When repairing cylinder heads with leaky valves, it is not enough to lap or replace the valves and valve seats. In addition, it is imperative to check the wear of the valve guides. This is especially important to do on engines that have high mileage. Worn guide bushings do not center the valves and cause high oil consumption. If wear is high, replace valve guides (service station work).
Remove valve.
Clean the guide bush with a cylindrical brush.
Insert the valve into the guide sleeve from the side of the combustion chamber and check its play when the valve is moved laterally in both directions.
Attach a steel ruler to check. The backlash should not exceed 0.8 mm; on a 4-valve M50 engine: 0.5 mm.
If required, replace valve guides (service station work).