Valve Seat Contact

After confirming that the dimensions of valve guides and valves are within specifications, perform this procedure.
Apply prussian blue (or white lead) onto contacting surface of valve seat to check the condition of the valve contact on the surface.
Check if the contact area band is continuous all around the circumference.
If not, grind to adjust valve fitting and check again. If the contacting surface still has NG conditions even after the re-check, replace valve seat.

Fig 1: Checking Valve Seat Contact Area
Courtesy of NISSAN MOTOR CO., U.S.A.