I highly doubt that (barring any abnormal use) your piston is hitting your head. its ver very unlikely. While the distances are INCREDIBLY close by human standards (less than a human hair thinkness), they are NOT touching. You would know it if it was hitting the head, because your engine would be destroyed. There are only a few ways this happens, all of which result in catastrophic engine failure. Unless you are a qualified mechanic, or at the very least, well versed in the Internal Combustion Engine and its workings, you are not gonna know where your tapping/knocking is coming from.
That being said, the ONLY way I could see you having a piston hitting the head and your engine isnt toast, is if your timing chain is stretched causing your timing to be ever so slightly off (happens after hundreds of thousands of miles of heavy use), or your piston connecting rod has stretched slightly, causing your piston to come high up on the exhaust stroke than tolerances allow. With tolerances we are talking Microns, not Milimeters, so theoretically, it is possible. Just incredibly unlikely. I have been wrenching since I was 15, and I have only seen it happen 1 time, and the engine had 420k on it (1990 Lexus LS400).