well, when i took my cooling head off a while ago and turned the flywheel, the same thing happened to me. the piston would just lift the sleeve out just like yours. i dont think thats a problem, at least i hope not. it hasn't changed anything about my engines performance since i found that out. the reason that happens is because every engine needs some pinch at the tdc. if not then your engine wont perform well. when you turn the flywheel and it gets to the tdc the piston hits that part of the sleeve where the pinch is, and it just lifts it up alittle. when the cooling head is on though, then it holds the piston down(obviously, lol). i dont think you should be worried about that. is this the 1st time you noticed it. cause im pretty sure that just means you have good pinch. sorry if i didn't explian well, lol.