If you don't do this and/or turn down the bolt heads the bolts will screw in at an angle and cross thread the case. I take the dremel and also grind a bit where the bolt heads seat so the bolts can be flush when screwed into the case. You can slot the holes in the magnum cylinder using a dremel or a file. The conversions to 038M I've done have used super cases and this was not an issue. The stihl service manual mentions that some metal may need to be removed on a 038 case to put a super cylinder on it (so it will fit flat on the case).
