A petcock is a valve used to control air and fluid flow through a vehicle's engine and coolant systems. The type of valve and the valve design differ per application. Some types of valves install within a fuel system to control the flow of gas between a fuel tank and a carburetor. Another type of petcock installs in a tank or radiator to allow a drain point for a vehicle's coolant system. Both types of valves operate using the same principles, although they differ in design.
Fuel petcocks install in an accessible location of a gravity-fed fuel system. Gravity fed fuel systems do not use fuel pumps, and theses petcocks have three positions. The positions determine the flow of the fuel through the system. When the petcock sits in the "closed" position, fuel will not flow to the engine's carburetor, and when the petcock is turned to "full," the fuel line will sit fully open. One last position on most fuel petcocks changes the flow of fuel from a main fuel tank to an auxiliary fuel tank.
Coolant system valves have one threaded and one smooth end. The threaded valve end secures in a hole located at the lowest point of the coolant tank. When the valve opens, coolant flows from the tank. These types of petcocks are often made of plastic or nylon with a brass handle. Over time, the components of coolant-style petcocks corrode and cause the valve to break instead of open.
Broken valves become difficult to remove without damaging the coolant reservoir. The process of removing a broken valve will vary based on the type of valve and the construction of the coolant tank or radiator. Improperly removing a broken petcock will strip the threads cut in the mounting hole threaded in the bottom of the tank or shear off a piece of the petcock in the body of the tank. Either situation results in an expensive repair to replace both components of the coolant system.
Even a non-broken valve will require replacement after years of exposure to coolants and fuels. The small size of a petcock does not allow the valve to be rebuilt like larger valve types. Signs of a worn fuel petcock include fuel leaking from the section of the valve located below the valve handle or fuel leaking from any part of the valve body. Worn radiator petcocks leak coolant from the center opening on the smooth end of the petcock body.