ICE MK A-School Practice Exam

The sealing surface is the valve seat, and when that surface is provided by a valve seat insert, it gives a hard, smooth, wear-resistant area for the valve face to rest against. As the valve closes, it lands on this seat and seals off the combustion chamber, stopping leakage. The valve spring simply supplies the closing force to push the valve against the seat, while the pushrod and valve bridge are part of the timing and actuation mechanism and do not create the sealing surface. If the seat becomes worn or damaged, leakage can occur, which is why the insert is used to maintain a reliable sealing surface.