I'll have to check the build date on my bike cause i have yet to have any problems with my valve cover gasket, even after the big bore install. might have been something they corrected mid model year.

 

when i was doing my valve check and adjustments, i noticed globs of silicon all over this area....most of my time on the valve adjustment was spent scraping and cleaning all the factory silicon off on the half moons to put the gasket back on..if you don't do this, i was warned that the gasket will not sit back correctly when putting it back together...kawasaki factory did a horrible silicon job on the valve cover...i just added a thin layer of new sealant before putting the rubber valve cover gasket back on....(little extra on the half moons)....there is nothing behind that rubber half moon...if you push it in with your fingers, that's enough to break the seal and cause it to leak...be careful not to push on that half moon when washing or something....