Do you have a battery load tester capable of putting a 300 amp load or more on the battery? When I'm diagnosing a starter I monitor the battery terminal voltage while the engine is cranking, and then load the battery to the same voltage and measure the amperage draw. A 6 volt starter in good condition should crank the engine at normal speed with approximately a 200-250 amp current draw. HAS ANYONE DONE A LOAD TEST ON THE "NEW" BATTERY?
Since most people don't have a heavy current capacity carbon pile load tester, cranking voltage is the only test that might give a clue about the condition of the starter. The battery terminals shouldn't drop below about 4.5 volts during cranking unless there's a problem with the starter, or the engine is almost seized up for some reason. (Rusty cylinder walls, etc.) Try spinning the engine with the spark plugs removed to see what speed you can get that way.
Have you done the 15 second cranking test with the ignition disabled like I suggested? Sometimes I think I'm shouting down an empty well because people tend to ignore my advice and make wild-donkey guesses. Maybe 60 years' experience at diagnosing electrical systems doesn't count!
www.youtube.com/watch?v=VeBb-gTjWKAJerry
