Even today, starter motors run at virtual dead short resistance. As current begins to flow and the armature spins up a "counter EMF" builds up in the windings to prevent overcurrent. There is really no practical way to measure impedance since it is constantly changing due to starter speed and load.

The quickest way to burn out a starter is to lug it down so the armature speed drops, and/or hit it with inadequate battery voltage. A slow-turning starter and a low-voltage battery will roast the windings very quickly. 12 volts against a 6V starter spins it up fast, and the starter is actually safer on 12 that it is on 6, particularly if the 6V battery loads down to 4V or less during cranking.
Jerry


"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!