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Model A Electrical and Ignition Technical Forum

Ignition, Electrical, Starting and Charging
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6V ~ 12V Starter conversion info

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  • 6V ~ 12V Starter conversion info

    Original Thread

    converting 6 volt starter to 12 volt? can the average Joe do this or is it a pro job, I don't have any new parts but several starters.

  • #2
    Just buy a modern bendix drive and your good to go. This one is in Bratton's catalog. Pat


    • canadian
      canadian commented
      Editing a comment
      Pat so is this the problem with these , starter turns to fast on 12 and damages bendix ?

    • pAAt
      pAAt commented
      Editing a comment
      Yes, the bendix will break off and fall into you flywheel housing. The starter will handle all the 12 volt you give it. Pat

      BNCHIEF commented
      Editing a comment
      The starter will handle 12 volt however it will really throw that bendix into the flywheel gear ring, it will work but that is not the best solution they do offer 12volt starters for a reason.

  • #3
    Let's slow this down just a tad and think thru some of the advice above. Not saying that it is wrong, but a smart man can make good decisions when he knows ALL the facts.

    To begin with, there are two sets of coils available for the Model-A starter. A set that is wound for 6 volts in my mind will typically spin twice as fast when powered by 12 volts. I will yield to Tom's opinion on this however the point I am trying to make is it initially spins the starter shaft at twice the speed before it is engaged. This causes the Bendix to slam into the ring gear which is brutal on the Bendix gear and the flywheel ring gear. The weak link in this is the Bendix bolts which often times shear and fall down into the flywheel housing, ...which left will generally catch between the flywheel and the flywheel housing and crack/break the housing.

    Since it has been mentioned above to consider using the modern Bendix, the issue they had (--and I am assuming they still do with these) is the modern Bendix teeth pitch does not properly match the gear teeth on the ring gear, and over time the teeth wear prematurely requiring the engine or transmission to come out for replacement.

    If I were looking to convert to a 12 volt starting system, I would look towards replacing the field coils first with ones manufactured to work on 12 volts, and then use the stock Bendix system with NOS or NORS Bendix bolts.


    • #4
      Brent you are exactly right I know can and do do it but the right way to go is converting the starter to twelve volt to begin with.


      • #5
        I had a local battery car electric shop convert mine. I bought 12v coils and they installed and tested the starter. Did not charge very much.



        • #6
          It depends on how hard you want the starter to slam the drive. A 6V starter on 12V will deliver four times the torque, not just twice as much. Power =amperage squared times resistance. The amperage will double for a 6V starter on 12V. When you square that, you get 4X engagement slam force.

          With the starter fields rewired from parallel to series, (making it a 12V starter) and then feeding that in series through the same armature, the average converted starter will still show almost 2X the power, but not so much as to severely abuse the drive.

          As far as 'barrel' drives go, they are not without their own problems. The can and do fail and are extremely difficult to change on-the-road unless you carry a special tool to compress and expose the shrouded set screw. When a barrel drive fails and locks in the extended position because the much finer helix shatters from poor off-shore manufacturing heat treating, the rear often cannot be compressed to access the set screw. It becomes hack saw and abrasive cutoff wheel time.

          People seem to forget an engine in good health and tune does not need a very fast crank to start. If it did, the hand crank would be useless.

          I say change the starter fields to 12V configuration, it will still perform much stronger and crank way faster than a 6V starter on 6V. You will also experience much less voltage drop to the ignition circuit during cranking.


          • #7
            About 10 years ago I did some tests using a 6 volt and a 12 volt battery on my 1928 Phaeton. I recorded the amps, battery voltage during cranking, and the engine cranking RPM. I posted the specs, but they were most likely on Shelly's forum, so may be lost now. I won't repeat the tests because I didn't even like doing it the first time, due to the harsh engagement of the starter drive while using 12 volts. At the very least you should change the field windings to series, as Mike suggested. Some of the dealers also sell a gear reduction 12 volt starter that fits the Model A.


            • #8
              Back to your original question, "...can the average Joe do this or is it a pro job...?" Unless you have the proper equipment to expand the field coil retainers and the are able to remove/tighten the retaining screws my opinion is no, the average Joe cannot do the job correctly and may even damage the armature.


              • canadian
                canadian commented
                Editing a comment
                Thanks Mike V, agreed.

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