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Skimming the head on a quench type combustion chamber needs more meat off than an open chamber, as part of the bore area is already flat so is not included in the CC calculation. Every head design is different but for a closed chamber clevo, 5 thou off removes 1CC of chamber. I should imagine crossys to be similar since although the bore is smaller, the quench area is proportionally less than on a clevo. Another very important aspect is quench distance. If your pisons are down the hole, no amount of head machining will fix the poor quench that results, regardless of how high your compression is. Example: If i had my time over again, the clevo would've copped a set of proper zero-deck pistons instead of the Hypertec rebuilders, which sit down the hole too far. Either that or skim the block to get to zero. I ended up taking 0.025" off the heads to get to a bit over 10:1. If I'd have taken the meat off the deck instead, it would have only needed 0.015" to achieve the same cc, with improved quench. Best quench distance is generally said to be 0.040", which happens to be the thickness of a standard head gasket. People do go thinner than this but are pushing the envelope for kissing the head with the piston top. Sent from my CPH1607 using Tapatalk

Ask old mate. Every engine builder has their own thoughts on this.

Well I guess the answer to that is, what is your current piston to valve clearance? Minus what you intend to cut off the head and is that going to cause piston to valve issues.

Yep, it's basically the same, but, the combustion area is shaped differently, all the combustion with flat tops, is done in the head cavity. Don't know how that would affect a Crossy, Slydog would prob be the best fella to answer this. Obviously pulling the head off would be easier than engine out, and piston swap. 2mm is a fair chunk off alloy to remove, and it's bit harder to put it back. Can use copper head gaskets, (Cometic ? I think) but they're pricey. And any head swap in the future will need the same machining, as the original.

Some MotoGP bike manufacturers actually take a stock engine and reconfigure it to run backwards, on purpose. !!

Yep, what he said. With a high lift cam, etc...may need eyebrows cut in the pistons.