An extension bar is a long, generally solid piece of metal piping that allows a user to extend the reach of a socket wrench by as much as 12 inches (30.48 cm) or more. They are commonly employed to help reach bolts that are recessed or otherwise unreachable with a standard wrench. They are often used by mechanics and other related professions, who must loosen stiff or rusted bolts and work in compact spaces, like engine compartments.
An extension bar fits between a socket bit and a wrench, and allows the full torque created by a turn of the wrench to be transmitted along its length. This is invaluable in cases where, for instance, a bolt at the bottom of an engine by may originally have been installed at the factory, before the engine itself was installed. With the engine in place that bolt would become impossible to reach by hand or with an unmodified socket wrench.
Extension bars are commonly confused with breaker bars, as they are both often associated with garage work. They are very different tools, however, and certainly not interchangeable. While an extension bar adds effective length to the socket bit to help with reach, a breaker bar extends the length of a wrench's handle, increasing the maximum torque possible with a turn.
A breaker bar plays upon the way in which torque works, which is rotational force calculated as power multiplied by distance. This is important because, with hand tools, there is a physical limit to how much power can be applied by a human, determined by his own strength. Once this limit has been reached, the only way to further increase torque is to increase distance. This is known as mechanical advantage.
While a breaker bar, in essence, can be any piece of long, hollow metal extrusion that fits over the handle of a wrench, an extension bar must be finely turned to fit both the wrench end and the socket. The most common diameter for the connectors between these various parts is 3/8 of an inch (1 cm), though larger and smaller sizes are available.
One common usage of an extension bar is with spark plug sockets. Spark plugs, which are an integral part of the ignition system in an internal combustion engine, require periodic replacement and must be unscrewed from their location in the engine, not unlike changing a light bulb. Spark plugs are not always easy to reach or put leverage on, however. In these cases, a special spark plug socket with rubber padding for grip may be connected through an extension bar, to a socket wrench, to easily loosen and pull out the plugs.