I had the same problem when I bought my 850 from a lame DIY owner.
The steel screws into the head to access the plugs and the sparkplugs
were partially seized.
It was a PIA to get them out safely, and once out,
the threads needed re-tapping.
You use the same 14 X 1.25mm tap for the plugs and the O2 sensor,
a handy item for your tool box.
Always apply hi-nickle/hi temp never-seez to spark plugs and steel screws going into aluminum to avoid this misery down the line. Anti-seize is your friend.
Hint: You'll wish the PO used anti seize on the thermostat screws and of course
the always stubborn O2 sensor.
Cheers, Bill
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