Valve Guides
When rebuilding a cylinder head one item that you can count on being worn are the valve guides. Valve guides a prone to wear because of the constant friction between the guide and stem due to side forces on the valve stem caused by changes in valvetrain geometry or by the direct action of overhead cams.
WORN VALVE GUIDES - When the valve guides wear there is too much clearance between the guide and valve stem, this will contribute to excessive oil use by the engine. This is is applicable to both intake and exhaust guides.
Oil consumption is more of a problem on the intake side because of constant exposure to engine vacuum. On the intake side, oil drawn into the engine past worn intake guides can foul spark plugs 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 back of an intake valve can create hesitation and idle problems because the deposits interfere with proper fuel delivery.
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.
Poor 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 valve travels to the valve seat, and the remaining 25% goes up the stem and out through the guide. On Suzuki engines with three-angle narrow valve 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 basically if the valve guide is worn, the valve may run hot and burn.
In addition worn valve guides can also pass unmetered air that is drawn into the intake ports past the guides that will create an phenomenon similar to worn throttle shafts on a carb. This extra air reduces the intake vacuum and upsets the air/fuel calibration of the engine at idle, resulting in a lean misfire problem and a rough (tickover) idle.
VALVE FAILURE - Worn valve guides can cause valve breakage. The guides support and centers the valve as it opens and closes, a worn valve guide will allow the valve to wobble as it opens, which will cause it to drift off-center of the valve seat. This will cause the head of the valve to flex slightly each time it closes, and after so many cycles, the metal will fatigue and the head of the valve breaks off from the stem.
VALVE GUIDE WEAR - In order to check guide wear, some machinists insert a valve stem into a guide and feel for looseness by wobbling the valve, while others may use a valve seat pilot tool to check the guides. Either technique will reveal badly worn guides, although neither is a very accurate method of gauging guide clearances or wear.
The optimum way to check guide wear is with a gauge set designed for this purpose. A gauge set will give you precise measurements and can be used to measure any portion of the guide. To check guide wear and taper the use of a telescoping or split ball gauge, measures the guide ID at both ends and in the middle. You then subtract the middle reading from the ends to determine taper wear. Comparing the smallest ID measurement which is usually in the middle of the guide to the factory specs in order to determine total wear.Valve stems should also be measured to check for wear and sizing.
HONING VALVE GUIDES - Honing is usually recommended to smooth the guide bore by knocking the peaks off the ridges left by a valve guide reamer. This will produce a superior bearing surface that will retain oil better and last longer than an unhoned guide. New valve guides can benefit from honing due to the fact that new guides are often rather rough. Honing provides a uniform surface finish which will reduce stem and guide wear, and generally extend the life of the guides, no matter if the valve guides are cast iron or bronze.
VALVE GUIDE REPLACEMENT - Replacement valve guides come in various alloys and varieties including bronze, cast iron and powdered metal. Phosphor/bronze, silicon/aluminum/bronze and manganese/bronze are generally more expensive than cast iron but are harder and usually provide superior wear resistance.
Concerning the use of Racing Gasoline, bronze guides typically last 3 to 5 times longer than cast iron. But with Pump Gasoline the difference in longevity between cast iron and bronze isn’t that different. Although bronze valve guides still provide superior heat transfer, resists seizing and can handle closer tolerances which will improves valve life and reduces oil consumption.