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Motorcycle Glossary of Terms


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Octane - The measure of a fuel's resistance to knock.

OEM - Original Equipment Manufacturer.

Off-road bike - Motorcycle designed for off-road use, not necessarily road-legal.

OHC - Overhead cam engine.

Ohm - The unit of electrical resistance. Ohms = Volts/Current.

Ohmeter - An instrument for measuring electrical resistance.

Oil cooler - System for diverting engine oil outside of the engine to a radiator for cooling purposes.

Oil injection - A system of two-stroke engine lubrication where oil is pump-fed to the engine in accordance with throttle position.

Open circiut - Am electrical condition where there is a break in the flow of electricity - no continuity (high resitance).

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.

Overhead cam - An engine with overhead camshaft or camshafts operating its valves.

Overhead valve - A valve mounted in the cylinder head.

Oversize (OS) - Term used for piston and ring size options fitted to a rebored cylinder.