I recently bought a hitachi 4.5 grinder, and put my old 4" dewalt disc on it, works fine. I went to put a new 4.5" delta cutting disc on it, and it is loose as heck! the 5/8" arbor (which was what the 4" disc hole was) was too small for the 7/8 disc. Checked the manual, and it says it's a 7/8!!! I don't get it. I even tried to put on the disc included with the grinder, and I can definitely feel an eccentricity when it runs, enough to make it uncomfortable to use.