Check the thickness of the valve head with a vernier caliper.
Standard intake valve thickness: 1.05–1.45 mm.
Minimum tolerance: 0.5 mm.
Standard thickness of exhaust valves: 1.20-1.60 mm.
Minimum tolerance: 0.5 mm.
If the thickness of the cylindrical part of the poppet is less than the minimum allowable value, replace the valve.