The valves should be lapped with a fine grinding paste. A rubber suction cup placed on the valve stem is used to provide the necessary rotational movements. The formation of scratches during the lapping process is prevented by frequently lifting and continuing to rotate the valve during the lapping process.
Note: After finishing lapping, carefully remove the grinding paste.
The grinding process can be controlled by the surface appearance or by fuel: insert the valve freely, fill the combustion chamber with fuel, fuel should not leak from the valve guide bushing. Otherwise, continue the grinding process.
(The original version is on the portal: www.bmwman.ru)