Why would you rework the head before selling?
Unless the car is perfect; seems you are throwing away money.
If Frank says the valves are the problem, take his diagnosis as fact.
Now; if you plan to keep it, the rework has good logic.
Sounds almost as if you would like to keep the car, if it would run right.
I am local, if you want to talk over coffee.

__________________
ASE Master Mechanic

Prototype R&D/testing:
Thermal & Aerodynamic System Engineering (TASE) Senior vehicle instrumentation technician.
Noise Vibration and Harshness (NVH).
Dynamometer.
Heat exchanger durability.
HV-A/C Climate Control.
Vehicle build.
Fleet Durability
Technical Quality Auditor.
Automotive Technical Writer

1985 300SD
1983 300D

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