Deck surfacing also affects CR

In this module on cylinder head machining, it stated that surfacing the head affects compression ratio. I don’t recall that being mentioned when discussing surfacing the deck in previous modules.

Machining the deck surface would affect CR correct? Since TDC and BDC positions haven’t changed, removing some deck height would mean the piston moves slightly more into the combustion chamber at TDC, increasing CR. Or is it normal to add a shim to the head gasket to compensate?

You are correct, removing material from the engine block deck will also reduce the unswept area, and it would normally be factored into the compression equation as the deck height (how far down, or above, the bore the piston top is).

Sometimes, because of manufacturing tolerances, the deck may be too high and be machined down, but as you suggest there are times it may be too low and, if required, this may be compensated for with a thicker head gasket - you may have noticed the more commonly tuned engines may have aftermarket gaskets available in different thicknesses, and these are to correct for excess skimming and/or tailor compression and/or quench/squish as desired by the engine builder.