6 Volt starters require a 139 tooth flywheel. a 168 tooth item WILL NOT work properly. There is a slight difference in the root diameter and the pitch of the 139 and 168 tooth flywheel ring gears, and they do not interchange with each other. Are you changing flywheels because of a bad clutch contact surface, or is there damage to the ring gear? Ring gears can be replaced without changing the entire flywheel.
Jerry

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