Yes, a long cranking session can damage a starter, particularly a 6V model running on 12 volts. the "official" word is 30 seconds of cranking, followed by a 20-minute cool down period. Shorten the crank time, and lengthen the cooldown for one running at twice its design voltage. Starters run at nearly dead-short resistance, and the cranking current can be tremendous. As the speed slows, the back-EMF or counter-voltage drops, allowing the starter to draw more current. Don't ever crank an engine until it won't turn any more- - - -it ruins batteries and starters.
Jerry
