Chevrolet Cars and Trucks
216-235-261 Engines 139 versus 168 tooth flywheels
Chevrolet introduced the 12 volt system in 1955 this means anything older will be a 6 volt vehicle. The 6 volt system was changed because 6 volts takes twice the current (amperage) as a 12 volt system and the entire auto industry was moving to 12 volts to make all components compatible. starter
When GM/Chevrolet went from 6 to 12 volts, they also reconfigured the starters. The starter drives (pinion gear) are not interchangeable. The new design for 1955 was not only a 12 volt starter, but more teeth on the flywheel ring gear. Since the starters have the same number of teeth they are not compatible. Both having 9 teeth on the starter drive gear however the pitch is different to compensate for the 168 teeth flywheel. This makes it very hard to tell by looking at just the starter. You can measure the pinion gear, 1" diameter= 168 and 1 1/8 = 139. this can be tricky to measure as the 9 teethThe 139-tooth flywheel ring gear will have 4 teeth per inch, while the newer 1955 12-volt systems that are generally associated with the 235 or 261 engine will have 168 teeth, or 5 teeth per inch.
Remember engines were commonly changed in cars and trucks, you may have a 1950 chevy car but someone along the way replaced with 1955 235 engine. Before selecting the starter, best to remove the starter and count the flywheel teeth or if you can measure the starter pinion gear that works too.