Most four-stroke motorcycle engines are fitted with a crankcase breathing system. These systems are required to moderate the pressure within the crankcase and to offset the volume variations caused by the normal movement of the piston(s). It is important to note that modern crankcase breathing systems allow combustion blowby gases to escape from the engine crankcase without getting released into the environment. Without a breather, these gases would otherwise build up pressure inside the motor causing poor performance and engine leaks – a good reason to keep your breather line unobstructed and breather filter clean (if equipped). Blowby gases contain atomized oil, unburnt fuel, water vapor and a host of other things the EPA does not want released into the atmosphere. While there are various systems that reclaim blowby gases, most consist of tubing that runs from the engine into the air cleaner box. In the airbox, these gases merge with fresh air and are recycled back into the engine through the normal intake system.

Padding, hard-shelled material or other impact-absorbing material fitted to a motorcyclist’s apparel. Usually placed in high impact areas such as shoulders, elbows, back and knees. Also common in sport bike gloves and boots.

The clearance between the meshed teeth of two gears. Usually this is measured by holding one of the gears stationary and measuring how far the other gear can move back and forth, using a dial indicator.

Motorcycles with a backbone frame use the engine as a stressed member. The backbone can be tubular or stamped steel or aluminum, and even carbon fiber is used in some exotic applications. The engine is suspended below the frame and adds rigidity and strength, allowing the frame to be smaller and lighter.

When a piston travels the length of its stroke, in normal operation, it is said to have reached bottom dead center when it is at the lowest position in the cylinder, closest to the crankshaft.