Valve guides are hardened steel tubes that the valves ride in. They are machined to ten thousandths of an inch to insure the valves stay centered in their seats and seal properly. The guides are pressed into the aluminum cylinder heads. On older engines, the cylinder heads were made of cast iron so the guides were just holes drilled into them. A lot of wear takes place over time in the guides. Cast iron holds up well to that wear. Aluminum does not.
Rubber valve guide seals are pressed onto the tops of the guides. Oil splashes onto other moving parts in the valve train. The seals prevent oil from running down between the valve guide and valve stem into the cylinder where it would be burned. It is common for the seals to become worn on high mileage engines. Most often you will not notice the oil smoke from the exhaust after the engine is warmed up, but it is common for oil to slowly run down the guides when the engine is stopped. It collects in the cylinders, then burns off as a puff of blue smoke when the engine is started. That usually doesn't cause any other problem as long as it isn't too severe.
Saturday, April 3rd, 2010 AT 3:45 PM