when you rent the ridge reamer keep it flat against the bore & pull the cutter around with a box wrench as opposed to pushing it (the cutter) ahead of you. Stop often & feel for the amt of ridge left. it will generally not cut even (close tho) so discipline your self to stop when the lowest part of the ridge (least amt of ridge left) is just a hair above flush as you dont want to gouge below. EDIT As you are working around the cyl in a circle change/shift your position often so you are always (pretty much) pulling in a straight line toward you. Gets tough in some of the clocking positions on some of the cyls especially but that is an opportunity to discipline yourself/be a professional. In the car you'd wanna take the hood off but you are ahead of an "over the fender" overhaul cuz the block is already out