Keep in mind, it is reported that the cylinder head comes new from the factory with a super thin nickel coating to protect the aluminum head surface from *galvanic corrosion due to it being in contact with the steel head gasket. So if you mill/machine the head to ensure flatness, you are removing the protective nickel coating. This may be why the FSM says the head is not machineable?
Also, as you described; if you machine the head, you are bringing the valves, when at full open, closer to the top of the piston. This is an "
interference" engine, so valve to piston clearance is critical.
If you do machine the head, be sure and use the thickest head gasket available to compensate for material removed.
Quote:
*Galvanic Corrosion
"When aluminum and steel are placed in direct contact, galvanic corrosion can occur. This is because aluminum and steel have different electrochemical properties. When aluminum and stainless steel are used in an assembly together, the electrons from the aluminum will begin to transfer into the stainless steel. This can result in aluminum weakening! This weakened aluminum can cause it to deteriorate at a faster rate."
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