Lapping (sanding) head down, within spec but still dirty. Keep going?

[MoVolvos]

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NICE WORK! More bold than I and in my confidence do such for sure. Continue sanding and I would rotate alternating 180 degrees. Though heavy we are not perfect in our push, pull and slight pressing pressure as you slide the cylinder head across the sand paper. Any tolerance is made up by the gasket as you torque and that spot is where a leak would most likely occur.
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[vtl]

If you remove too much thickness from the head, you can increase the compression ratio too much. This can cause ping and the need for higher octane fuel.

This is a turbo engine, it already has too much compression ECM knows how to detect pings and advance ignition/ move VVT.