I'm no expert, but I'll take a guess.
An 11 tooth gear has a larger pitch diameter than a 9 tooth, so its centerline needs to be further from the other gear's centerline for the teeth to mesh properly. Using the mounting position for the 9 tooth cog will bind if an 11 tooth cog is on there, since you're trying to impose the same center to center distance between gears.
Not sure if I'm right in this case but it's something to think about.
|