did you.... replace the piston rings correctly in the piston lined up with the piston dowls fit new head o ring and gaskets tighten all bolts generally in a star shape so as to tighten it evenly fefit all electric connections like earth etc if it ran before then its something you have done. so trace your steps back and find the issue!
As creamsodauk said, its something you have done. Piston rings could of moved ever so slightly out of place, No gaskets or a crack in them. and the bolts should be fuc**** tight otherwise you will get air leaks and therefore will not start. Ive replaced loads of cyclinders etc but the first one i EVER done came out spot on, Was abit tricky but started first time and still NOT 1 Problem theres nothing to it mate, just take it back off and what ever you done when u took it off make sure its all how it should be. Good luck
