A poppet or mushroom valve has 2 main parts, a stem and a head. It fits into a port in the head. Its face makes a gas-tight seal against the seat.
During operation, the head near the face of the valve transfers heat to the seat. Some is conducted up into the valve stem. The stem transfers heat on to the guide, so the stem is the valve’s coolest part.
The valve seat and guide are also cooled by coolant in passages around the valve ports.
When a valve does not seat properly, there’s a smaller area where heat transfer can occur. That means the face will overheat. Local hot spots can reach such extreme temperatures that the edge of the valve can actually burn.
The width of the valve seat is important. A narrow seat is desirable because a thin circular contact with the valve face forms an efficient seal.
But a wider seat is better for transferring heat from the valve to the cylinder head.
A common compromise is for the inlet valve to have a narrower seat than the exhaust valve.
