you have to measure the port size and work out how much your gona take off you would idealy need to do it on a bench were you could clamp the head down so that you get it acurate how ever getting the size you whant my be a littel hard but ther are some good cutting tool you can by that adjust the cut but then your gona have to grade down from 40 grit to 80 grit and if it isnt shiny then use a past and wheel ther lots of ways off dowing it you can get flap wheels and keep going up in size but ther about £3 each and you will need about 3 per port to make perfect gooood luck