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Free length The no-load state of a component
when measured. Clutch. valve and fork spring
lengths are measured at rest, without any
preload.
Decarbonisation
The process
of removing
carbon deposits -
typically from the combustion
chamber, valves and exhaust port/system.
Deton.tion
Destructive
and
damaging
explosion
of fuel/air
mixture
in combustion
chamber instead of controlled burning.
Diode
An electrical
valve which only allows
current to flow in one direction. Commonly used
in rectifiers and starter interlock systems.
Disc velve (or rot8ry
velve)
A induction system
used on some two-stroke engines.
Doubie-overtM8d
C8m8haft (DOHC) An engine
that uses two overhead camshafts. one for the
intake valves and one for the exhaust valves.
DrtvebeIt
A toothed belt used to transmit drive
to the rear wheel on some motorcycles.
A
drivebelt
has also been used to drive the
camshafts. Drivebetts~
usually made of Keylar.
Drive8haft
Any shaft used to transmit motion.
Commonly
used when referring
to the final
driveshaft on shaft driVe motorcycles.
E
Earth return The return path of an electrical
circuit, utilising the motorcycle's frame.
ECU (Electronic Control Unit) A computer
which controls (for instance) an ignition system,
or an anti-lock braking system.
EGO Exhaust Gas Oxygen sensor. Sometimes
called a Lambda sensor.
Electrolyte The fluid in a lead-acid battery.
EMS (Engine Management
System)
A
computer controlled system which manages the
fuel injection and the ignition systems in an
integrated fashion.
Endftoat The amount of lengthways movement
between two parts. As applied to a crankshaft,
the distance that the crankshaft can move side-
to-side in the crankcase.
Endl..s chain A chain having no joining link.
Common use for cam chains and final drive
chains.
EP (Extreme
Pressure)
Oil type
used in
locations where high loads are applied. such as
between gear teeth.
Evaporetlve
emieeion
control
eyetem
Describes a charcoal filled canister which stores
fuel vapours from the tank rather than allowing
them to vent to the atmosphere.
Usually only
fitted to California models and referred to as an
EVAP system.
Expan8ion
chamber
Section
of two-stroke
engine exhaust system so designed to improve
engine efficiency and boost power.
F
Feeler blade or gauge A thin
strip or blade of
hardened steel, ground to an exact thickness,
used to check or measure clearances between
parts.
Final drive
Description
of the drive from the
transmission to the rear wheel. Usually by chain
or shaft, but sometimes by bett.
Firing
order
The order in which the engine
cylinders
fire, or deliver their power strokes,
beginning with the number one cylinder.
Flooding
Term used to describe a high fuel level
in the carburettor float chambers, leading to fuel
overflow.
Also refers to excess
fuel in the
combustion
chamber due to incorrect starting
technique.
Technical
Terms Explained
REF-25
Hellcoil
A thread insert repair system.
Commonly used as a repair for stripped spark
plug threads (... III.-tratIon).
Honing A process used to break down the glaze
on a cylinder bore (also called glaze-busting).
Can also be carried out to roughen a rebored
cylinder to aid ring bedding-in.
HT High Tension Description of the electrical
circuit from the secondary winding of the ignition
coil to the spark plug.
Hydr8uIic A liquid filled system used to ~It
pressure from one component to another.
Common uses on motorcycles are brakes and
clutches.
Hydrometer An instrument for measuring the
specific gravity of a lead-Kid
battery.
Hygroscopic Water absorbing. In motorcycle
applications, braking efficiency will be reduced if
DOT 3 or 4 hydraulic fluid absorbs water from the
aJr
-
care must be taken to keep new brake fluid
in tightly sealed containers.
FreepiBY The amount of travel before any action
takes place. The looseness in a linkage, or an
assembly of parts, between the initial application
of force and actual movement. For example, the
distance the rear brake pedal moves before the
rear brake is actuated.
Fuel Injection The fuel/air mixture is metered
electronically and directed Into the engine intake
ports (Indirect injection) or into the cylinders
(direct injection). Sensors supply information on
engine speed and conditions.
Fuel/air mixture
The charge of fuel and air
going into the engine. See Stolchl~
ratio.
Fuse An electrical device which protects a
circuit against accidental overload. The typical
fuse contains a soft piece of metal which is
calibrated to melt at a predetermined current
flow (expressed as amps) and break the circuit.
G
Ibf ft
Pounds-force feet. An imperial unit of
torque. Sometimes written as ft-IOs,
Ibf In Pound-force inch, An imperial unit of
torque, applied to components w~
a very low
torque is required. Sometimes written as in-ibs.
IC Abbreviation for Integrated Circuit.
Ignition advance
Means of increasing the
timing of the spark at higher engine speeds.
Done by mechanical means (ATU) on early
engines or electronically by the ignition control
unit on later engines.
Ignition timing The moment at which the spark
plug fires, expressed In the rRxnber of cr81k8haft
degrees before the piston r88che8 the top of Its
stroke, or in the number of mllHmetres before the
piston reaches the top of Its stroke,
Infinity (~)
Description of an open-circuit
electrical state. where no continuity exists.
Inverted fora (~
down fora)
The sliders
or lower legs are held in the yokes and the fork
tubes or stanchions are connected to the wheel
axle (spindle), Less unsprung weight and stiffer
construction than conventional forks.
G8p The distance the spark must travel in
jumping from the centre electrode to the side
electrode in a spark plug. Also refers to the
distance between the ignition rotor and the
pickup ~I in an electronic ignition system.
G..ket
Any thin, soft material
- usually cort,
cardboard, asbestos or soft metal. installed
between two metal surfaces to ensure a good
seal. For instance, the cylinder head gasket seals
the joint between the block and the cylinder
head.
Gauge An instrument panel display used to
monitor engine conditions. A gauge with a
movable pointer on a dial or a fixed scale is an
analogue gauge. A gauge with a numerical
readout is called a digital gauge.
Gear ratios The drive rstio of a pair of geers in a
gearbox, calculated on their number of teeth.
Glaze-busting see Honing
Grinding Process for renovating the valve face
and valve seat contact area in the cylinder head.
Gudgeon pin The shaft which connects the
connecting rod small-end with the piston. Oft~
called a piston pin or wrist pin.
J
H
JASO Quality standard for 2-stroke oiI8.
Joule The
unit
of electrical energy.
Journal
The b88r1ng slriace of a shaft.
Helical gee,.
Gear teeth are slightly curved BOO
produce less gear noise that straight-cut gears.
Often used for primary drives.
K
Klckstart
Mechanical means of tuming the
engine over for starting purpoHS. Only usually
fitted to mopeds, small capacity motorcycles and
off-road motorcycles.
Kill switch
Handebar-mounted switch for
emergency ignition cut-out. Cuts the ignition
circuit on all models. and additionally prevent
starter motor operation on others.
kin Symbol for kilometre.
kph Abbreviation for kilometres per hour.
L
Lambda (A) sen8or
A sensor fitted in the
exhaust system to measure the exhaust gas
oxygen content (excess air
factor).
Installing a Helicoil thread insert in a
cylinder head
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