Check the number of teeth. IIRC they use a "hunting tooth" so that the gears don't wear out unevenly. There's one more tooth that there should be on one wheel and that way they move relative to one another but because the diameters are the same, they stay in synch but the marks "move".
Now I could be completely wrong with that. I am trying to remember something I haven't had to think about for decades. And I sold all my T120 stuff a very long time ago, so I don't even have the manuals to check.
Now I could be completely wrong with that. I am trying to remember something I haven't had to think about for decades. And I sold all my T120 stuff a very long time ago, so I don't even have the manuals to check.