Motorcycle and Vehicle Technical Terms Glossary

So what's a cardan? Or an easy out? Or a burr? Or lateral runout? Or an oil slinger? Or a zener diode? It's all in here the HU Motorcycle and Vehicle Technical Terms Glossary!

(an alphabetical list of technical terms in some specialized field of knowledge!)

Got any more to add? Please contact me!

This is a permanent work in progress! And you can add to it! Please contact me with your additions.

Also see:

Motorcycle and Vehicle Tech Terms Translation

All those tech words you NEED for vehicle travel, but just aren't in the common phrasebook! Brakes, engine parts and much much more!

Language: Common Phrases for the road

"On the road" phrases; Where's a gas station? The engine won't start! Can I park here?
All the "road" phrases you're unlikely to find in the common phrases book for the backpacker!

Motorcycle Technical Terms Glossary


alternating current (output from the alternator is AC, same as your household wall socket) - electrical current which constantly reverses direction and polarity.

AC welding

arc welding welding method which uses alternating current to provide heat for melting and fusion of metal parts.

accelerator pump

a small pump that squirts fuel into throat of carburetor as throttle is opened.

accessory system

part of electrical system consisting of lights, horn, electrical starter, turn signals, and warning systems.


chemical compounds used to alter characteristics of lubricating oils and fuels.

air box

plastic, fibreglass or metal box mounted between carburetor and air filter. This box provides a volume of still, filtered air for induction into the engine.

air cooling

use of fins to dissipate heat from part into air.

air filter

a device with paper, oiled foam, oiled gauze or wire mesh to prevent entry of dirt or foreign particles into engine through air intake.

air-fuel mixture

finely atomized mist of air and fuel necessary for combustion. This mixture consists of approximately 15 parts air to 1 part fuel (15:1) at cruising speeds.

air jet

a small jet in air passage of a carburetor. This jet meters amount of air fed to diffuser in an air-bleed type carburetor.

alternating current

(AC) electrical current which constantly reverses direction and polarity.


a crankshaft driven electrical generator that produces alternating current which must be rectified to DC current.


an instrument for measuring current flow in electrical circuits.


(amp) electrical unit used to measure flow.

arc welding

a type of welding which uses an electric arc to produce necessary heat. Melting of base metal and welding rod to join two parts. Also known as electric-arc welding.


portion of a DC generator housing generating coils.


heat resistant material commonly used in brake and clutch lining. It is a known cancer causing substance if inhaled.


tiny particles of fuel mixed with air, making fine mist.

automatic transmission

a transmission not using a manually operated clutch.

backbone frame

frame which uses the engine as a structural member, with the frame over the top of the engine.


the clearance between mesh of two gears.

ball and ramp

a clutch release mechanism made of two stamped plates with three or four ramps. As one plate is rotated by clutch cable, balls climb ramps, forcing plates apart. This movement disengages clutch.

ball bearing

an antifriction bearing consisting of inner and outer races separated by hardened steel balls.

battery and coil ignition system

an ignition system with a battery as the source of primary ignition current.

battery-powered electric system

an electrical system having a lead-acid battery as a source of power. The battery is recharged by a charging system using either a generator or alternator.

battery-supported CDI

capacitive discharge ignition system which uses a battery to supply primary ignition current.


a part in which a journal, shaft or pivot turns or moves.

bearing preload

amount of static pressure exerted on a bearing or a set of bearings. Preload is usually adjusted by a threaded collar or shims.

bearing spacer

a piece of tubing used between wheel bearing inner races to prevent unwanted bearing preload as the axle is tightened.

bench grinder

a power grinder with round abrasive stones mounted on either end of a special electric motor, for rough grinding of ferrous metals.

bleeder-type needle circuit

air from carburetor air jet is mixed with fuel in body of needle jet making an air-fuel froth. The mixture is then metered into throat of carburetor by jet needle and needle jet.


when piston rings do not effectively seal combustion pressure, allowing hot gases to blow between rings and cylinder wall. This causes overheating of piston and poor performance.

boring bar

a machine tool used to accurately enlarge a cylinder bore.

bottom dead centre (BDC)

lowest piston position in cylinder.

brake actuator cam

small cam that pivots in brake backing plate and forces brake shoe into brake drum.

brake caliper

part of a disc brake which holds friction pads and encloses disc. As brake is applied, hydraulic fluid forces a piston in calipers toward disc, causing disc to be pinched between brake pads.

brake disc

a round, flat disc made of steel or cast iron. It is mounted on outside of wheel hub.

brake drum

a circular ring of cast iron that is part of wheel hub. It provides a place for brake lining to be applied.

brake line

special hydraulic tubing made of steel, plastic and reinforced rubber. Hydraulic brake lines must be capable of withstanding extreme pressure without deforming.

brake lining

a special high friction material made of asbestos and other materials bonded to brake shoes and brake pad plates. Brake lining produces friction and heat when it is forced against brake drum or disc.

brake pads

small circular or square asbestos inserts in a wheel caliper. They produce friction and heat when forced against the disc.

brake shoe

a cast aluminum, half-circular shoe that holds a bonded brake lining material. When brake is applied, shoe forces lining into brake drum.

brake wear indicator

index grooves, tabs, or reference lines to indicate amount of brake lining or pad wear.


type of welding using a brass alloy filler rod.

bridged ports

a vertical port division in a two-stroke cycle engine cylinder which allows use of a large port without danger of ring or piston catching.


a small, rotating cutter mounted in a rotary grinder and used for metal removal (rotary file). Also, sharp, rough area around a drilled hole.

butterfly-controlled carburetor

a carburetor using a flat plate between venturi and intake manifold to regulate airflow through carburetor.

bypass valve

valve which allows fluid to flow around normal path of flow, used in oil filters.

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cam follower

components in valve train that rides on cam lobe.

cam ground

oval shape of a piston to control and compensate for expansion.

cam lobe

protrusion on a camshaft that causes valve train parts to move as camshaft rotates.

cam plate

flat plate with slots that engage pins on shift forks. As plate is rotated, slots cause shift forks to move sliding gears or dogs, causing engagement and disengagement of gearbox ratios.


shaft with protruding eccentric lobes. As camshaft rotates, lobes cause reciprocating movement of other valve train parts.


device that mixes and delivers proper amount of air and fuel to engine at a ratio of approximately 15 parts air to 1 part fuel.

cast alloy wheel

a one piece wheel made of cast aluminum or magnesium alloy. This design is more rigid than wire spoked wheel.

cast-in sleeve

an aluminum cylinder block cast around an iron cylinder sleeve.

cast iron cylinder

a one piece cylinder assembly made of cast iron with a machined bore.

cast piston

a piston made by pouring molten aluminum alloy into a mould of desired shape.


inadequate lubrication caused by air taking place of oil in pump.

CDI voltage amplifier

a device, used in a battery powered capacitive discharge ignition system, that steps up battery voltage to provide high primary ignition voltage.

centre of gravity

point at which any object is perfectly balanced.

centrifugal clutch

clutch engaged by centrifugal force as engine speeds up.

centrifugal oil slinger

cup-shaped centrifugal oil filter mounted to end of camshaft. As oil passes through slinger, centrifugal force removes impurities that are heavier than oil.

chain drive

use of a chain and sprocket to connect gearbox output shaft to rear wheel.

chain stretch

wear of pins and bushings of a roller or hy-vo chain, causing chain to lengthen.


to bevel an edge of an object or to chamfer edges of port openings in a two-stroke cycle cylinder to prevent piston ring breakage.

check valve

a spring-loaded ball or piston valve that allows flow only in one direction.

chemical regulator

voltage regulator with solid state electronic devices to regulate charging system output.


a circular clip or snap ring that fits into a groove, used to locate or retain a shaft or component.


an electrical path from a power source, through wire, to components, and back to source.


amount of space between two adjacent parts.

close-ratio gearbox

a gearbox with gear ratios spaced close together.


device used to connect and disconnect engine power to gearbox input shaft.

clutch basket

part of clutch assembly containing clutch plates. Primary drive gear engages teeth on outside of clutch basket.

clutch holder

tool to secure clutch basket and clutch hub while loosening or tightening clutch security nut or primary drive gear nut.

clutch hub

part of clutch that engages with plain driven clutch plates. Clutch hub is mounted on gearbox input shaft.

clutch pressure plate

part of a clutch assembly providing pressure against clutch disc or clutch plates.

clutch release mechanism

mechanism that moves clutch pressure plate away from clutch pack, allowing clutch to slip.

coated bore

thin coating of chrome or iron applied to inside of cylinder by electroplating or wire explosion spray coating.

coil build-up

build-up of a magnetic field while current is flowing through primary windings of coil.

cold soak cleaner

a strong cleaning solvent used to dissolve and remove varnish on carburetor parts.

colour code

use of different base colours and coloured tracers on insulation of electrical wire for purpose of identification.


burning of air-fuel mixture in combustion chamber.

combustion chamber

area of cylinder head and cylinder above piston where combustion of air-fuel mixture takes place.

common sump

same oil is used to lubricate engine, gearbox, and primary drive.


increased pressure caused as volume is reduced. Also, movement of suspension components against spring pressure caused by a force against the wheel.

compression gauge

gauge which measures cranking pressure between piston and upper cylinder.

compression ring

piston ring designed to seal pressure between piston and upper cylinder.

compression stroke

movement of piston from BDC (bottom dead centre) to TDC (top dead centre) with valves closed, compressing air-fuel mixture for more violent combustion.


when two or more circular parts have same centre line.

condenser (capacitor)

two metal sheets separated by an insulator used to store an electrical charge.

conical hub

a wheel hub (wire wheel) with spoke holes on brake side of wheel at a greater distance from centre of hub than spoke holes on opposite side of hub.

connecting rod

a rod made of steel or aluminum or titanium, usually having an I-beam cross section. It connects piston to crankshaft.

connecting rod kit

a parts kit consisting of connecting rod, crank pin, thrust washers and roller bearings, used in reconditioning of assembled crankshafts.

connecting rod tip

amount of radial (side) play at top of connecting rod. Measurement of rod tip is one way of determining condition of rod big end bearing.

contact points

switching device used to start and stop flow of current.


a continuous path for current flow.

conventional rear suspension

suspension used on dual-purpose and road bikes which provides less than 6 in. (152 mm) of suspension travel.

cooling fins

projections on cylinder heads, cylinders and crankcases to increase surface area for more efficient heat dissipation into air.

countershaft sprocket

output sprocket from gearbox. Mounted on output shaft in indirect drive gearbox and on high gear pinion in direct drive gearbox.

cradle frame

frame built of tubing which supports and surrounds engine.


castings that support and contain the crankshaft, primary drive and gearbox.

crankcase leak test

pressure test done to a two-stroke cycle engine to determine if crankcase is properly sealed.


pin or journal on which big-end of connecting rod rides.

crankshaft axles

extensions at each end of crankshaft to provide a mounting place for main bearings, primary drive or sprocket, and alternator rotor or magneto flywheel.

crankshaft reconditioning

replacement of worn lower-end components in an assembled crankshaft. This involves pressing crankshaft apart, replacing crankpin, roller bearing, thrust washer, and connecting rod, pressing crankshaft back together and trueing it (assembled crankshaft).

crankshaft wheel

portions of an assembled crankshaft that provides a mounting place for crankpin and crank axles.

current flow

movement of electrons through a conductor.


a series of events which take place during a specific interval.


a machined hole in cylinder block for piston, open at both ends.

cylinder block

casting attached to crankcase which contains cylinder bore, cooling fins or water jacket, and provides a means of mounting cylinder head.

cylinder bore

diameter of cylinder opening.

cylinder boring

bore diameter machined (bored) to accept oversized piston. This renews a worn cylinder.

cylinder deglazing

use of a hone to slightly roughen walls of cylinder. It produces a crosshatch pattern which aids in seating of new rings.

cylinder head

casting that seals top of cylinder and provides a mounting place for spark plug. In four-stroke cycle engine, cylinder head also contains intake and exhaust ports. Both two- and four-stroke cycle engines also have combustion chamber built into cylinder head.

cylinder honing

use of a parallel type cylinder hone to size a cylinder after boring. Proper cylinder honing provides proper texture and crosshatch.

cylinder liner

cast iron sleeve or tube pressed or cast into cylinder block to provide bore which piston moves in.


device which uses oil metered through orifices to control abrupt suspension movement during extension and compression.

damper rod

tube secured to bottom of each fork slider to hold slider onto fork leg. Damper rod controls movement of front suspension by metering hydraulic fluid through orifices in rod.


to remove carbon build-up on piston, combustion chamber, and other parts.

depth micrometer

a precision measuring tool used to take measurements of stepped surfaces.

detergent oil

an oil which keeps particles and contaminants in suspension and has ability to neutralize acids resulting from combustion process.


a condition where excessive temperature of air-fuel mixture in combustion chamber causes uncontrolled explosive burning. As detonated flame front collides with flame front initiated by spark plug, extreme pressure is often heard as "pinging" or "knocking." Detonation can be caused by incorrect ignition timing, lean air-fuel ratio, and improper fuel.


process of determining cause of a failure.

dial bore gauge

a precision measuring tool which combines a telescoping gauge and dial indicator to give readings of inside diameter measurements.

dial caliper

a precision measuring tool used to determine inside, outside, or depth measurements. Measurements are displayed on a dial index.

dial indicator

a precision measuring tool using a dial index to show linear movement of components being measured.


a thin flexible disc of rubberized fabric which separates two cavities and uses vacuum or pressure for activation.

diaphragm spring

a slightly cone shaped metal disc which acts as a clutch pressure plate spring when flattened.


a projection in base of carburetor venturi and at top of needle jet that aids in fuel atomization.


a solid state electronic device that permits current flow in only one direction.

direct bearing lubrication

an oil injection system which feeds undiluted oil to two-stroke cycle engine main bearings and rod big-end nearing.

direct current

a continuous flow of current in same direction.

direct drive gearbox

power is transferred from clutch to input shaft (main shaft), to layshaft, to high gear pinion, which has output sprocket mounted on it.

disc brake

a brake consisting of a flat circular disc attached to a wheel. A hydraulic or mechanical caliper applies pressure to two brake pads to slow or stop disc rotation.

DOHC (double-overhead camshaft)

two camshafts located in cylinder head (one for intake, one for exhaust).

double-leading shoe

a drum brake having two leading shoes and no trailing shoes. Each shoe has its own activating cam and pivot.

double-piston caliper

a hydraulic brake caliper with two pistons and provisions for applying hydraulic pressure equally to both pistons. The caliper body is fixed solidly.

double-row chain

a chain having two rows of rollers. Duplex (double-row) chains are used for primary drives.

drive plate

a clutch plate which is indexed into clutch basket (outer hub) by tabs. Drive plate has friction material bonded to its surface. When clutch is engaged, drive plate transfers power to driven plate.

driven plate

a clutch plate which is indexed onto clutch inner hub by tabs or splines around its inside diameter. Driven plate is usually a plain plate (no friction material) and drives gearbox input shaft through clutch inner hub.

drum brake

a brake consisting of two brake shoes mounted on a backing plate. One or two cams cause shoes to expand against inside of brake drum. Brake drum is part of hub or is bolted to hub.

dry clutch

a clutch assembly that does not run in an oil bath.

dry sump

in this system, oil is gravity fed to supply side of oil pump from a remote oil tank. After oil has been pumped through fourstroke cycle engine, it is returned to oil tank by return side of oil pump.

dual-rate charging system

a charging system that switches extra coils into charging system when lights are turned on.


length of time, in crankshaft degrees, that ignition contact points are closed.

dykes piston ring

a piston ring with an L-shaped cross section designed to use combustion pressure to improve sealing.

dynamic ignition timing

use of a strobe light to check ignition timing with engine running.

electric arc welding

see arc welding.


conductors at centre and side of spark plug that provide an air gap for an electric arc to start combustion process in engine.


a solution of dilute sulphuric acid used to provide a chemical reaction in a lead-acid battery.


a magnet which produces a magnetic field by passing electrical current through a coil wire wrapped around a soft iron core.


alternator an alternator which uses electromagnet to produce a magnetic field.

electromotive force (EMF)

force that causes electricity to flow because of a difference in potential between two points.

electronic ignition system

an ignition system which uses a magnetic triggering device and solid state amplifier rather than conventional contact points.

element filter

a disposable oil or air filter that uses gauze or paper as filtering material.

elliptical port shape

rounded port shape designed to prevent ring catching in large ports of two-stroke cycle engine.


byproducts of a running engine, including: exhaust pollutants, noise emissions.

endless chain

a roller chain without a master link for connections of ends. All pin links are permanently riveted.


a machine that converts chemical energy into mechanical energy.

English measurement system

a measurement using the foot (12 in.) as a standard.

enrichment circuit

a carburetor system with a plunger to open and close an air-fuel circuit which discharges a rich mixture into throat of carburetor for cold starting.

eroded piston (crown)

a condition caused by detonation or pre-ignition where temperatures are raised so high that part of piston crown is melted away.

exhaust port

an opening or passage which directs flow of exhaust gases out of engine. In a four-stroke cycle, it is located in the head. With a two-stroke cycle, it is in the cylinder.

exhaust (port) timing

amount of time two-stroke cycle exhaust port is open, expressed in crankshaft degrees or piston travel.

exhaust stroke

with a four-stroke cycle engine, it is movement of piston from BDC to TDC with exhaust valve open, pushing burned gases out of cylinder.


the return or stretching outward of suspension components (after compression) caused by spring pressure.

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a device used to attach one part or assembly to another (nut and bolt, screw, rivet, etc.).

feeler gauge

a measuring tool made of steel blades of precise thickness used for measuring distance between surfaces.

field coil

a electromagnet used in DC generator or AC generator to produce a magnetic field.

filler rod

a metal rod that is melted into welding puddle to provide necessary bead thickness.

film strength

ability of an oil to keep moving parts from making contact with each other.

final drive

chains and sprockets or shafts and gears used to connect the gearbox output shaft to rear wheel.

flange-mounted carburetor

a carburetor mounted by a flange. The flange is bolted to a manifold on cylinder or cylinder head. An insulator block and a gasket are used to seal and insulate carburetor.

float assembly

a carburetor device consisting of plastic or hollow brass floats attached to an arm. This assembly pivots on a pin and raises and lowers, opening and closing float needle to control fuel level in float bowl.

float level

level of fuel maintained in carburetor float bowl. Controlled by float, float needle and seat.

float needle and seat

a carburetor valve, opened and closed by the float, that lets fuel into float bowl from fuel line.

fluid pressure gauge

gauge for measuring fluid pressure in a system such as oil pressure or fuel pressure.


a weight attached to an engine crankshaft, providing crankshaft with added inertia. This helps to smooth out abrupt movement of crankshaft during power stroke and to keep crankshaft turning during non-power producing events.

flywheel magnet

magnet mounted on side of flywheel in a flywheel magneto.

flywheel magneto ignition system

an AC ignition system using a generating coil or a magnetic trigger (CDI) or contact points as well as a flywheel to provide primary ignition current and triggering. An external ignition coil is used to provide secondary current.


undesirable characteristic of oil being whipped into a froth (air and oil solution).


amount of work required to lift one pound one foot, used to express torque.

forged piston

a piston made by hammering hot aluminum into a mould of desired shape.

fork sliders

outer portion of fork which slides over fork leg inner.

fork tubes

long sturdy tubes usually attached to triple clamps and fitted inside fork sliders. Can also be fitted to front wheel on some modern bikes (upside down forks)

four-stroke cycle

one cycle consisting of four distinct events, intake, compression, power, exhaust. One cycle requires two revolutions of crankshaft.


resistance to movement between two objects contacting each other, causes heat.

friction plate

see drive plate.

fuel filter

a small filter designed to remove dirt and water from fuel before it reaches carburetor or injection system. Filters may be made of metal or plastic screen, paper and gauze. Filters may be located at fuel tank, in fuel line, or at carburetor.

fuel line

a flexible fuel resistant hose that carries fuel from fuel petcock to carburetor.

fuel petcock

an on-off valve located at bottom of fuel tank. It may provide for reserve fuel supply and may have a filter screen and sediment bowl. Fuel line to carburetor is attached to fuel petcock.

fuel pump

a diaphragm or electric device that maintains proper fuel pressure to carburetor or injection system. A fuel pump must be used when fuel tank is located lower than carburetor and with all injection systems.

fuel system

system which stores, filters, and regulates flow of fuel to engine. It consists of fuel tank, fuel valve, fuel filters, fuel lines, air filter, carburetor, and carburetor mounting.

fuel tank

a reservoir used to store fuel for delivery to engine.

full-wave rectifier

a rectifier that converts AC to DC by inverting negative portion of AC sine wave.


a device used to protect electrical circuits from overloading. A link in fuse melts and opens circuit if current is above normal rating.

gas-charged shock absorber

a shock absorber using a pressurized gas such as nitrogen to help prevent changes in damping as shock absorber heats up.


a series of shafts and gears which varies ratio of engine to rear wheel speed. Motorcycle gearboxes use from two to seven speeds or ratios.

gearbox indexer

a plunger and spring or pivoted lever that indexes into dimples or slots in shift drum or cam plate. It provides positive positioning of shift drum or cam plate in each gear.

gear oil pump

an oil pump with gear type teeth to move oil.

gear ratio

ratio of rotation of rotating part compared to another rotating part.


a device which converts mechanical energy into electrical energy, producing direct current (DC).

graduated cylinder

a container, calibrated in cubic centimetres or millimetres used for accurate measurement of fluids such as fork oil and gearbox oil.


a reinforcing plate or boxed section used to prevent flexing of frame or swing arm.

half-wave rectifier

a rectifier which blocks one-half of AC sine wave to convert AC into DC.

heat transfer

movement of heat from one area to another.

heliarc welding

a type of electric arc welding which uses inert gas to shield area around arc.

helical gear

a gear having teeth that are slightly angled. Helical gears (used for primary drives) are quiet in operation, but absorb a slight amount of power due to side thrust.

hemispherical combustion chamber

a combustion chamber shaped like a round dome, allowing use of large valves placed opposite each other in chamber.

high-gear pinion

top gear on main shaft in direct drive gearbox. All ratios drive through high-gear pinion, which has output sprocket mounted to it.

holed piston

a condition caused by severe detonation or pre-ignition, where a hole is eaten or burned through crown of piston as a result of extreme heat and pressure.

horsepower (h.p.)

a unit for measuring the power of engines, motors, etc.; 1 h.p. equals 550 foot-pounds per second.

hydraulic brake

a braking system using hydraulic fluid, piston, and cylinders to provide extremely high pressure for brake application.

hydraulic clutch

a system which uses hydraulic pressure to disengage clutch.

hy-vo chain

a very strong chain made up of toothed plates positioned side by side and held together by pins. Advantage of this type of chain is great strength and quiet operation.

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idle mixture

air-fuel ratio with throttle twist grip closed. Idle mixture is controlled by pilot jet and pilot adjustment screw in carburetor or computer in electronic fuel injection system.

ignition coil

a step-up transformer that uses induction to increase battery voltage to over 20, 000 volts.

ignition-generating coil

coil in a flywheel magneto that provides primary ignition current.

ignition system

part of the electrical system responsible for providing a high voltage at proper time to ignite air-fuel mixture. Battery and coil, electronic, magneto, and CDI are the four systems commonly used.


finned wheel that produces pressure and flow when spun in enclosed housing of oil pump or water pump.

indirect drive gearbox

a gearbox where power is transferred from clutch to input shaft to output shaft. Output shaft has a sprocket mounted on one end.


production of current flow resulting from magnetic field cutting through a conductor.


tendency of a stationary object to resist movement or tendency of a moving object to continue moving in same direction.

injection system

an oil pump and oil lines that deliver oil to a two-stroke cycle engine as it is needed. Also, fuel system which meters gasoline into engine intake ports.

in-line cylinders

cylinders positioned in a row or side by side.

in-line filter

a small fuel or oil filter that replaces a section of line or hose.

input shaft

shaft of a gear box which carries power into gearbox. Clutch assembly is mounted on gearbox input shaft.

inside micrometer

a precision measurement tool used to take accurate measurements of inside surfaces.

insulator block

a fibre or rubber block that insulates carburetor from engine heat, used with flange-mounted carburetors.

intake manifold

a sleeve or flange made of rubber or metal to attach carburetor to intake port.

intake port

an opening or passage that directs flow of air-fuel mixture into engine. In a four-stroke cycle engine, intake port is located in cylinder head. In a two-stroke cycle engine, intake port is located in cylinder or crankcase.

intake stroke

in a four-stroke cycle engine, movement of piston from TDC to BDC with intake valve open, drawing air-fuel mixture into cylinder.

intake timing

amount of time two-stroke cycle engine intake port is open, expressed in crankshaft degrees or piston position.

internal combustion engine

an engine that burns fuel inside rather than outside engine, as does a steam engine (external combustion engine).


a carburetor component, usually made of brass, which meters flow of fuel to venturi. Fuel is metered by a small hole drilled through centre of jet.

junction block

a device that transfers action of a single cable to two or more cables.


parallel-sided piece inserted into groove cut part way into each of two parts, which locates and prevents slippage between parts. Also known as a woodruff key.

keystone piston ring

a piston ring with a tapered cross-section designed to use combustion pressure to aid in sealing.

kinetic energy

energy associated with motion. An internal combustion engine produces kinetic energy (crankshaft rotation).

lateral runout

side-to-side movement (wobble) of a wheel rim.


a machine tool used to spin, cut, and shape a metal part with a movable cutting tool.

lay shaft

second shaft in a direct drive gearbox. It transfers power from input shaft to high gear pinion.


an air-fuel ratio that contains extra air compared to fuel.

lever and pivot

a clutch release mechanism in which a pivoted lever attached to clutch cable disengages clutch by pushing plate away from clutch plates, allowing them to slip. Lever and pivot may act directly on pressure plate or through a push rod.


a hard surface shaft with a radius or roller at one end that rides on cam lobe.

liquid cooling

use of liquid piped through water jackets, thermostat, and radiator to dissipate heat.

long travel rear suspension

suspension used on current competition dirt bikes to provide more than 6 in. (15 cm) of travel.

lower end

portion of an engine from cylinder base downward, including connecting rods.

lubricating film

a thin coating of lubricant (oil) that prevents contact between moving parts.


use of lubricants (oil, grease) to reduce friction.

magnetic field

invisible lines of force surrounding a magnet or a conductor with current flowing through it.


an invisible force that attracts ferrous metals.


see flywheel magneto.

magneto-supported CDI

a capacitive discharge ignition using a generating coil in magneto to produce primary ignition current.

main fuel circuit

carburetor circuit that controls air-fuel ratio from three-fourths to full throttle opening. Main jet meters fuel in this circuit.

main jet

a carburetor fuel metering jet, usually mounted at base of carburetor body, to control air-fuel ratio from three-fourths to full throttle.

manifold injection

injector oil is pumped into the intake port of two-cycle engine.

master cylinder

components in a braking system that produces hydraulic pressure for system.

master link

a pin link that has one removable side plate located by a clip, or is staked (punched) in place. This allows a convenient way of separating chain.

mating parts

two or more parts that contact each other during operation and set up wear patterns.

mechanical brake

a braking system that uses a mechanical advantage by way of levers and cables or rods to apply brakes. A braking system not using hydraulic fluids or hydraulics.

mechanical caliper

a disc brake caliper actuated by a lever and cam rather than hydraulic fluid.

mechanical voltage regulator

a voltage regulator using an electromagnet to open or close contact points, varying output of a DC generator or electromagnet alternator.

metric system

a measurement system that uses the meter (39.37 in.) as a standard.

mild port timing

two-stroke cycle engine ports open for a relatively short time, providing for a broad power band.


a two-wheeled vehicle powered by an internal combustion engine.

multi-grade oil

an oil that flows like a thin oil when cold, but lubricates like a thicker oil when hot. For example, 10W40 at 0<F (-18<C) flows like a 10W oil; at 210< F (99<C), it flows like a 40W oil.

multi-plate clutch

a clutch assembly using more than one driving plate and more than one driven plate.

multiple ports

use of many small transfer ports rather than two large ports in two-stroke cycle cylinder. This can give improved scavenging.

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needle bearing

an antifriction bearing utilizing hardened steel rollers between hardened races or parts.

needle circuit

carburetor circuit that controls air-fuel ratio from one-fourth to three-fourths throttle opening. The jet needle and needle jet meter fuel flow in this circuit.

negative terminal

with an excess amount of electrons which flow toward positive terminal.

neutral indicator switch

a switch, usually mounted on end of shift drum, which completes circuit of neutral indicator light when gearbox is in neutral.

non-primary kickstart

a kickstart system using gearbox input shaft and clutch hub to connect kickstart lever to crankshaft. For starting, gearbox must be in neutral and clutch engaged.

non-unit construction

engine design with separate engine crankcase and gearbox case.

nylon screw clutch release

a clutch release mechanism that uses a coarse, square threaded nylon screw for clutch disengagement.


unit used to measure resistance in an electrical circuit or component.


an instrument that measures resistance.

oil bath

oil level is just high enough so it splashes around inside case or housing. Lubricates gearbox, primary drive ass well as ring and piston.

oil control ring

piston ring designed to remove excess oil from cylinder wall, usually bottom ring.

oil pump

a device that provides oil under pressure for engine lubrication.

oil pump bleeding

removal of air from supply line and pump in a two-stroke cycle oil injection system.

oil slinger

a circular, lipped disc used in direct bearing oil injection to feed oil to rod big end.

opposed cylinders

cylinders positioned opposite each other in same plane.


a ring made of neoprene that is used to provide a positive seal. It usually fits into a groove slightly shallower than O-ring, and mated against a flat surface to provide a seal for oil, fuel, or air.

O-ring chain

roller chain that uses O-rings to permanently seal lubricant into area between pins and bushings.

out of roundness

cylinder wear caused by piston thrust. This results in wear to front and back of cylinder. Cylinder wears oval shaped rather than round.

output shaft

gearbox shaft that transmits power to final drive at a selected ratio.

outside micrometer

a precision measuring tool for taking accurate measurements between outside surfaces.

overall gear ratio

ratio of crankshaft revolutions to rear wheel revolutions.

overflow tube

a tube that is open at both ends and mounted in bottom of carburetor float bowl. If float sticks or float needle does not seat properly, excess fuel flow out overflow tube rather than into throat of carburetor.

oxidized oil

oil that has been chemically combined with oxygen as a result of excessive heat, oil agitation or exposure to combustion contaminants.

oxyacetylene (gas) welding

a type of welding (metal fusion) which burns a mixture of oxygen and acetylene to provide a high temperature flame.

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parallel circuit

a circuit where current is provided to each component independently (each component has own ground). The failure of one component does not affect rest of circuit.

permanent magnet alternator

an alternator using permanent magnets to produce a magnetic field for AC production.

pilot air screw

a screw mounted on side of carburetor that allows fine adjustment of air-fuel ratio in pilot circuit of slide type carburetor.

pilot circuit

a carburetor circuit that provides proper air-fuel ratio from approximately zero to one-eighth throttle opening. It basically consists of a pilot jet, pilot air screw or pilot fuel screw, and a pilot discharge port.

pilot jet

jet that meters fuel for pilot circuit.

pinion gear

a small gear that either drives or is driven by a larger gear.

piston block

a slotted block used to support pistons for cylinder installation. Also, may be used in conjunction with a piston pin to lock camshaft.

piston (braking system)

a moveable part of a master cylinder or wheel caliper. Master cylinder piston produces pressure and caliper piston applies that pressure to brake disc, slowing or stopping wheel.

piston crown

top of piston, directly exposed to combustion pressure and heat.

piston (engine)

a cylindrical part, closed at one end, which moves up and down in cylinder. Open end is attached to connecting rod. Combustion pressure is exerted on closed end of piston, causing connecting rod to move and camshaft to turn.

piston pin boss

a strengthened section of piston extending to inside of piston crown. It supports piston pin.

piston pin hole

machined hole through piston wall where piston pin and retaining circlips are mounted.

piston port

two-stroke cycle piston skirt used to control intake port opening and closing.

piston ring (compression)

a split cast iron band, sometimes chrome plated, designed to seal combustion pressure above piston.

piston ring end gap

distance between ends of a piston ring when installed in cylinder. This clearance is measured with a feeler gauge. Ring must be straight in cylinder as if installed on piston.

piston ring groove

groove machined in piston to accept piston rings.

piston ring (oil control)

a split cast iron or steel band used to scrape excess oil from cylinder wall. Sometimes designed as a three-piece unit.

piston seizure

overheating of piston to point where it will no longer move freely in cylinder.

piston skirt

part of piston below bottom ring and pin boss.

plain bearing

a split or circular type bearing in which a part slides, turns, or pivots.

plain plate

see driven plate.

plunger oil pump

an oil pump that uses a piston, or plunger, and check valves to move oil.


direction of current flow.

port window

port opening in a two-stroke cycle engine cylinder wall.


electrical terminal having a deficiency of electrons and attracts electrons from negative terminal, causing current flow.

positive carburetor linkage

a carburetor linkage designed to positively open and close one or more carburetor throttles, actuated by one cable for opening and another cable for closing.


ability to do work or rate of work being done.

power stroke

in four-stroke cycle engine, movement of piston from TDC to BDC with intake and exhaust valves closed, after ignition and combustion of compressed air-fuel mixture.

power transmission

a system of gears, chains, sprockets, and shafts that transfers power from crankshaft to rear wheel in varying ratios.

power transmission shock absorber

a compensator cam, rubber pads, or springs in primary drive, gearbox, drive shaft, or rear wheel, provides means of cushioning loading of gearbox.


premature ignition cause by "hot spots" in combustion chamber that ignite air-fuel mixture before spark plug "fires." This causes uncontrolled combustion and rapidly raises piston and cylinder head temperatures (similar to detonation).


two-stroke cycle engine lubrication provided by oil mixed with fuel in fuel tank.

pressed-in sleeve

cast iron cylinder sleeve in interference fit into aluminum cylinder block.

primary chain drive

a primary drive design that uses a chain and sprocket to transfer engine power from crankshaft to clutch.

primary drive

use of chain, gear, or belt drive to connect crankshaft to clutch and gearbox into shaft.

primary ignition circuit

section of ignition circuit including contact points, condenser, and primary windings of ignition coil, power supply (battery or ignition generating coil), kill switch, ignition switch, and related wiring.

primary kickstart

a kickstart system that connects kickstart lever to crankshaft through clutch basket. Gearbox can be in gear for starting if clutch is disengaged.

primary reduction

a gear reduction usually about three to one, accomplished by primary drive.

primary-type needle circuit

fuel metered by clearance between jet needle and needle jet is partially mixed with air from air jet and discharged into venturi of carburetor.

primary winding

hundreds of turns of wire in an ignition coil to provide build-up and collapse of a magnetic field, including voltage in secondary.


contour or shape of camshaft lobe.

push rod (pushrod)

in four-stroke cycle engine, push rods provide means of transferring tappet movement to rocker arm, which opens valves.

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rack and pinion clutch release

a clutch release mechanism using pinion gear and a rack gear to disengage clutch.

radial runout

up and down movement of a rotating part (wheel rim, flywheel, etc.).


a heat exchanger that reduces coolant temperature in a liquid cooling system.

radical port timing

ports open for a relatively long period of time.

rear housing

an aluminum case which provides a mounting place for shaft drive ring and pinion gears. This housing is sealed and carries oil that lubricates ring and pinion gears.

reciprocating weight

weight of any engine parts moving back and forth while engine is running: pistons, rings, piston pin and circlips, push rods, valves.


a device that converts AC (alternating current) into DC (direct current) for battery charging.

reed cage

an aluminum frame providing a mounting place for reed petals in two-stroke cycle engine.

reed petals

movable parts of reed valve assembly.

reed valve

a one-way valve place in intake port of a two-stroke cycle engine. It prevents backward flow of crankcase air-fuel mixture.


a device that controls alternator or generator charging current to battery.

relief valve

a spring-loaded valve that regulates maximum pressure of enclosed system.


characteristic of an electrical circuit to resist flow of current, measured in ohms.


an air-fuel ratio that contains extra fuel compared to air.


an unworn portion or ledge at the top of cylinder formed above end of ring travel.

rim offset

distance a wheel rim is set off-centre (lateral) to a wheel hub. Rim offset is sometimes needed for drive sprocket clearance.

rim strip

a rubber strip installed around centre of a wire wheel rim to protect inner tube from being punctured by spokes.

ring and pinion contact pattern

area of contact between teeth of ring gear and teeth of pinion gear. Checked with red lead.

ring gear

a large gear in shaft drive housing. It is driven by pinion gear.

ring land

solid area of piston which supports rings, located between ring grooves.

rocker arm

a pivoted arm that converts upward movement of push rod to downward movement of valve.

roller bearing

an antifriction bearing consisting of hardened steel inner and outer races, separated by hardened steel rollers.

roller chain

a chain made up of pins, side plates, bushings and rollers. Roller links are connected by pin links to achieve desired length. Ends are usually connected by a pin link which has a removable side plate, called a master link.

rotary grinder

a high rpm hand held grinder using small grinding stones or burrs for metal removal in cramped or hard to get at areas.

rotary valve

a circular disc with a portion removed. It covers and uncovers intake port of two-stroke cycle engine. Disc is keyed to crankshaft.

rotor oil pump

an oil pump that uses an inner and outer rotor to move oil.


out-of-round condition of a rotating part.


inertia-caused movement of gases through cylinder and exhaust system of a two-stroke cycle or four-stroke cycle engine.


a diagram representing an electrical system.

scraper ring

dual-purpose piston ring that helps seal combustion pressure as well as control oil.

secondary ignition circuit

part of ignition systems consisting of secondary winding of ignition coil, spark plug wire, spark plug terminal, and spark plug.

secondary winding

thousands of turns of fine wire in an ignition coil. It uses induction to step up voltage as a magnetic field is rapidly collapsed in primary windings.

sediment bowl

a cup located at bottom of fuel petcock designed to prevent flow of dirt and water into fuel line.

selenium rectifier

a device that converts AC to DC through use of selenium plates.

series circuit

a circuit in which current must follow a single path through each component in sequence before reaching ground. If one component fails, entire circuit is open.

shaft drive

use of a drive shaft, universal joints and gears to connect gearbox output shaft to rear wheel.

shift drum

a drum shaped gearbox component with slots around its outside diameter. It engages with shift mechanism and shift forks. As drum is rotated, drum slots cause shift forks to move sliding gears or dogs causing engagement and disengagement of various gearbox ratios.

shifting fork

a flat forked gearbox component that engages with a slot in a sliding gear or dog. Shifting forks slide back and forth on lateral shafts. Rotation of shift drum or cam plate causes shift fork to move its sliding gear or dog to engage with another gear, locking both gears to shaft.

shift linkage

hinged lever or levers that transfer movement of shift lever to shift drum or cam plate.

shift stopper

a fork or pin that limits movement of shift drum to prevent overshifting.


a spacer used between two parts to achieve proper clearance.

shim bucket

a cylindrical shaped part in DOHC (double overhead camshaft) valve train. It is located between valve and camshaft and provides a receptacle for valve lash adjustment shims.

shock absorber

rear suspension unit made up of a coil spring and a hydraulic damper. It prevents spring oscillations. See also power transmission shock absorber.

siamese crankshaft

crankshaft configuration where two rods are mounted on same crank pin (journal). One rod is forked the other is mounted on inside of fork.

side-by-side crankshaft

crankshaft configuration where connecting rods are mounted next to each other on same crankpin (journal).

sine wave

a graphical representation of 360 degrees of rotation, representing change in direction of single-phase AC.

single-leading shoe

a drum brake having two shoes: one is leading and one is trailing. Leading shoe tends to wedge itself into drum providing more braking action than trailing shoe.

single-phase AC

alternating current that reverses direction of flow every 180 degrees.

single-piston caliper

a laterally floating disc brake caliper that uses one hydraulic piston to apply both brake pads. As pressure is applied to piston, it pushes one pad against disc. Other pad is solidly mounted on opposite side of caliper and makes contact with disc as caliper floats.

single-row chain

a conventional roller chain with one row of rollers.

sleeve-mount carburetor

a carburetor mounted on an aluminum manifold by a pinch bolt that clamps carburetor spigot over a plastic or fibre- insulating sleeve.

slide-controlled carburetor

a carburetor using a throttle slide (plunger) to regulate venturi restriction (throttle opening).

slide cutaway circuit

carburetor circuit which controls air-fuel ratio from one-eighth to one-quarter throttle opening. Height of cutaway portion of slide controls how much air is mixed with fuel. Fuel is metered by pilot jet, needle jet and jet needle.

sliding gear (dog gear)

a gear splined to its shaft and moved side to side by a shifting fork. A sliding gear has dogs on its lateral face that engage with holes or dogs in adjacent gears.

SOHC (single overhead camshaft)

a single overhead camshaft located in top of cylinder head.

solid roller chain

roller chain using a one-piece roller and bushing assembly.

spigot-mount carburetor

a carburetor mounted by clamping spigot into a rubber sleeve manifold.

spindle (steering fork)

a shaft that is secured to upper and lower triple clamps. It pivots on bearings mounted in steering head.

spoke-crossing pattern

number of spokes crossed on same side of hub by any one spoke.

spoked wheel

a wheel consisting of a rim, spokes, nipples and hub. Spokes are laced between hub and rim and are attached to rim by nipples. Thirty-six or forty spokes are termly used.

spoke torque wrench

a small torque wrench used to accurately adjust spoke tension.

spring preload

static installed length of a spring subtracted from its free length. In suspension system, spring preload determines ride height of motorcycle.

spring tester

a device that measures spring pressure in relation to spring length.


a circular plate with teeth machined around outside to engage links of a chain.

stamped frame

a frame stamped from pieces of sheet metal that are welded together to provide support for engine and suspension.

stamped wheel

a wheel assembly using stamped sheet metal spokes in place of small wire type spokes. A stamped wheel resembles a cast alloy wheel in appearance.


backward flow of air-fuel mixture in intake tract caused by radical port or cam timing.

starter system

carburetor system that provides a rich mixture for cold starting (tickler, choke, enrichment circuit). Also, electrical circuit for cranking engine, including: starter switch, relay, battery, and starting motor.

static ignition timing

use of a buzz box, test light or ohmmeter to check ignition timing without running engine.

steering damper

a device that uses friction or a hydraulic damper to reduce steering oscillation.

steering head

forward part of frame providing a mounting place for bearings that locate and support steering spindle and fork assembly.

steering offset

distance between steering axis and axis on which axle pivots. Steering offset is accomplished by offsetting axle or triple clamps.

steering rake

angle of steering axis from vertical, given in degrees.

steering trail

distance along ground between lines drawn vertically through front axle and down centre of fork tubes and sliders.

straight-cut gear drive

a gear drive which uses gears having straight-cut teeth, used for one type of primary drive.

straight-cut offset gear

a gear design using straight-cut gears installed side-by-side and one-half tooth out of phase. This gear design will eliminate noise caused by backlash common to conventional straight-cut gears.


movement of piston from TDC to BDC or from BDC to TDC.


lowest position of crankcase, containing oil.

surface plate

a block of granite ground and lapped to provide a very smooth, flat surface. It is used for checking straightness of cylinders, cylinder heads, and crankcase joints.


components that absorb road surface irregularities to smooth motorcycle ride. It is designed to permit controlled wheel movement over irregular surfaces. Basic parts include forks, swing arm and shock absorbers.

swing arm

main member of rear suspension that provides a mounting place for rear wheel and one end of shock absorbers.

swing arm pivot

forward part of swing arm, where it is attached to frame and pivots on bushings or bearings.


two sides or events being identical. For instance, two-stroke exhaust timing is symmetrical. Exhaust port opens and closes at same crankshaft position before and after TDC.

symmetrical hub

a wheel hub (wire wheel) that has spoke holes on each side of hub at same distance from hub center.


adjustment of carburetor throttles on two or more carburetors, so that all throttles are open same amount at any position of throttle twist grip.

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a small diameter at one end of part, for instance: tapered piston or taper cylinder bore.

telescopic fork

front suspension unit made up of two fork tubes and two sliders that telescope up and down tubes against spring pressure.

telescoping gauge

a "T" shaped instrument used to duplicate distance between inside parallel surfaces (cylinder bore for example).


a temperature sensitive device used in a liquid cooling system to adjust flow of coolant as coolant temperature changes.

three-phase AC

alternating current with a 120-degree phase difference. Three wires are needed to carry current.

throttle cable

a cable consisting of an outer housing and an inner cable that connects carburetor to throttle twist grip.

throttle stop screw

an idle speed adjustment screw used in slide type carburetor. This screw contacts base of throttle slide. As screw is turned in, throttle slide is raised, increasing idle speed.

throttle twist grip

a device mounted on end of handlebar that locates one end of outer throttle cable and pulls inner cable as twist grip is rotated.

thrust washer

a washer used to bear side thrust loads or provide lateral location.

tickler system

a spring-loaded plunger in carburetor that can be depressed to push float assembly down. This allows fuel to flow into float bowl, raising float level temporarily for cold starting.

tire pressure gauge

a pressure gauge used to check tire air pressure.

top dead center (TDC)

uppermost piston travel in cylinder.

top end

part of an engine from cylinder base up, including pistons but not connecting rods.


a twisting force measured in foot-pounds or newton-metres.

torque converter

a fluid turbine that takes place of clutch in primary drive. Oil is used to transmit power through torque converter depending upon engine rpm. Used in automatic transmissions.

torque pattern

a specific sequence of tightening fasteners to help prevent warping of components.

torque wrench

a wrench calibrated to allow for accurate application of tightening force to fasteners.

transfer port

opening in cylinder wall of a two-stroke cycle engine that connects cylinder to crankcase.

transfer (port) timing

amount of time transfer port is open, expressed in crankcase degrees or piston position.


see gearbox.

triple clamps

a pair of sturdy brackets that provide a mounting place for fork legs and steering spindle. Triple clamps attach forks to frame through spindle, steering head and steering head bearings.

triple-row chain

a chain having three rows of rollers. Triplex chains are used for primary drives.

two-stroke cycle

a cycle of overlapping events (intake, transfer, compression, power, exhaust) which all occur during one revolution of crankcase.

unit construction

engine design has one crankcase to house gearbox, clutch, primary drive, and engine.

universal joint

a flexible joint that allows changes in angle of drive shaft.


pressure that is less than atmospheric pressure.

vacuum-controlled carburetor

a carburetor using a butterfly to regulate airflow and a vacuum controlled piston to regulate venturi restriction.

vacuum gauge

a gauge that measures pressures which are lower than 14.7 lb. per square inch (101 kPa) at sea level.

vacuum piston

throttle piston used in a vacuum operated CV carburetor. One type uses a diaphragm at top with a small diameter piston. Other type uses a large diameter piston in a vacuum cylinder.


a device that controls flow by opening and closing. For instance, fuel valve controls flow of fuel from fuel tank to carburetor. Intake valve controls flow of air-fuel mixture into cylinder.

valve overlap

period of time when intake and exhaust valves in a four-stroke cycle engine are open at same time.

valve stem

portion of valve that slides in valve guide and provides a mounting place for valve spring upper collar and keepers.

valve train

all components which directly influence valve operation (cam, cam chain, cam followers, valves, valve springs, valve collars, and keepers in SOHC engine).


a build-up of oxidized oil or fuel.


cylinders positioned at angles to each other forming a "V".


a tube or drillway going from top of float chamber to atmosphere. This prevents build-up of pressure or vacuum in float chamber.


a restriction in a tube that cause an increase in air velocity, resulting in decrease of pressure (increase in vacuum) at restriction.

vernier caliper

a precision measuring tool used to determine inside, outside, or depth measurements. A linear vernier scale is used.

vertical mill

a machine tool that removes metal with a rotating cutter mounted vertically.


the resistance that oil offers to flow when it is subjected to stress (thickness or weight).


force causing electron flow in conductor.


an instrument that measures electrical pressure (EMF, voltage) in electrical circuits.


unit of measurement of electrical power (rate of doing work).

wear limits

minimum and maximum acceptable size of a component.


use of heat to fuse two pieces of metal together.

wet clutch

a multi-plate clutch that runs in an oil bath (primary drive).

wet sump

four-stroke cycle engine sump (crankcase) in oil reservoir.

wheel bearing grease

a special, heavy duty, high temperature grease used to lubricate wheel bearings.

wide ratio gearbox

a gearbox having wide ratio spacing between gears. A wide ratio gearbox is commonly used in enduro (off-road) and trials motorcycles.

wire gauge

a measuring tool made of precisely sized wire. It is for measuring distance between irregular surfaces (spark plug gap for example).

wire wheel

see "spoked wheel"

woodruff key

parallel-sided piece inserted into groove cut part way into each of two parts, which locates and prevents slippage between parts.

Zener diode

a solid-state device used to regulate voltage in motorcycle charging systems.

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