I was curious when was the last time you changed your Conrod? My experience similar to this was the cause of a loose bushing on the rod. Do to the play everytime it rotates the rod seems to move around inside freely causing it to hit. If you think about it the slightest millimeter off can make your piston head extend itself going to far or down making the travel of your piston & sleeve a little longer either way. I don't know but this is what I experienced so thought you might want to know if it will help.