| OHV |
See
Overhead valve. |
|
|
| Oil
ring |
The
lowest ring on an engine's piston that scrapes oil away from the
cylinder wall as the piston moves downward and prevents oil from
getting into the combustion chamber. An oil ring is actually three
rings assembled together; two thin, flat rings are separated by
an expansion ring. As an engine operates, oil is splashed about
by the crankshaft and connecting rods. Oil gets onto the cylinder
walls when the piston is at the top of it's travel. The oil ring
removes the majority of oil on the cylinder walls but a tiny film
remains trapped in the fine crosshatching (basically fine sanding
marks left by honing) that helps the compression rings seal. If
an oil ring wears excessively or cracks, excessive oil can get into
the combustion chamber resulting in engine oil use and sparkplug
fouling. Also called an oil control ring. |
|
|
| O-ringed
block |
A
block that has been grooved around the top of each cylinder bore
to accept a wire insert or crush washer, to form a tight seal
between the head and block. Used with high compression engines
and where severe combustion conditions such as forced induction
and nitrous oxide injection may exist. In some applications, a
copper washer is placed in the groove at the top of each cylinder
and used with a modified head gasket (one that leaves room for
the washer but seals the water and oil passages. The washer becomes
crushed when the head is installed and forms a durable combustion
chamber seal between the head and block.
Another
application involves the use of a ring of fine stainless steel
piano wire in a thin groove used with a high-performance head
gasket. The wire imbeds itself in the head gasket's metal ring
(fire ring) and adds integrity to it. Also see Compression and
Four-stroke engine.
|
| |
|
| Oil-cooled
piston |
A
piston cooled by oil sprayed on it's underside. Used on some commercial
diesel engines. |
|
|
| Oil
cooler |
A
small heat exchanger, similar to a small radiator, that is used
to cool engine, or transmission oil. Aftermarket transmission oil
coolers are commonly installed and can extend the life of a transmission;
especially if installed on a vehicle used for towing. |
|
|
| Oil
filter |
A
device that removes suspended dust and other harmful microscopic
particles from and engine's oil which can increase friction and
thus engine wear. Modern oil filters are a spin-on canister type.
Inside the metal housing is a pleated paper filter cartridge and
a bypass valve system that allows engine oil bypass the filter element
should the filter become clogged. At the top of most filters is
a rubber anti-drainback valve which prevents engine oil from draining
out of the filter into the crankcase when the engine is turned off.
Keeping the filter cartridge full of oil allows instant lubrication
when the engine is started. Most manufacturers recommend installing
a new, quality oil filter during each oil change. |
|
|
| Oil
galleries |
Small
passages in an engine block and cylinder head that oil is circulated
through for lubricating and cooling the engine. Many of the passages
are cast right into the block or heads. Other passages are machined
during construction. An oil pump pumps oil throughout the motor
to critical moving parts. Oil flows through passages from the top
of the motor back to the oil pan. |
|
|
| Oil
pan |
A
metal object that attaches to the bottom of an engine and acts as
the oil reservoir. When the engine is turned off, the oil drains
back down to the oil pan. Most oil pans are made of stamped steel
or cast aluminum. |
|
|
| Oil
pressure |
Pressurized
oil (created by the oil pump) that is forced throughout passages
in the motor to lubricate and cool critical moving parts and prevent
wear. |
|
|
| Oil
pump |
A
device that draws oil from the oil pan it throughout the engine
under pressure. Most oil pumps are driven by a gear that comes from
the camshaft or distributor, but many four cylinder engines have
oil pumps located at the front of the motor and are driven off the
crankshaft. |
|
|
| Oil
scraper ring |
A
piston ring positioned between the oil ring (bottom ring) and the
compression ring (top ring). Removes residual oil left on the cylinder
wall by the oil ring. |
|
|
| Oil
slinger |
A
thin metal collar, usually cone-shaped, commonly installed at the
front end of a crankshaft that splashes oil onto the timing chain
and associated components, and also reduces the amount of oil that
splashed onto the crankshaft seal. |
|
|
| Onboard air compressor |
An
air compressor, usually miniature, permanently mounted on a vehicle
for use of operating an air-ride suspension or inflating tires,
and operating tools. Luxury vehicles that have an air-ride suspension
use an onboard air compressor and valves to adjust the air in
shock absorbers to meet changing road, load and suspension changes.
Many trucks also use airbags for overload springs.
Off-roaders
often convert air-conditioning compressors to air compressors
or install custom 12V compressors in their engine compartments
to inflate tires (when airing up) or operate air tools to make
repairs on the trail. For reserve air, many off-roaders go one
step further and custom-make their tube bumpers and roll cages
to be airtight; they can be inflated and act as an air tank.
|
| |
|
| Onboard
diagnostics |
See
diagnostics |
|
|
| Onboard welder |
A
miniature DC stick welder that can be installed on the vehicle (usually
in the engine compartment). Operates off 12V and frequently used
by off-roaders, adventurers and expeditions to make trail repairs. |
| |
|
| On-center
feel |
The
characteristic of a steering wheel to return to center after a turn
and stay at center while driving. Helps keep a vehicle in a straight
line and reduces sway and wander due to uneven roads and wind. The
effect is created by steering caster. See Caster. |
|
|
| One-way
clutch |
A
clutch mechanism that allows torque to be applied to a shaft in
one direction but allows freewheeling in the opposite direction.
A one-way clutch is used on starter drives. When the starter is
engaged it "holds" and the starter motor turns the engine
over until it begins to run. If not for a one-way clutch, a starter
motor could easily become damaged because the engine, once started,
would spin the starter motor faster than it is designed for. A one-way
clutch on the starter drive allows the gear on the starter drive
to turn freely applying no torque to the starter motor. Once the
key is backed off from the "start" position, the tiny
starter gear is retracted from the engine's flywheel. |
|
|
| One-way
valve |
A
valve that allows air or liquids to flow in only one direction.
A one-way valve is typically located in the fuel line at some point
to prevent fuel from draining back into the tank which would create
starting problems. See Check Valve. |
|
|
| One-wire alternator |
See
Marine-style alternator. |
| |
|
| Opera lights |
Lights
which are typically mounted in the rear sides of the interior, or
sides of the exterior for decoration. |
| |
|
| Opera
window |
A
small window, sometimes round, located in the rear pillar of an
automobile. Found on some 1970's full-sized sedans. |
|
|
| Opposed
engine |
An
engine where the banks of cylinders lie 180 degrees opposite of
each other on the same plane. The Volkswagen engine used in the
famed VW Beetle was a flat engine. Also called Flat Engine. |
|
|
| Opposite
lock |
A
term for a driving technique where the front wheels are steered
in the opposite direction of a skid (but the same direction of travel)
in order to "counter" rear wheel skid and keep the car
under control. Also known as Counter Steering. |
|
|
| Original
equipment |
Parts
and components on a vehicle that were installed by the manufacturer
when new. Does not include dealer installed parts. |
|
|
| Original
Equipment Manufacturer (OEM) |
Used
to describe vehicle manufacturers but also applies to any parts
or suppliers that produced parts or equipment that was original
when a vehicle was manufactured. Restorers of classic cars frequently
use the term OEM when talking about replacement parts. OEM parts
are replacement parts that were made or sold by the manufacturer
as direct parts replacements. They typically fit better and are
of higher quality than aftermarket parts.
Another
term, NOS means New Old Stock. It refers to new replacement parts
(OEM) made by the vehicle manufacturer that were never used and
have been discontinued; and usually sold in bulk from dealer shelves
to private sellers, collectors or individuals. NOS parts are coveted
by restorers for their originality and fit.
|
|
|
| O-ring |
A
circular ring typically made of rubber or soft copper used to create
a seal between two parts. O-rings are used extensively in automobiles
to maintain a good seal between air-conditioning connections and
fuel and transmission line connections. |
|
|
| Oscilloscope |
A
diagnostic instrument that uses a TV-like display to illustrate
electrical frequencies such as ignition timing. Sometimes referred
to as just, Scope. |
|
|
| Otto
cycle |
See
Four-stroke cycle. |
|
|
| Outboard
brakes |
Brakes
that are installed at or near the wheels. Nearly all vehicles use
outboard brakes. Few high-performance production vehicles and many
race cars use an inboard brake design where the brake drums or rotors,
and associated brake components are mounted on the axles near the
differential which is near the center of the vehicle. |
|
|
| Output
shaft |
A
shaft that carries power from one component to another. The transmission
has an output shaft that connects to and sends power to the drive
shaft, or axles (FWD). |
|
|
| Outward vision |
Describes
the field of vision of the driver and passengers. More glass area
and smaller roof pillars contribute to greater outward vision. |
|
|
| Overall
diameter |
When
applied to a tire; the maximum diameter of an unloaded tire (not
contacting the ground) when it's mounted on a rim and fully inflated.
When
applied to bearing journals it is the maximum diameter.
|
|
|
| Overall
length |
The
complete length of a vehicle from the furthestmost point on the
front bumper to the furthest point at the rear. Often cited by
manufacturers in new vehicle specifications.
|
|
|
| Overall width |
The
maximum width of a vehicle including side trim and fender flares.
When
applied to tires, it is the maximum width of an unloaded, fully
inflated tire.
|
|
|
| Overdrive |
A
transmission with a gear ratio less than 1:1. Modern vehicles typically
use an overdrive top gear to improve fuel economy. In a 1:1 output
ratio, the output of the transmission is equal to engine speed;
for each revolution of the engine, the transmission makes one revolution.
With an overdrive transmission, in overdrive, the transmission output
actually turns more than engine speed. Overdrive is considered a
highway cruising gear because it required more engine power. |
|
|
| Overflow
tank |
A
reservoir commonly used in the engine cooling system to contain
extra coolant, maintain coolant levels and collect coolant overflows.
The expansion tank on most vehicles is made of translucent plastic
and mounted near the fender. It is connected to the radiator (near
radiator cap) via a small hose. If the engine becomes excessively
hot or overheats, expanding coolant overcomes the radiator cap pressure
and flows through the tube into the expansion tank. When, the vehicle
cools down, a vacuum will be created in the cooling system and coolant
will be drawn back into the radiator from the expansion tank. This
way the radiator will always be full of coolant. |
|
|
| Overhang |
The
part of the vehicle that protrudes beyond the front and rear of
the wheelbase. For example, in the front of a car, the area from
the center of the front wheels to the furthestmost part of the front
bumper is the front overhang. Full-sized sedans with large trunks
typically have a large rear overhang. You can determine the average
front/rear overhang by subtracting the wheelbase measurement from
the total length. |
|
|
| Overhaul |
Replacement
of a major automotive component that usually involves replacing
critical main wear parts. An engine overhaul usually means replacing
piston rings, rod and crankshaft bearings, performing a valve job,
honing the cylinders, installing a new timing chain/gears and installing
new gaskets. A major overhaul would involve replacing all
the previously mentioned parts plus replacing the crankshaft, camshaft
& lifters, pistons, oil pump as well as having the engine block
cleaned and bored to correct for cylinder wear. |
|
|
| Overhead
cam (OHC) |
An
engine arrangement where the camshaft is positioned at the top of
the cylinder head above the valves. Traditionally, camshafts were
located in the center of the engine block requiring lifters, long
push rods and rocker arms to open valves. An overhead cam arrangement
substantially reduces the number of moving parts which increases
responsiveness and allows higher valvetrain speeds. |
|
|
| Overhead console |
An
interior roof-mounted panel, usually colored to complement interior
colors that may contain switches, lamps (i.e. map/dome light), radio
or CB, compass, travel computer, and/or storage. |
| |
|
| Overhead
valve (OHV) |
An
engine design used by nearly all manufacturers where intake and
exhaust valves are located directly above the piston. Engines uses
in vintage vehicles often had valves located in the cylinder block
next to the pistons; camshafts were always located in the cylinder
block. The overhead valve design that used pushrods and rocker arms
was considered an engineering achievement in it's day thus lending
to the term overhead valve. |
|
|
| Overlap |
The
point of time where both the intake and exhaust valves are open
at the same time, measured in degrees of crankshaft rotation. Between
the end of the exhaust stroke and the beginning of the next intake
stroke (piston at top) the exhaust valve is timed to close late
to allow a little extra time for burned gasses to leave the cylinder.
At the same time, the intake valve is opened slightly early in order
to allow the fresh air/fuel mix a little extra time to fill the
combustion chamber. |
|
|
| Oversize |
Applied
to installing replacement crankshaft and rod bearings that are
slightly larger in order to accommodate a using an existing crankshaft,
or a crankshaft that has been reground and has journals that are
smaller diameter. Also applies to installing slightly larger piston
rings to an engine to accommodate cylinder wear.
Also,
can be used to describe installing larger than original tires
and rims on a vehicle. Off-road enthusiasts commonly install larger
off-road tires to their 4x4 trucks and SUV's to increase off-road
traction and vehicle height.
|
|
|
| Overslung |
A
term applied to vintage automobiles where the vehicle frame is above
the axles. Underslug was a technique where the frame was attached
below the axle. It allowed lower frame height. |
|
|
| Oversquare |
Applied
to an engine where the cylinder bore is wider than the stroke of
the piston. |
|
|
| Oversteer |
An
undesirable handling condition where the rear wheels of a vehicle
tend to want to swing out around a corner ahead of the front wheels.
When oversteer is excessive, minor cornering requires the driver
to counter steer to prevent the rear wheels from swinging out of
control. Engineers design understeer into automobiles because it
results in more predictable steering. See Understeer. |
|
|
| Owner's manual |
An
instruction booklet that accompanies a new vehicle that explains
how to operate the vehicle's safety and optional equipment and other
important information such as fluid capacities, driving tips and
service guidelines. |
| |
|
| Oxides
of nitrogen (NOx) |
Compounds
containing nitrogen and oxygen which result from combustion at high
cylinder temperatures. It is a major pollutant and contributes to
smog. Common ways of reducing NOx emissions include lower compression
ratios and recirculating exhaust gasses using an EGR valve. An EGR
system recirculates a small portion of exhaust gasses back into
the combustion chamber. The result is significantly lower upper
cylinder temperatures and reduced formation of NOx compounds. |
|
|
| Oxygen
sensor |
A
sensor used in nearly every modern automobile which senses oxygen
levels in exhaust gasses. A signal is sent from the oxygen sensor
to the engine management computer which precisely controls the air/fuel
ratio to maintain the "best" mix to yield the lowest level
of emissions. |
