Standard bore is 65mm. stock compression is 7.6 to 1 . The high compression piston is no larger in bore size unless you order it that way. A high compression piston only sticks up into the head further to smash your fuel and air more. When you get your cylinder checked only bore it out enough to clean the cylinder and then order the piston to fit. For example take your cylinder to a shop and have them check it out, if they say it needs taken out 20 thousants, then order the piston. Don't do like some new people do and "punch" it out to the max. If you do; the next time you go for a rebuild, you will need to get a new cylinder. Hope my rambling-on helps [img]i/expressions/face-icon-small-happy.gif[/img][img]i/expressions/face-icon-small-happy.gif[/img]