Valve Seat
A hardened metal ring pressed into the cylinder head that the engine valve seals against when closed.
The valve seat is a hardened metal ring pressed into the combustion chamber ceiling of the cylinder head. When an engine valve closes, the valve head rests tightly against the valve seat, sealing the combustion chamber to ensure full compression and transferring heat from the hot valve to the cylinder head cooling jackets.
Valve seats are made of durable alloys like steel or stellite to withstand millions of valve impacts and extreme combustion temperatures. Over time, the seats can wear, erode, or accumulate carbon deposits, preventing the valves from sealing fully.
If a valve seat is damaged or if a valve does not seal completely, compression will leak out of the cylinder (known as valve blow-by), causing a rough idle, power loss, and misfires. In aluminum heads, overheating can cause a valve seat to drop out of its pocket, leading to catastrophic cylinder and piston damage.