suggest supporting the alt. with the shaft horizontal with an opening at the bottom so as not to wash the grease out of the bearings. Old alt. bearings would have worn seals
if any at all.
If it were mine I'd take it apart, replace the brushes
and bearings after a good cleaning
inside and out.
The few things that damage alternators are: vibration, overloading (bad batteries), short brushes
, bad bearings and contamination. And of course PEOPLE that hook them up wrong.
Some alt's are just lemons (bad windings).