[MikeK]

Originally Posted by Logan

You can use the stock 6 volt starter on the 12 volts. It won't hurt it, it'll just spin twice as fast

Twice as fast? I don't think so. Given 6V and 125A cranking draw, that works out to 0.048 Ohms and 750 watts. 746 watts = one horsepower, so that's what you have on 6V. That same 0.048 Ohm starter motor on 12V will draw 250A, but the output power, calculated as Amps squared x Ohms, = 3000. Divided by 746 that's a bit over FOUR horsepower. Four times as much as the same starter on 6V. Some guys do it, but it really slams the starter drive and ring gear. Also, a 5/8" shaft is pretty weenie for 4 HP.

Changing the starter field wiring from two parallel coils to series is not a big deal, and you'll still end up with way more power. After this conversion you will draw a bit over 150A @ 12V, giving you 1800 watts, about 2.4 horsepower. That's way more than those spendy, fancy gear drive converts.

You can get away with using a 6V starter on 12V if you use a 4' x 6ga lead wire to the starter. Your draw will be about 165A, ~2.6 hp. You'll have to fold up and wire-tie the excess. The disadvantage here is you can only crank 20-30 seconds. That 6ga wire will get hot! Of course, that's not a real problem 'cause A's are known for starting right up on the first crank.