Specific Gravity
Refers to the relative weight of a fluid as compared to an equal volume of distilled water. It is common to check the specific gravity of engine coolant and battery electrolyte to determine if they are within specification using a tool called a hydrometer. There’s even a type of hydrometer that tells you when your homemade beer is ready to be bottled!
Stroke
When the piston in a motorcycle engine moves from TDC (Top Dead Center) to BDC (Bottom Dead Center) or from BDC to TDC - the distance traveled is referred to as the stroke. The stroke takes one half of a revolution of the crankshaft.
Tachometer
An instrument used to determine engine speed in revolutions per minute (rpm). Most motorcycles have a tachometer that sits next to the speedometer in the instrument cluster.
Trailing Shoe
The rear facing brake shoe on a drum brake system.
Voltage Regulator
A mechanical or electrical device used to control the output of a motorcycles charging system to protect the battery from being over charged. Over charging can cause permanent damage to a battery.
Zener Diode
A solid state electronic device used to regulate charging voltage in motorcycle charging systems.
Wiring Harness
The electrical wires that make up the electrical systems of a motorcycle are bundled together and encased in a protective sleeve (known as loom or conduit) to form what is known as the wiring harness. Different color wires are used for different components to make electrical testing and troubleshooting easier, and bundling the wires keep them protected and compact.
Backlash
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.
Piston Skirt
The part of a piston that is below the bottom piston ring and piston pin boss. The skirt acts as a guide as the piston goes up and down in the cylinder. It is often the part that shatters when the piston-to-cylinder clearance becomes excessive allowing the piston to rock in the bore.
Parasitic Draw Test
A diagnostic test used to check the electrical system of a motorcycle for current flow (amperage) when the key is off. The purpose of this test is to ascertain whether the battery is being drained while the vehicle is not in use. Some devices with memories such as clocks, radios and digital odometers draw a small amount of current with the key off, but if the motorcycle does not have any devices that must maintain a memory of information, there should be no current flow with the key off.
