I'm not sure there is a vertical spec...or even a horizontal one for that matter. If you look at the sensor, you'll see that there is a wave washer installed down near the drive gear which allows for a decent amount of up and down play. However, if you have any lateral or horizontal movement of the shaft, that would be more of a concern. That would indicate the bronze bushing has worn or some other issue. That said, they can be rebuilt. I'm sure GNS Performance will chime in here.