Our 15MM 180149 Stop Cock Light is a particular kind of valve used in plumbing systems to regulate water flow Similar to the heavy stop cock, it is made for lighter applications such as temporary use or low-pressure systems. The diameter of the valve which is commonly used in many plumbing systems is 15 mm. The 15MM 180149 Stop Cock Light is lighter and thus not intended for heavy use or high-pressure applications.