Johno, it sounds as if you may have fallen into a trap. The piston rings do not reach the top of the bore, that is why I said you need to measure the bore at right angles to the crankshaft, 10 mm down the bore. By using vernier calipers, you have actually measured the top edge of the bore, which has never experienced any wear. This part has what is commonly called a "ridge" which is of standard diameter, and prevents the piston from being removed from the cylinder by pushing it upward, since the rings catch on the ridge. It is standard automotive practice to remove the ridge with a special tool at the beginning of engine overhaul processes.
You need to measure the bore in the place I said, preferably by using an inside micrometer, but failing that, by using inside calipers and transferring the dimension to your vernier calipers. I think you will find it is at least at the Honda service limit for diameter, but probably quite a bit larger than that.
