1987 Mazda Truck Starter Grinds will not start

When I turn the key it makes a terrible grinding sound, it sounds like the whole engine is breaking apart. Then you can hear the starter spinning but it will not engage. I changed the starter, but the problem persists. I took the starter out and visually inspected the flywheel, it is in perfect shape, no wear on the teeth and no broken teeth. I then tested the started again to make sure it the bedix was engaging all the way, it is.
What could cause this?