I have one of these but they won't pull an old corrioded aluminum head loose. They just tear it apart. A 7/16 ID X 1/2" OD hollow shell drill works best. Someone said this removes to much material but it don't. Measure the OD of the stud and the ID of the hole in the head. It cuts the crud between the stud and head whuch is what holds the head from comming loose. If a stud is not centered in the hole of the head it will remove a lillte material but not enough to hurt anything, What's the differance if the clearance between the head and stud is .030 or .040 on one side it is not a sealing joint anyway and the studs are not all centered in the exact center of the head holes. They are all off center slightly so with 21 or 48 studs the head can't move sidewards anyway. When the hole saw is used it should be turned by hand or at very slow speed with slight pressure, thin lubricant and the dirt blown out as the drill is rotated. With only a .030 wall thickness the drill will break if you get rough with them. G.M.