Tough motor and casing, but a small bump can tweak the base out of alignment. If you're wanting to use a clamped straight edge as a guide, you'll have to measure to make sure the base isn't tweaked, then you'll have to measure an offset of 1 5/16" to one side, or 3 21/32" on the other. Come on -- what's wrong with 1.5 or 3.5?  Who the heck designed this base anyway. A mixed bag for a lot of bucks.