Strange looking wear on my 5.9 cylinders

Take a compression ring and push it down square just past the ridge with a piston and then mic the gap, then mic it at the middle and bottom of piston travel. The difference is the bore taper. .005 is usually the max allowable. use the circumference of 12.56637 as a STD 4.00 diameter piston. plug the deviation of the ring gap (.016 is max) into a calculator and add it to this diameter. devide it by pi (3.14159) and you'll get new diameter. Lets say you got a gap difference of 0.0160 thats about the limit. 12.582 is the bogey of a .005 cylinder diameter taper.. I had a stain like that and I used a stone glaze breaker but couldnt get rid of it. It was then that I realized...it was only a stain.....a smooth stain.....