You do not need a flex hone. You need an adjustable hone which stays at a fixed diameter after it is set. Otherwise, you wil be following the walls as they are with taper and out of round. With a proper hone you will cut a round hole the same diameter from top to bottom. Cut first with 80 grit to 120 depending on how much you need to remove. Finish with 220. If you took it into a professional machine shop they would plateau hone with a diamond embedded stone to do the final finish.