REMOVAL & INSTALLATION
The valve guides are so named because they guide the stem of the valve during its motion. By properly locating the stem of the valve, the head and face of the valve are kept in proper relationship to the head. Occasionally, the valve guides will wear causing poor valve sealing and poor engine performance. If this problem is suspected, the valve stem-to-guide clearance must be checked.
If the wear is minimal, it may not be necessary to replace the guides. A competent machine shop can knurl the guides. This process raises a spiral ridge on the inside of the guide (without removing it), thereby eliminating the wobble in the stem. This is not a cure-all; it can only be done to correct minimal wear.
Excessive wear of the valve guides is indicated by excessive stem-to-guide clearance. You can try inserting a new valve into the old guide and measuring stem-to-guide clearance again. If this brings tolerances well within specifications, it may be considered satisfactory to just to replace the valve.
If this does not bring the clearance to within specifications, the guide must be replaced. Provided the old valve is in good condition, it may be checked in the new guide. However, if the engine is receiving a complete overhaul, maximum service life will be achieved by replacing both the guide and valve. Valve guides should be replaced by an automotive machine shop equipped with the proper tools and presses.
The guide must be pressed out of the cylinder head using special tools and a hydraulic press. The tool number varies by specific motor (due to the diameter of the guide) and equivalent tools should be available from most suppliers. Use the tool backed up by a large press to press the guide out the bottom of the cylinder head.
Do NOT replace the guide with one of the same size, but use a guide that is one size oversize. The guide bore in the cylinder head must be machined to the exact dimensions of the outside diameter of the new guide. Press the new guide in, from the top, using the same tool. All these operations should be performed with the parts at room temperature. For each motor, the amount of projection of the valve guide is critical. Use of the correct special tool will ensure the guide is pressed in the correct distance.
The new or re-used valves should operate freely inside the new guides. A new guide may require reaming to achieve the correct stem-to-guide clearance. Once the guide is correctly mated to the valve, the valve face and seat must be lapped and then checked for correct seating. This must be done because the new guide may change the exact angle of the valve stem.