Why Do Exhaust Valves Break?

An exhaust valve allows cylinder head gasses to escape, Expensive engine work is often necessary when an exhaust valve breaks. Though repair is needed once this occurs, having information on what caused the valve breakage might be important.

Types
There are two types of breakage to which exhaust valves can fall victim. One type involves a breakage where the head and the stem connect. A break can also occur at the point of the stem where the keeper-grooves have been built in.
Head Breakage
Exhaust valves can experience this type of breakage because of stretching that occurs due to constant heat. Thermal shock negatively effects exhaust valves, because sudden drops in the engine's temperature causes them to break.
Stem Breakage
Heavy wear in the keeper grooves will cause the stem to snap off an exhaust valve. Constant sideloading can cause stem breakage if the stem height has not been installed correctly.