so what is the thickness difference of using a head gasket and using o-rings (assuming the grooves are cut to proper depth/depth = RS250 grooves)?

My assumption is that it will be less clearance using o-rings.To make up for the less clearance, you would want to use a larger base gasket to make up for that difference.

Depending on the difference between o-rings and gaskets, you could be adding more clearance from stock, or less clearance from stock (but greater clearance then stock base gasket with o-rings).

Of course you would also need to take into account the tolerance (or lack of) in paper/material base gaskets and a .6 may be a .59 or possible a .62. Then you take into account the differences in size of pistons.

This is why I measure my base gaskets (and pistons) and hand pick base gaskets to match the pistons I am using to make sure my deck height is what I want/need. I may go with a fat .6 base gasket, or a thin .6 to produce the deck height I am going after.
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Charles Gallant