such as iodine, bromine, etc. Bulb glass is quartz to withstand intense heat.

HAND BRAKE: Hand operated brake which prevents vehicle movement while parked by applying rear wheel brakes or transmission brake.

HARMONIC BALANCER: See Vibration Damper.

HC: Symbol for hydrocarbon.

HEADLIGHTS: Main driving lights used on front of vehicle.

HEAD PRESSURE: See Discharge Pressure.

HEAT CROSSOVER (V-Type Engine): Pas sage from one exhaust manifold up, over, and under carburetor and on to other manifold. Crossover provides heat to carburetor during engine warmup.

HEAT ENGINE: Engine operated by heat energy released from burning fuel.

HEAT EXCHANGER: Device, such as radiator, either used to cool or heat by transferring heat from one object to another.

HEAT RANGE (Spark Plugs): Refers to operating temperature of given style plug. Plugs are made to operate at different temperatures depending upon thickness and length of porcelain insulator as measured from sealing ring down to tip.

HEAT RISER: Area, surrounding portion of the intake manifold, through which exhaust gases can pass to heat fuel mixture during warmup.

HEAT SINK: Device used to prevent over heating of electrical device by absorbing heat and transferring it to atmosphere.

HEAT STOVE: Sheet metal housing around a portion of exhaust manifold. An intake pipe from housing provides hot air to carburetor air cleaner when needed. Can also mean a small shrouded depression in exhaust manifold from which hot air may be drawn to automatic choke housing.

HEAT TREATMENT (Metal): Application of controlled heat to metal object in order to alter its characteristics (toughness, hardness, etc.).

HEEL (Brake): End of brake shoe which rests against anchor pin.

HEEL (Gear Tooth): Wide end of tapered gear tooth such as found in differential gears.

Hg: Abbreviation for the word MERCURY. Vacuum is measured in inches of mercury.

HELICAL: Spiraling shape such as that made by a coil spring.

HELICAL GEAR: Gear that has teeth cut at an angle to center line of gear.

HEMI: Engine using hemispherical-shaped (half of globe) combustion chambers.

HEMISPHERICAL COMBUSTION CHAMBER: A round, dome-shaped combustion chamber that's considered by many to be one of the finest shapes ever developed. Hemispherical-shape lends itself to use of large valves for improved breathing and suffers somewhat less heat loss than other shapes.

HERRINGBONE GEARS: Two helical gears operating together and so placed that angle of the teeth form a “V” shape.

HIGH COMPRESSION HEADS: Cylinder head with smaller combustion chamber area thereby raising compression. Head can be custom built or can be a stock head milled (cut) down.

HIGH-RISE MANIFOLD: Intake manifold designed to mount carburetor or carburetors, considerably higher above engine than is done in standard manifold, done to improve angle at which fuel is delivered.

HIGH TENSION: High voltage from ignition coil. May also indicate secondary wire from the coil to distributor and wires from distributor to plugs.

HONE: To remove metal with fine grit abrasive stone to precise tolerances.

HOOD PINS: Pins designed to hold hood closed.

HOPPING UP: Increasing engine performance through various modifications.

HORIZONTAL-OPPOSED ENGINE: Engine possessing two banks of cylinders that are placed flat or 180 deg. apart.

HORSEPOWER: Measurement of engine’s ability to perform work. One horsepower is defined as ability to lift 33,000 pounds one foot in one minute. To find horsepower, 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. Dividing this by 33,000 foot pounds (1 horsepower) you find that the machine is rated as 2 horsepower (hp).

HORSEPOWER (Brake): See Brake Horse power.

HORSEPOWER (Frictional): Amount of horsepower lost to engine friction.

HORSEPOWER (Gross): Maximum horse power developed by engine without a fan, air cleaner, alternator, exhaust system, etc.

HORSEPOWER (Net): Maximum horse power developed by engine equipped with fan, air conditioning, air cleaner, exhaust system, and all other systems and items normally present when engine is installed in car.

HORSEPOWER — WEIGHT FACTOR: Relationship between total weight of car and horsepower available. By dividing weight by horsepower, 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 CLAMPS: Devices used to secure hoses to their fittings.

HOSES: Flexible rubber tubes for carrying water, oil, air, and other fluids.

HOTCHKISS DRIVE: Method of connecting transmission output shaft to differential pinion by using open drive shafts. Driving force of rear wheels is transmitted to frame through rear springs or through link arms connecting rear axle housing to frame.

HOT SPOT: Localized area in which temperature is considerably higher than surrounding area.

HOT WIRE: Wiring around key switch so as to start car without key. Wire connected to battery or to some part of electrical system in which a direct connection to battery is present. A current-carrying wire.

HP: Horsepower.

HUB (Wheel): Unit to which wheel is bolted.

HYATF ROLLER BEARING (Antifriction):

Similar to conventional roller bearing except that rollers are hollow and are split in a spiral fashion from end to end.

HYDRAULIC: Refers to fluids in motion. Hydraulics is science of fluid in motion.

HYDRAULIC BRAKES: Brakes operated by hydraulic pressure. Master cylinder provides operating pressure transmitted via steel tubing to wheel cylinders or pistons that in turn apply brake shoes to brake drums and/or discs.

HYDRAULIC LIFTER: Valve lifter that utilizes hydraulic pressure from engine’s oiling system to keep it in constant contact with both camshaft and valve stem. They automatically adjust to any variation in valve stem length.

HYDRAULICS: The science of liquid in motion.

HYDROCARBON: A mixture of hydrogen and carbon.

HYDROCARBON — UNBURNED: Hydro carbons that were not burned during the normal engine combustion process. Un burned hydrocarbons make up about 0.1 percent of engine exhaust emission.

HYDROCARBONS: Combination of hydrogen and carbon atoms. All petroleum based fuels (gasoline, kerosene, etc.) consist of hydrocarbons.

HYDROMETER: Float device for determining specific gravity of electrolyte in a battery. This will determine the state of charge.

HYDROPNEUMATIC SUSPENSION: Suspension system using both a liquid (oil) and compressed air for springing.

HYGROSCOPIC: Ability to absorb moisture from air.

HYPOID GEARING: System of gearing wherein pinion gear meshes with ring gear below center line of ring gear. This allows a somewhat lower drive line thus reducing hump in the floor of car. For this reason hypoid gearing is used in differential on many cars.