You didn't mention if it's a new or used piston/sleeve. On a used p/s, if you can push the piston to the top of the sleeve it's probably worn out. If it's a new p/s and seems like a tight fit, that's o.k. Just break it in with a few runs at a rich needle-valve setting. Be sure not to over rev the engine before getting it on the water. I've seen a few 11/13cc K&B OBs go KBOOM when first being started.
JD