I was told (read it somewhere) that a new top ring might hit the ridge (right or wrong) with it still there. If you use one, put the box wrench on it & pull the cutter toward you as opposed to pushing it & change your position around the eng frequently so that you are pulling it at a good comfortable angle & press down on it at all times so it stays concentric down on the deck. It will never cut dead evenly so frequent fingernail checks & never go past flush at any point in the circumference which will likely leave a very slight ridge at some point but that is as good as it can get.