boring a cylinder is needed when ur rings cannot make propewr fitr and the surface of cylinder is worem out,,so even replacing new rings womnt make it fit seal,,
therefore the cylinder is bored out,,,means a small thin layer is cut fron inside ,,that increases the dia and new bigger sied rings and piston is needed,,
now companies have standardized when how much to cut,,
first overbore means that u r going to over bore the current to the first larges oversized piston, which was .5 or .5mm ,, itni layer cylinder sa cut hogi,,
second overbore was 1 jiska matlab ha 1mm,,
now if u go for first over bore then u use it fo another 50-60k kms and when again bore is needed u over bore to 1mm,,,
suppose u overedore it directly now to 1mm the u would be missing one life on ur cylinder,,
so it is recomemnnded to over bore to .5mm