[TAQuest]

Originally Posted by lust4speed

The critical thing is the depth of the starter pinion teeth with the ring gear teeth. The aluminum casting size can be all over the map -- or not even there on most aftermarket starters. Even for stock starters, the 1/8" measurement is dependent on how frisky the guy happens to have been with the deburring tool that day when cleaning up the raw casting.

You didn't understand the measurement. It is between the flexplate tooth tip and the starter shaft. Nothing to do with the casting.
What the measurement does is match the manufacturing call out for how the starter was made as per the print.
When they make the starter gear they make it so that if the shaft it slides on is 1/8" from the tip of the flexplate tooth tip the gear will mesh correctly.
It's how the starter is checked at quality control when they make them.
The variance in the castings is why they use the gear shaft as a reference.