ca20e pistons?

[800horsepower]

hi guys i have just pulled the head off my 12 and have found that it has pistons to sleeve 16 valvles,why? do theses come out standard in them or what? and why do they allow 16v's for are they the same as 18det pistons, im currently in the prosses of having the head folwed and have sourced some bigger inlet valvles from a bmw and then gonna pull the block down and so on.and im not looking for any excuses of doing any 18det transfers.