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A petcock is a small valve used to control the flow of liquid or gas.

Use on motorcycles[edit]

Fuel petcock on a 1978 Yamaha XS400.

Most older motorcycles have a fuel petcock valve mounted on or nearby the fuel tank to control the supply of gasoline. In the United Kingdom it is known as a petrol tap. Meriden Triumph Bonnevilles have two petrol taps, one on each side of the tank. The petcock typically has three positions: on, off, and reserve. The reserve position accesses the bottom portion of the fuel tank. Its functionality is especially useful on older or more basic motorcycles, which may not possess a fuel gauge. Many motorcycles now have an automatic, vacuum operated, petcock, with on and reserve as well as sometimes a prime position, which bypasses the vacuum operation and allows fuel to flow to the carburetor without the engine turning over. Another common option is to have a vacuum operated petcock with no reserve, and instead use a sensor in the tank to turn a light on when low on fuel. In most cases these will not have an off option either, and the petcock will be entirely transparent to the rider and not accessible without removing the fuel tank.

When operating a motorcycle the fuel management process often proceeds as follows: when regarding vintage motorcycles, the petcock is set to the off position when the motorcycle is not being operated. This is to eliminate fuel overflow and leakage via the carburetor. Before starting the engine the petcock is turned to the on position in order to provide gasoline to the fuel delivery system.

While operating the engine there will reach a point at which fuel consumption causes the level of gasoline in the fuel tank to fall below that which can be accessed by the petcock in the on position. At that time continued operation of the engine can be maintained. This operation is achieved by accessing the remaining fuel in the fuel tank via rotating the valve in the petcock to the reserve position.

Use on automobiles[edit]

Few modern automobiles currently in use have a petcock. This is because fuel pumps are used, rather than a gravity feed fuel system.

The Porsche 356 and early Volkswagens had a petcock with off, on, and reserve positions, much like a motorcycle petcock.

Limousines may use a petcock to control fuel flow, since fuel travels several feet farther than on typical automobiles. However, electronic petcocks are available on most prefabricated limousines.

Many automotive radiators use a stopcock to allow for the draining of coolant, but this bears very little resemblance to fuel supply type petcocks.

Use on farm equipment[edit]

On John Deere tractors and stationary power units produced from the late 1920s to the early 1940s, compression relief petcocks were placed on the engine block, one for each cylinder. These petcocks were opened during starting to aid in spinning the flywheel by hand. Once the engine started they were closed and the engine would begin running smoothly.

Use on large marine engines[edit]

The large MAN six-cylinder diesel engines used on German U-boats had petcocks that enabled the engineers to verify combustion in each cylinder. Opening the petcock with the engine running would result in a long blue-white flame if the cylinder was firing correctly. There is a description of the procedure in the novel Das Boot and it is also shown in the film version.