DO NOT fit oversize pistons to a standard bore, it will sieze up solid after about 5 minutes running. The ridge at the top is caused by the top compression ring gradually wearing the bore over time,( tens of thousands miles). To check it correctly the bore needs to be checked for ovality and taper using a precision bore dial gauge...can be too expensive for the average home mechanic.
However you can use a standard compression ring pushed down a little at a time and checking the end gap using feelers to give you a rough idea of the condition, using the piston to push the ring down keeping it square in the bore, you won't be able to check for ovality using this method - only taper
The ridge can be removed using a metal scraper and careful use of fine emery paper, fit standard pistons and rings making sure the end gap of the rings is within spec, and don't forget to deglaze the bore.
