I'm sure there is a method to achieve what you want like using a knife sharpening stone. I would search YouTube for examples. Probably use some blue dye to determine progress.

I would probably secure some thin metal shim on the sides of the high spirits and use a stone to shave it down to the level of the shims then remove the shims to finish the job.

Just an educated guess of what would be done. I would try to find some YouTube videos first.

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2011 E70 • N55 (me)
2012 E70 • N63 (wife)