after 9 years fixing starters...
#1 RULE....

 take starter APART...
 take shaft of starter and insert  the bendix end of the shaft
into the nose, which is held  in a vice.
 IF THE SHAFT CAN BE MOVED at the opposite end  (commutator end)
close to OR OVER  1/2 INCH SIDEWAYS... the nose bushing IS BAD...
(sometimes it was the whole nose, but thats a different problem
 replace nose bushing as needed.. EVEN IF YOU HAVE TO HAND MAKE
THEM...BTDT too

 I had a guy bring in a 23 Ford, had babbitt bearings in the
starter, and his complaint was he was tired of hand cranking
it...  stuck a composite brass bushing into the nose end (it DID
NOT show from the outside) and he had one of the few starters
that would turn over at a car show, AND start the car.
 this applies ESPECIALLY to 6 volt starters...

 we need x power to turn over the engine.
lets say at 6 volts it needs 200 amps to turn it over...
at 12 volts, the same amount of power only requires 100 amps..

 but if that 6 v armature is AT ALL LOOSE in the nose, it will
take 3 or 4 hundred amps QUICKLY- battery dies pretty quick.