Petcock

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

The most common application of a petcock in an automobile is as a drain valve[1] for a radiator.

[citation needed] Petcocks may be used in vehicle heating and cooling systems, such as limousines and tractor trailers.

[3] Many John Deere and other tractors and stationary power units produced from the late 1920s to the early 1940s had compression relief petcocks on the engine block, one per cylinder.

Opening one with the engine running would result in a long blue-white flame if the cylinder was firing correctly.