A list of car parts/automotive terms beginning with the letters M through R . . .

A casting of passages or set of pipes that connect the cylinders to an inlet or outlet source. 
Manual Transmission
Also known as "stick-shift", and a few other names besides, this is a much less common gearing method in New Zealand, where most cars are "automatic".  You change gears by depressing the clutch (which disengages the gear mechanism) and shifting the gear-stick to the appropriate gear.
Master Cylinder
The primary fluid pressurising device in a hydraulic system. In automotive use, it is found in brake and, hydraulic clutch systems and is pedal activated, either directly or, in a power brake system, through the power booster. 
Multigrade Oil
Oil that meets certain low-temperature and high-temperature requirements simultaneously (e.g., SAE l0W-40). 
The gear position in which the engine is unable to move the wheels. 
Octane Number
The rating number that indicates a gasoline's ability to resist knocking. 
Instrument that measures and records the number of kilometers or miles a vehicle travels. 
The substance used to lubricate and cool an engine's moving parts and reduce rust and corrosion. 
Oil Consumption
All engines use a small amount of oil over time. It gets past the piston rings and valve guide seals and is burned in the combustion chamber. A small amount escapes through the PCV system and a few drops usually managed to seep through a gasket or seal. The question is at what point should one consider oil consumption to be a problem? Any engine that consumes less than a litre of oil every 5000 kms is in excellent mechanical condition. If it uses less than a litre in 2500 kms, it's still in pretty good condition. But once oil consumption exceeds a litre every 1500 kms, it signals the engine is approaching retirement. Blue smoke in the exhaust or oil consumption on the order of a litre or more every 750 kms indicates serious oil burning problems (usually due to worn or broken piston rings, a cracked piston, or worn valve guides and/or seals). Sometimes a leaky seal or gasket can make an otherwise good engine use oil. The most frequent leak points are valve cover gaskets, crankshaft end seals and oil pan gaskets. Tightening the valve cover or pan bolts can sometimes stop a leak but usually the only cure is to replace the gasket .
Oil Filter
The filter used to strain oil moving through the engine. Oil filters should be replaced when the oil is changed. 
Oil Gauge
The dashboard gauge that indicates oil pressure. When this gauge lights up, it indicates a very low level of oil in the engine, which should immediately be replenished. 
Oil Pressure
The amount of pressure created in an engine's oil system by the oil pump. A certain amount of oil pressure is needed to circulate oil throughout the engine and to maintain adequate lubrication. Low oil pressure or loss of  pressure is dangerous because it can lead to expensive engine damage. A low oil level in the oil pan, oil leaks, dirty oil, diluted oil (with gasoline), too low a viscosity oil, a plugged oil pickup screen or oil filter, a worn oil pump or worn main bearings can all contribute to low oil pressure. Complete loss of oil pressure usually results from a broken oil pump drive shaft (if the pump is driven off the camshaft). Unless the engine is shut off immediately, it will be ruined. A sending unit mounted on the engine block monitors oil pressure. Oil pushes against a spring-loaded diaphragm, which in turn is connected to a resistor or set of contacts that trigger a warning light if pressure drops below about 4 or 5 psi.
Output Shaft
The shaft that transmits torque from a device, such as a transmission. 
Over Inflation
The condition that exists when a tyre is inflated beyond the pressure corresponding to the actual load or beyond the vehicle manufacturer's recommendation. 
Any gearset in which the output shaft turns faster than the input shaft. Overdrive gears are used in most modern transmissions because they reduce engine rpm and improve fuel economy. Occasionally, a separate gearbox with an overdrive gearset is coupled to a conventional transmission. 
Overhead Cam Shaft
The type of valvetrain arrangement in which the engine's camshaft(s) is in its cylinder head(s). When the camshaft(s) is placed close to the valves, the valvetrain components can be stiffer and lighter, allowing the valves to open and close more rapidly and the engine to run at higher rpm. In a single-overhead-cam (SOHC) layout, one camshaft actuates all of the valves in a cylinder head. In a double-overhead-camshaft (DOHC) layout, one camshaft actuates the intake valves, and one camshaft operates the exhaust valves. 
Overhead Valve (OHV)
An engine configuration in which all of the valves are located in the cylinder head and the camshaft is located in the cylinder block. The camshaft operates the valves via lifters and pushrods.
A handling condition in which the slip angles of the rear tires are greater than the slip angles of the front tires. An oversteering car is sometimes said to be "loose," because its tail tends to swing wide. 
The tyre construction utilising plies that run radially from bead to bead under the tread. This construction requires a belt to stabilize the tread and define the tyre diameter. 
The radiator is the most prominent part of the cooling system. Coolant that has travelled through the engine is pumped through the tubes of the radiator and is cooled off for another round.
Rear Toe
The toe setting of the rear wheels. Rear toe is not adjustable on rear-wheel drive cars with solid axle housings but is adjustable on many front-wheel drive cars and minivans. If rear toe is unequal, it can produce a diagonal wear pattern (heel and toe wear) on the rear tyres. 
Rear Wheel Drive (RWD)
A method of driving a vehicle whereby engine power is applied to the rear wheels. Power from the engine flows through the transmission, down the drive shaft, through the differential to the rear axles and wheels. 
A part of an alternator that used diodes to convert alternating current into direct current. It usually consists of three pairs of diodes. 
The working agent in an A/C system that absorbs, carries and releases heat. The two primary automotive refrigerants are R12 and R134a, but many other substances have similar properties (primarily a low boiling temperature) that allow them to be used as "alternative" refrigerants. But most of these substances are not "approved" for use in mobile A/C systems because of safety (flammability) or incompatibility concerns. 
A device that maintains the amperage and/or voltage levels of a circuit at predetermined values. 
The process of applying rust-inhibiting chemicals, waxes or sealers to the underside and inside of the vehicle's body as well as any other rust-prone areas. Not to be confused with undercoating which treats only the underside of the vehicle. Commercial rustproofing treatments usually include a guarantee for a certain number of years. Be aware that some guarantees require annual "checkups" to touch up any areas where the rustproofing may have been damaged.