Hello, and wellcome, I have a 21r bottom end and i change the gear box/wet clutch for a sp/se box and dry clutch, Durning the change it was pointed out to me to watch this seal as you can NIC it and cuase damage when you replace the box, did you mechanics take the box out? as i cant see how they would change a seal with it in, On the inside there is a bearing and a seal with a spring in it, I would take the box out and change the bearing and seal at the same time someone here will give you some run out figures for the shaft to make sure its not worn or ndamaged!