One thing you can almost always count on when rebuilding a cylinder head is worn valve guides. The guides experience a lot of wear because of the constant friction between the guide and stem. To make matters worse, positive valve seals on late model engines prevent the guides from receiving much lubrication. Side forces on the valve stem caused by changes in valvetrain geometry or by direct acting overhead cams further contribute to guide wear.

WORN VALVE GUIDES
When the guides are worn or there is too much clearance between the guide and valve stem, the engine will use oil. This applies to both intake and exhaust guides. Though oil consumption can be more of a problem on the intake side because of constant exposure to engine vacuum, oil can also be pulled down the exhaust guides by suction in the exhaust port. The flow of exhaust past the exhaust guide creates a venturi effect that can pull oil down the guide.

Oil in the exhaust system of late model vehicles with catalytic converters may cause the converter to overheat and suffer damage. On the intake side, oil drawn into the engine past worn intake guides can foul spark plugs, cause the engine to emit higher than normal unburned hydrocarbon (HC) emissions, and contribute to a rapid buildup of carbon deposits on the backs of the intake valves and in the combustion chamber. Carbon deposits in the combustion chamber can raise compression to the point where detonation occurs under load. Deposits on the backs of the intake valves in engines equipped with multipoint fuel injection can cause hesitation and idle problems because the deposits interfere with proper fuel delivery.

Inadequate valve cooling and premature valve failure is another problem that can be caused by worn guides or ones with excessive clearance. About 75% of the heat from a typical valve is conducted to the seat, and the remaining 25% goes up the stem and out through the guide. On late mode engines with three-angle narrow seats, the amount of heat transfer that takes place through the stem is even higher because less heat can be dissipated through the seat. So if the guide is worn, the valve may run hot and burn.

Worn guides can also pass air. "Unmetered" air drawn into the intake ports past the guides creates an effect similar to worn throttle shafts on a carburetor. The extra air reduces intake vacuum and upsets the air/fuel calibration of the engine at idle. The result may be a lean misfire problem and rough idle.

VALVE FAILURE
Worn guides can also contribute to valve breakage. The guides support and center the valves as they open and close. A worn guide will allow the valve to wobble slightly as it opens, which cause it to drift off-center with respect to the seat. This can cause the head of the valve to flex slightly each time it closes (much like a valve with a nonconcentric seat). After so many cycles, the metal fatigues and the head of the valve breaks off from the stem.

Generally speaking, the intake valve stem-to guide clearance for most passenger cars ranges from .001 to .003 in., and .002 to .004 in. for exhaust guides (which generally require .0005 to .001 in. more clearance than the intakes for thermal expansion). Diesel engines as a rule have looser specs on both intake and exhaust guides than gasoline engines, and heads with sodium-filled exhaust valves usually require an extra .001 in. of clearance to handle the additional heat conducted up through the valve stems.