[MikeK]

There is a reason things are breaking! 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.

You can buy new 12V configuration starter field coils to make the conversion from the vendors. With the starter motor fields rewired from parallel to series, (making it a 12V starter) 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 excessive motor torque and poor off-shore manufacture and heat treating, the rear of the drive often cannot be compressed, even with the special removal tool, to access the set screw. It becomes hack saw and abrasive cutoff wheel time.

The motor field conversion is the only way to go. If you continue to slam the flywheel ring gear with a 6V starter on 12V you risk walking the ring gear off the flywheel.