There was a rubber pad on the shaft, yes. It seemed ok, but I've replaced it anyway because I had a generic starter rebuild kit handy. Didn't make a difference unfortunately.
I'm not sure if there are any differences between Chevy and GMC starters; that's simple to test though, I'll try it tomorrow with a negative ground instead.
Thanks fellas. I have a sinking feeling the armature is grounded internally though. I need to find a way to verify that without one of these "growlers".
