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Nail-Guard Tires See self-sealing tires. Normally Aspirated An engine that intakes air or "breathes" without the assistance of a supercharger or turbocharger.
O Odometer see Speedometer OEM See original equipment manufacturer. Ohm The unit of electrical resistance. One volt applied to a resistance of one ohm will produce a current of one amp. Ohmmeter An instrument for measuring electrical resistance. Oil Filler Cap The oil filler cap is a plastic or metal cap that covers an opening into the valve cover. It allows you to add oil when the dipstick indicates that you need it. Some cars have the crankcase vented through the filler cap. Oil which is added through the filler passes down through openings in the head into the oil sump at the bottom of the engine. Oil Gauge The Oil Gauge lets you know the pressure in the oil pump; and is a direct indicator of the condition of your engine. If the oil gauge gives very low readings, it means that your oil pressure has dropped. The engine would soon grind to a halt if the oil is not forced into the nooks and crannies that need to be oiled. Oil Pan The pan that is bolted to the bottom of an engine. Oil returns to this pan after circulating throughout an engine to provide lubrication, as well as some cooling effects. Most oil pans are open inside, but high-performance pans can feature trap doors and baffles to control the oil during aggressive driving and keep it close to the oil pump pickup. Another product that can help control engine oil is a windage tray or a crank scraper. Oil Pump An engine-driven pump that delivers oil, under pressure, to the engine's moving parts. Oil Ring The lowermost piston ring that scrapes off excess oil from the cylinder walls and returns it to the oil pan via vents in the ring and piston. Operating Coil see Limiting Coil O-Ring A type of sealing ring made of a special rubber-like material; in use, the O-ring is compressed into a groove to provide the sealing action.
Generally speaking, overdrive (O/D) is the highest gear in the transmission. On most cars the automatic transmission has 3 speeds and overdrive (fourth speed). Overdrive allows the engine to have less rpm with higher speed for better fuel efficiency. When you switch it on, you allow the transmission to shift into overdrive mode after a certain speed has been reached (usually 30-40 mph depending on the load). When it's off, you limit transmission shifting to third speed.In normal driving conditions the overdrive should be always on.You may need to switch it off if you drive in a mountainous area. [The automatic transmission automatically shifts from O/D to 3rd gear when it feels more load. When it feels less load it shifts back to O/D, but under certain conditions, e.g., driving uphill or towing a trailer, the transmission cannot decide whether to stay in O/D or shift into 3rd speed and it starts shifting back and forth. That's the time when you may switch the O/D off and help the transmission to decide] . You also may need to switch it off when you want to slow down using the engine braking, for example, if you're driving downhill. For more details, check your owner's manual. Overhead Cam (ohc) Engine An engine with the camshaft(s) located on top of the cylinder head(s). Overhead Valve (ohv) Engine An engine with the valves located in the cylinder head, but with the camshaft located in the engine block.
P Parking Brake The parking brake holds one or more brakes continuously in the applied position. The parking brake employs the regular drum brakes on the rear wheel. Instead of hydraulic pressure, however, a simple mechanical linkage is used to engage the brake shoes. When the parking-brake pedal is depressed (or, in some cars, a hand lever is raised), a steel cable pulls taut a tension lever; other cables draw the brake shoes firmly against the drums. The release knob slackens the cables and disengages the brake shoes. The parking brake is self adjusting. An automatic adjuster in the piston moves on the thrust screw to compensate for lining wear. PCV valve See positive crankcase vent valve. Pinging The sound an engine makes when it is suffering from detonation or preignition. This is often a warning that the gasoline being burned has insufficient octane, and it can be a very hazardous condition for an engine. Pinion A small diameter gear with a small number of teeth designed to mesh with a much larger gear wheel or a toothed rod (rack). Used in rack-and-pinion steering and for speed reduction with an increase in power. Piston, Rings, and Wrist Pin The piston converts the potential energy of the fuel into the kinetic energy that turns the crankshaft. The piston is a cylindrical shaped hollow part that moves up and down inside the engine's cylinder. It has grooves around its perimeter near the top where rings are placed. The piston fits snugly in the cylinder. The piston rings are used to ensure a snug "air tight" fit. The wrist pin connects the piston to the connecting rod. Pistons are made of aluminum, because it is light and a good heat conductor. The piston head or "crown" is the top surface against which the explosive force is exerted. It may be flat, concave, convex or any one of a great variety of shapes to promote turbulence or help control combustion. In some, a narrow groove is cut into the piston above the top ring to serve as a "heat dam" to reduce the amount of heat reaching the top ring. Piston A partly hollow, cylindrical metal engine part that is closed at one end and fits into the engine cylinder. Connected to the crankshaft via the connecting rod and usually fitted with rings to seal it in the cylinder.
Plastigage
A thin strip of plastic thread, available in different sizes, used
for measuring clearances. Ply Rating A measure of the strength of tires based upon the strength of a single ply of designated construction. An eight-ply rating does not necessarily mean the tire has eight plies, but rather that the tires has the strength of eight standard plies. Positraction General Motors' trade name for a limited-slip differential. Pneumatic Tire A circular tube of rubber or synthetic rubber and fabric, and sometimes also steel, attached to the rim of the car's wheel, having resilience due to its containing air under pressure.
Power Brakes Power brake are of four general types: vacuum suspended; air suspended; hydraulic booster, and electro-hydraulic booster. Most power brakes use vacuum suspended units, which contain a large vacuum-powered booster device to provide the added thrust to the typical power-brake. Pressure on the brake pedal pushes forward a rod connected to the pistons of the two master cylinders. The pistons begin forcing fluid into the front and rear brake lines. At the same time, the brake-pedal pushrod positions the vacuum-control valve so that it closes the vacuum port and seals off the forward half of the booster unit. The engine vacuum line then draws off the air, creating a low-pressure vacuum chamber. Atmospheric pressure in the control chamber then pushes against the diaphragm, dividing the two chambers. The pressure on the diaphragm, which is locked to the pushrod, forces it forward, supplying even more pressure on the pistons. The safe driver is always ready to apply the total force needed to stop their vehicle, even if the engine quits (removing the power assist). Predetonation The undesirable "knock" or "ping" that occurs when the ignition of the air-fuel mixture occurs before the ignition spark. Also known as "pre-ignition". Propeller Shaft The long hollow tube with universal joints at both ends that carries power from the transmission to the differential on front-engined rear wheel drive vehicles. Proportioning Valve A hydraulic control valve which limits the amount of pressure to the rear brakes during panic stops to prevent wheel lock-up.
Pushrod
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