The armature windings are attached to the commutator bars in what appears to be a parallel arrangement, but electrically it isn't that. But that doesn't matter right now. As the armature rotates, a pair of brushes connects one end of a specific armature coil to power and the other end to ground. With your starter, you have two sets of brushes, two sets of field coils, and maybe two separate sets of armature windings.
You should not have any continuity between the windings/bars and the shaft or core, but you should have continuity from one bar to any other. Although, if your armature has two separate winding circuits, and everything is normal, you would likely find no continuity between two specific bars, probably those about 90° apart from each other, which would indicate two separate windings- in effect, two motors on the same shaft, inside one housing. Measuring bar-to-bar with an ohmmeter, you will find very low resistance- I'm not sure how much, but probably a few tenths of an ohm, but increasing as you increase the number of windings (number of bars between connections)in the measurement.
Another possibility for your trouble is that the insulating bushing on the power connection is broken and the field windings and insulated brushes are grounded to the case that way. It may be that when you pull the end frame away from the case, the ground connection is broken, by virtue of moving things apart. You should be able to take nut and washer off the power terminal and see the bushing is completely intact. If not, that may be where your trouble lies- or at least part of it.
I don't know how much all this helps, but maybe it will.
Harvester
