Glossary of Technical Terms
Catalytic converter A silencer-like device in
the exhaust system which converts certain
pollutants in the exhaust gases into less
Circlip A ring-shaped clip used to prevent
endwise movement of cylindrical parts and
shafts. An internal circlip is installed in a
groove in a housing; an external circlip fits
into a groove on the outside of a cylindrical
piece such as a shaft.
Clearance The amount of space between two
parts. For example, between a piston and a
cylinder, between a bearing and a journal, etc.
Coil spring A spiral of elastic steel found in
various sizes throughout a vehicle, for
example as a springing medium in the
suspension and in the valve train.
Compression Reduction in volume, and
increase in pressure and temperature, of a
gas, caused by squeezing it into a smaller
Compression ratio The relationship between
cylinder volume when the piston is at top
dead centre and cylinder volume when the
piston is at bottom dead centre.
Constant velocity (CV) joint A type of
universal joint that cancels out vibrations
caused by driving power being transmitted
through an angle.
Core plug A disc or cup-shaped metal
device inserted in a hole in a casting through
which core was removed when the casting
was formed. Also known as a freeze plug or
Crankcase The lower part of the engine
block in which the crankshaft rotates.
Crankshaft The main rotating member, or
shaft, running the length of the crankcase,
with offset "throws" to which the connecting
rods are attached.
Diagnostic code Code numbers obtained by
accessing the diagnostic mode of an engine
management computer. This code can be
used to determine the area in the system
where a malfunction may be located.
Disc brake A brake design incorporating a
rotating disc onto which brake pads are
squeezed. The resulting friction converts the
energy of a moving vehicle into heat.
Double-overhead cam (DOHC) An engine
that uses two overhead camshafts, usually
one for the intake valves and one for the
Drivebelt(s) The belt(s) used to drive
accessories such as the alternator, water
pump, power steering pump, air conditioning
compressor, etc. off the crankshaft pulley.
Crocodile clip See Alligator clip
Driveshaft Any shaft used to transmit
motion. Commonly used when referring to the
axleshafts on a front wheel drive vehicle.
Drum brake A type of brake using a drum-
shaped metal cylinder attached to the inner
surface of the wheel. When the brake pedal is
pressed, curved brake shoes with friction
linings press against the inside of the drum to
slow or stop the vehicle.
EGR valve A valve used to introduce exhaust
gases into the intake air stream.
Electronic control unit (ECU) A computer
which controls (for instance) ignition and fuel
injection systems, or an anti-lock braking
system. For more information refer to the
Haynes Automotive Electrical and Electronic
Electronic Fuel Injection (EFI) A computer
controlled fuel system that distributes fuel
through an injector located in each intake port
of the engine.
Emergency brake A braking system,
independent of the main hydraulic system,
that can be used to slow or stop the vehicle if
the primary brakes fail, or to hold the vehicle
stationary even though the brake pedal isn't
depressed. It usually consists of a hand lever
that actuates either front or rear brakes
mechanically through a series of cables and
linkages. Also known as a handbrake or
Endfloat The amount of lengthwise
movement between two parts. As applied to a
crankshaft, the distance that the crankshaft
can move forward and back in the cylinder
Engine management system (EMS) A
computer controlled system which manages
the fuel injection and the ignition systems in
an integrated fashion.
Exhaust manifold A part with several
passages through which exhaust gases leave
the engine combustion chambers and enter
the exhaust pipe.
Fan clutch A viscous (fluid) drive coupling
device which permits variable engine fan
speeds in relation to engine speeds.
Feeler blade A thin strip or blade of hardened
steel, ground to an exact thickness, used to
check or measure clearances between parts.
Firing order The order in which the engine
cylinders fire, or deliver their power strokes,
beginning with the number one cylinder.
Flywheel A heavy spinning wheel in which
energy is absorbed and stored by means of
momentum. On cars, the flywheel is attached
to the crankshaft to smooth out firing
Free play 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
brake pedal moves before the pistons in the
master cylinder are actuated.
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
Fusible link A circuit protection device
consisting of a conductor surrounded by
heat-resistant insulation. The conductor is
smaller than the wire it protects, so it acts as
the weakest link in the circuit. Unlike a blown
fuse, a failed fusible link must frequently be
cut from the wire for replacement.