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REF•24 Glossary of technical terms
automatic transmission, a switch that
prevents starting if the vehicle is not in Neutral
or Park.
Strut See MacPherson strut.
T
Tappet A cylindrical component which
transmits motion from the cam to the valve
stem, either directly or via a pushrod and
rocker arm. Also called a cam follower.
Thermostat A heat-controlled valve that
regulates the flow of coolant between the
cylinder block and the radiator, so maintaining
optimum engine operating temperature. A
thermostat is also used in some air cleaners in
which the temperature is regulated.
Thrust bearing The bearing in the clutch
assembly that is moved in to the release levers
by clutch pedal action to disengage the
clutch. Also referred to as a release bearing.
Timing belt A toothed belt which drives the
camshaft. Serious engine damage may result
if it breaks in service.
Timing chain A chain which drives the
camshaft.
Toe-in The amount the front wheels are
closer together at the front than at the rear. On
rear wheel drive vehicles, a slight amount of
toe-in is usually specified to keep the front
wheels running parallel on the road by
offsetting other forces that tend to spread the
wheels apart.
Toe-out The amount the front wheels are
closer together at the rear than at the front. On
front wheel drive vehicles, a slight amount of
toe-out is usually specified.
Tools For full information on choosing and
using tools, refer to the Haynes Automotive
Tools Manual.
Tracer A stripe of a second colour applied to
a wire insulator to distinguish that wire from
another one with the same colour insulator.
Tune-up A process of accurate and careful
adjustments and parts replacement to obtain
the best possible engine performance.
Turbocharger A centrifugal device, driven by
exhaust gases, that pressurises the intake air.
Normally used to increase the power output
from a given engine displacement, but can
also be used primarily to reduce exhaust
emissions (as on VW’s “Umwelt” Diesel
engine).
U
Universal joint or U-joint A double-pivoted
connection for transmitting power from a
driving to a driven shaft through an angle. A U-
joint consists of two Y-shaped yokes and a
cross-shaped member called the spider.
V
Valve A device through which the flow of
liquid, gas, vacuum, or loose material in bulk
may be started, stopped, or regulated by a
movable part that opens, shuts, or partially
obstructs one or more ports or passageways.
A valve is also the movable part of such a
device.
Valve clearance The clearance between the
valve tip (the end of the valve stem) and the
rocker arm or tappet. The valve clearance is
measured when the valve is closed.
Vernier caliper A precision measuring
instrument that measures inside and outside
dimensions. Not quite as accurate as a
micrometer, but more convenient.
Viscosity The thickness of a liquid or its
resistance to flow.
Volt A unit for expressing electrical “pressure”
in a circuit. One volt that will produce a current
of one ampere through a resistance of one
ohm.
W
Welding Various processes used to join metal
items by heating the areas to be joined to a
molten state and fusing them together. For
more information refer to the Haynes
Automotive Welding Manual.
Wiring diagram A drawing portraying the
components and wires in a vehicle’s electrical
system, using standardised symbols. For
more information refer to the Haynes
Automotive Electrical and Electronic Systems
Manual.