R-bikes
R-bikes come in two flavors: Airheads and Oilheads. These two flavors share a basic engine layout in common, the Boxer, and not a whole lot else. They both look like motorcycles, and aside from basic things like wheels and handlebars, aren't too much alike. The Oilheads are clearly descended from the Airheads, but in the same way that a housecat is clearly descended from a tiger. (I won't hazard a guess which way 'round that goes.)

The term "R-bike" comes from the fact that motorcycles of this type are all named as R and then a number, like R65, R100 or R1200. Most of the motorcycles have a suffix, like LS, GS, R, RT, RS, etc. Each of these suffixes has a meaning, usually indicating a "body type" or "trim type" based on the same model. For example, an R100 RT is an R100 motorcycle with touring trim on it (big fairing, hard bags, etc.). An R80 GS is an R80 with modifications to make it more offroad worthy (GS stands for "Gelände/Strasse", which means "Field/Street" or "Country/Street").

Rat Bike
A motorcycle that's been kept running by any means possible, usually with mismatched parts and minimal maintenance. (See: Get your Sportster running Parts I and II)

Reciprocating Motion
Reciprocating Motion of an object between two limiting positions. Applied to piston engines because of the limited up and down motion of the pistons.

Redline
Redline The maximum recommended revolutions per minute for an engine. In cars equipped with a tachometer: an instrument that measures engine rpm: the redline is usually indicated by, surprisingly enough, a red line. Some tachometers mark the redline with a colored sector. Others have two lines: the lower one marking the maximum allowable sustained engine rpm, the higher line indicating the absolute maximum rpm.

REVOLUTION
The Revolution® engine, Harley-Davidson's first water-cooled engine (V-Twin, produced from 2002 – Current Day).

RICH MIXTURE
The designation L has been selected to identify the excess-air factor used to quantify the spread between the actual current mass A/F ratio and the theoretical optimum (14.7:1): lambda= Ratio of induction air mass to air requirement for stoichiometer combustion. lambda=1: The inducted air mass corresponds to the theoretical requirment.
lamda<1: Indicates an air deficiency, producing a corresponding rich mixture. Maximum power is derived from lambda=0.85...0.95.

ROD KNOCK
This term sends chills through every motorist that hears it but most people don’t know what it means. All they know is that it costs a lot to fix! When an engine develops a rod knock it’s because the connecting rod bearings have worn out. The crankshaft is what moves the piston up and down in its cylinder the piston is connected to the crankshaft via a connecting rod. This rod has at the end of it a round yoke split in half that bolts together. The yoke has Teflon bearing inserts that sit within it; these bearings ride smoothly on the crankshaft as the piston is pumped up and down. When the bearings wear out from lack of lubrication or just age and wear, a knocking sound starts to emanate from the crankcase. At first it might sound like a light pecking, but as time goes on, it will eventually sound like the Hammers of Hell! When this occurs, the only thing to do is either rebuild the engine or replace it, it all depends on the extent of the damage.

RPM
An acronymn that means: Revolutions Per Minute-- a unit of measure used to express the rotational speed of an engine. A term used to describe how fast a motor is spinning. Also known as "revs." Horsepower, Torque, and RPM are interrelated by the formula: HP = Torque * RPM/5252.