Engine valve

• Valves are used to control gas flow to and from cylinders in automotive internal combustion engines. The most common type of valve used is the poppet valve.
• The valve itself consists of a disc-shaped head having a stem extending from its centre at one side. The edge of the head on the side nearest the stem is accurately ground at an angle – usually 45 degrees, but sometimes 30 degrees, to form the seating face. 
• When the valve is closed, the face is pressed in contact with a similarly ground seat.