Oil entering the exhaust or intake due to wear in a cylinder head is most likely caused by wear in which part?

• A. Valve margin
• B. Valve face
• C. Valve seat
• D. Valve guide

Answer: (D)

Oil entering the exhaust or intake from wear in a cylinder head is most likely caused by wear in the valve guides. The valve guides keep the valve stem aligned and provide the oil that lubricates the stem as it moves. When a guide becomes worn, the clearance around the valve stem increases and oil can seep along the stem into the port—into the intake side or into the combustion chamber and then out the exhaust as the valve opens and closes. This burning oil shows up as blue smoke and higher oil consumption.

The other parts—valve margin, valve face, and valve seat—are mainly involved in sealing the combustion chamber. Wear there leads to compression or leakage issues between the chamber and the ports, not a direct path for oil to travel into the intake or exhaust like a worn valve guide does.